How many mammal species are there now? Updates and trends in taxonomic, nomenclatural, and geographic knowledge

Journal of Mammalogy, 2025, 106, 1082–1117 https://doi.org/10.1093/jmammal/gyaf047 Advance access publication 14 September 2025 Research Article How many mammal species are there now? Updates and trends in taxonomic, nomenclatural, and geographic knowledge Connor J. Burgin1,*,, Jelle S. Zijlstra2,, Madeleine A. Becker3,4,5,, Heru Handika6,, Jesse M. Alston7,, Jane Widness8,, Schuyler Liphardt9,, David G. Huckaby10,, Nathan S. Upham11,*, 1Department of Biology, University of New Mexico, Albuquerque, NM 87106, United States 2Hesperomys Project, Orinda, CA 94563, United States 3Smithsonian-Mason School of Conservation, Smithsonian Institution and George Mason University, Front Royal, VA 22630, United States 4Center for Conservation Genomics, Smithsonian’s National Zoo and Conservation Biology Institute, Washington, DC 20008, United States 5College of Science, George Mason University, Fairfax, VA 22030, United States 6Museum of Natural Science and Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, United States 7School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721, United States 8Department of Anthropology, Yale University, New Haven, CT 06511, United States 9Division of Biological Sciences, University of Montana, Missoula, MT 59812, United States 10Department of Biological Sciences, California State University, Long Beach, Long Beach, CA 90840-3702, United States 11School of Life Sciences, Arizona State University, Tempe, AZ 85287, United States *Corresponding authors. Connor J. Burgin, Department of Biology, University of New Mexico, Castetter Hall, 219 Yale Blvd NE, Albuquerque, NM 87106, United States. Email: [email protected]; Nathan S. Upham, School of Life Sciences, Arizona State University, 777 E. University Dr., Tempe, AZ 85287, United States. Email: [email protected] Associate Editor: Alexandre Percequillo Abstract The Mammal Diversity Database (MDD) is an open-access resource providing up-to-date taxonomic, nomenclatural, and geographic data for global mammal species. Since its launch in 2018, the MDD has transformed the traditionally static process of updating mammalian taxonomy into regular online releases reflecting the latest published research. To build on this foundation, we here present version 2.0 of the MDD (MDD2), which catalogs 6,759 living and recently extinct mammal species, representing net increases of 4.1% and 24.8% over MDD version 1.0 and Mammal Species of the World, 3rd edition (MSW3), respectively. Additionally, we identify a net increase of 68.8% (+2,754; 3,149 splits + de novo, 395 lumps) species since 1980 at a rate of ∼65 species/yr based on past totals from 14 mammalian compendia, leading to projections of ∼7,079 species by 2030 and ∼8,376 by 2050 if these trends continue. Key updates in MDD2 include: (i) codings of US state, country, continent, and biogeographic realm geographic categories for each species; (ii) a comprehensive nomenclatural dataset for 50,230 valid and synonymous species-rank names, curated with type locality and specimen information for the first time; and (iii) integration between the MDD and the databases Hesperomys and Batnames for greater data accuracy and completeness. These updates bridge critical gaps in the taxonomic and nomenclatural information needed for ongoing revisions and assessments of mammalian species diversity. Using these data, we evaluate temporal and geographic trends over the past 267 yr, identifying 4 major time periods of change in mammalian taxonomy and nomenclature: (i) the initial monographic description of traditionally charismatic species (1758 to 1880); (ii) the peak of descriptive taxonomy, describing subspecies, and publishing in journals (1881 to 1939); (iii) the shift toward revisionary taxonomy and recognizing polytypic species (1940 to 1999); and (iv) the current technology-driven period of integrative revisionary taxonomy (2000 to present). Geographically, new species recognition since MSW3 has been concentrated in equatorial, mountainous, and island regions—highlighting areas of high mammal endemism (e.g., Madagascar, Philippines, Andes, East Africa, Himalayas, Atlantic Forest). However, gaps in 21st-century taxonomic activity are identified in West and Central Africa, India, and some parts of Indonesia. Additionally, lagging conservation assessments are alarming, with 25% of the MDD2-recognized mammal species allocated to the “understudied” conservation threat categories of Data Deficient (11%) or Not Evaluated (14%), underscoring the need for greater taxonomic integration with conservation organizations. Governance advancements in MDD2 include the establishment of external taxonomic subcommittees to guide data collection and curation, a rewritten website that improves access and scalability, a cross-platform mobile application that provides offline access, and new partnerships to continue linking MDD data to global biodiversity infrastructure. By providing up-to-date mammalian taxonomic and nomenclatural data—including links to the text of original name descriptions, type localities, and type specimen collections—the MDD provides an integrative resource for mammalogists and conservationists to more easily track the status of their study organisms. Key words: biodiversity, biodiversity infrastructure, conservation, database, distribution, extinction, integrative taxonomy, mammal, nomenclature, species. Received: January 21, 2025. Revised: June 10, 2025. Revised: June 11, 2025. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/ by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact [email protected] for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact [email protected] . © The Author(s) 2025. Published by Oxford University Press on behalf of the American Society of Mammalogists, www.mammalogy.org. Research Article Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1083 “¿Cuántas especies de mamíferos hay ahora? Actualizaciones y tendencias en el conocimiento taxonómico, nomenclatural, y geográfico” Resumen La Mammal Diversity Database (MDD) es un recurso de acceso abierto que proporciona datos taxonómicos, nomenclaturales y geográficos actualizados para las especies de mamíferos a nivel global. Desde su lanzamiento en 2018, la MDD ha transformado el proceso tradicionalmente estático de actualizar la taxonomía de mamíferos en lanzamientos en línea periódicos que reflejan la investigación publicada más reciente. Para ampliar esta base, presentamos aquí la versión 2.0 de la MDD (MDD2), que cataloga 6,759 especies de mamíferos vivos y recientemente extintos, representando incrementos netos del 4.1% y 24.8% con respecto a la versión 1.0 de la MDD y a Mammal Species of the World, 3.a edición (MSW3), respectivamente. Además, identificamos un aumento neto del 68.8% (+2,754; 3,149 escisiones + de novo, 395 fusiones) en el número de especies desde 1980 a una tasa de ∼65 especies/año según totales históricos de 14 compendios de mamíferos, lo que lleva a proyecciones de ∼7,079 especies para 2030 y ∼8,376 para 2050 si estas tendencias continúan. Las principales novedades en MDD2 incluyen: (i) codificaciones de categorías geográficas a nivel de estado de EE. UU., país, continente y reino biogeográfico para cada especie; (ii) un conjunto integral de datos nomenclaturales para 50,230 nombres de rango de especie válidos y sinónimos, curado por primera vez con información de localidad y ejemplares tipo; y (iii) la integración entre la MDD y las bases de datos Hesperomys y Batnames para lograr una mayor precisión y exhaustividad de los datos. Estas actualizaciones cubren brechas críticas en la información taxonómica y nomenclatural necesaria para revisiones y evaluaciones en curso de la diversidad de especies de mamíferos. Usando estos datos, evaluamos las tendencias temporales y geográficas a lo largo de los últimos 267 años, identificando cuatro grandes períodos de cambio en la taxonomía y nomenclatura de mamíferos: (i) la descripción monográfica inicial de especies tradicionalmente carismáticas (1758–1880); (ii) el pico de la taxonomía descriptiva, describiendo subespecies y publicando en revistas (1881–1939); (iii) el cambio hacia la taxonomía revisionista y las especies politípicas (1940–1999); y (iv) la etapa actual impulsada por la tecnología de la taxonomía revisionista integrativa (2000–presente). Geográficamente, el reconocimiento de nuevas especies desde MSW3 se ha concentrado en regiones ecuatoriales, montañosas e insulares, destacando áreas con alto endemismo de mamíferos (p. ej., Madagascar, Filipinas, Andes, África Oriental, Himalaya, Bosques Atlántico). Sin embargo, se identifican brechas en la actividad taxonómica del siglo XXI en África Occidental y Central, India y algunas partes de Indonesia. Las evaluaciones de conservación rezagadas son alarmantes, pues el 25% de las especies de mamíferos reconocidas en MDD2 se asignan a categorías de amenaza de conservación «poco estudiadas»: Datos Insuficientes (11%) o No Evaluadas (14%), lo que subraya la necesidad de una mayor integración taxonómica con organizaciones de conservación. Los avances en la gobernanza de MDD2 incluyen el establecimiento de subcomités taxonómicos externos para guiar la recolección y curación de datos, un sitio web reescrito que mejora el acceso y la escalabilidad, una aplicación multiplataforma que ofrece acceso sin conexión, y nuevas alianzas para continuar vinculando los datos de MDD con la infraestructura mundial de la biodiversidad. Al proporcionar datos taxonómicos y nomenclaturales de mamíferos actualizados—incluyendo enlaces al texto de las descripciones originales de los nombres, localidades tipo y colecciones de especímenes tipo—la MDD constituye un recurso integrador para que mastozoólogos y conservacionistas puedan rastrear más fácilmente el estado de los organismos que estudian. “Quantas espécies de mamíferos existem agora? Atualizações e tendências em o conhecimento taxonômico, nomenclatura, e geográfico” Resumo O Mammal Diversity Database (MDD) é um recurso de acesso aberto que fornece dados taxonômicos, nomenclaturais e geográficos atualizados para as espécies de mamíferos em escala global. Desde seu lançamento em 2018, o MDD transformou o processo tradicionalmente estático de atualizar a taxonomia de mamíferos em lançamentos on-line periódicos que refletem as pesquisas publicadas mais recentes. Para expandir essa base, apresentamos aqui a versão 2.0 do MDD (MDD2), que cataloga 6.759 espécies de mamíferos vivos e recentemente extintos, representando aumentos líquidos de 4,1% e 24,8% em relação à versão 1.0 do MDD e ao Mammal Species of the World, 3a edição (MSW3), respectivamente. Além disso, identificamos um aumento líquido de 68,8% (+2.754; 3.149 cisões + de novo, 395 fusões) no número de espécies desde 1980, a uma taxa de ∼65 espécies/ano com base em totais de 14 compêndios de mamíferos, levando a projeções de ∼7.079 espécies até 2030 e ∼8.376 até 2050, caso essas tendências continuem. As principais atualizações no MDD2 incluem: (i) codificações de categorias geográficas por estado dos EUA, país, continente e reino biogeográfico para cada espécie; (ii) um conjunto abrangente de dados nomenclaturais para 50.230 nomes em nível de espécie (válidos e sinônimos), organizado pela primeira vez com informações sobre localidades e espécimes-tipo; e (iii) a integração entre o MDD e os bancos de dados Hesperomys e Batnames para maior precisão e completude dos dados. Essas atualizações preenchem lacunas críticas nas informações taxonômicas e nomenclaturais necessárias para revisões e avaliações em curso da diversidade de espécies de mamíferos. Com base nesses dados, avaliamos as tendências temporais e geográficas ao longo dos últimos 267 anos, identificando quatro grandes períodos de mudança na taxonomia e na nomenclatura de mamíferos: (i) a descrição monográfica inicial de espécies tradicionalmente carismáticas (1758–1880); (ii) o ápice da taxonomia descritiva, descrevendo subespécies e publicando em periódicos (1881–1939); (iii) a transição para a taxonomia revisionista e espécies politípicas (1940–1999); e (iv) o período atual, impulsionado pela tecnologia, de taxonomia revisionista integrativa (2000–presente). Geograficamente, o reconhecimento de novas espécies desde o MSW3 concentrou-se em regiões equatoriais, montanhosas e insulares, ressaltando áreas de alto endemismo de mamíferos (por exemplo, Madagascar, Filipinas, Andes, Leste da África, Himalaia, Mata Atlântica). No entanto, foram identificadas lacunas na atividade taxonômica do século XXI na África Ocidental e Central, na ndia e em algumas partes da Indonésia. Atrasos nas avaliações de conservação são preocupantes, pois 25% das espécies reconhecidas no MDD2 estão atribuídas às categorias de ameaça de conservação “pouco estudadas”: Dados Insuficientes (11%) ou Não Avaliado (14%), destacando a necessidade de maior integração taxonômica com organizações de conservação. Os avanços de governança no MDD2 incluem o estabelecimento de subcomitês taxonômicos externos para orientar a coleta e a curadoria de dados, a reformulação do site que aprimora o acesso e a escalabilidade, um aplicativo multiplataforma que oferece acesso off-line e novas parcerias para continuar vinculando os dados do MDD à infraestrutura global de biodiversidade. Ao fornecer dados taxonômicos e nomenclaturais de mamíferos atualizados—incluindo links para o texto das descrições originais dos nomes, localidades-tipo e coleções de espécimes-tipo—o MDD oferece um recurso integrador para que mastozoólogos e conservacionistas possam acompanhar mais facilmente o status de seus organismos de estudo. Palabras clave: base de datos, biodiversidad, conservación, distribución, especies, extinción, infraestructura de biodiversidad, mamíferos, nomenclatura, taxonomía integradora Palabras Clave: base de datos, biodiversidad, conservación, distribución, especies, extinción, infraestructura de biodiversidad, mamíferos, nomenclatura, taxonomía integradora Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1084 | Burgin et al. Species are the central unit of biodiversity research, providing evolutionary and ecological context for zoonotic disease monitoring, conservation policymaking, and other societally critical topics (Cook et al. 2020; Upham et al. 2021; Wang et al. 2021; Colella et al. 2023). Delimiting species is fundamental for understanding biodiversity, allowing researchers to conceptualize meaningful discontinuities among organisms, communities, and ecosystems. Species form the foundation of biodiversity infrastructure, ranging from physical biocollections and digital databases to the knowledge contained in publications (Cook and Light 2019; Galbreath et al. 2019; Heberling et al. 2021). However, species boundaries are constantly changing over time via advances in taxonomy-related fields, leading to the lumping, splitting, and new description of species. In mammals, recent advances in the morphometric, molecular, statistical, and ecological tools used to distinguish previously cryptic taxa—coupled with the increasing globalization of mammalogical research efforts—have led to widespread taxonomic changes (Patterson 2000; Reeder et al. 2007; Burgin et al. 2018; Álvarez-Castañeda et al. 2019). Although modifying species boundaries to match the current evidence of evolutionary relationships is an essential step for studying that history (Sterner 2019), conflict has arisen in mammalogy when various authors have presented different evidence and perspectives on the same taxa (e.g., Frankham et al. 2012; Groves et al. 2017; Zachos 2018a,b; Padial and De la Riva 2021). This plurality of taxonomic views is not unique to mammalogy; instead, this dynamic aspect of biodiversity science emphasizes the importance of rigorous real-time tracking of accepted species names and the associated biological data that signifies their taxonomic meaning (Franz and Sterner 2018; Miralles et al. 2020; Sterner et al. 2020b; Upham et al. 2022a; Sterner et al. 2023). Progress in this arena is critical because identifying which species are recognized and how they are defined affects inferred biodiversity patterns at local, regional, and global scales, including assessments of conservation priorities and threats (Mace 2004; Thomson et al. 2018). Thus, broad dissemination of the latest knowledge of species designations and their geographic distributions provides an essential foundation for advancing our collective understanding of mammals globally. To further this mission, the American Society of Mammalogists launched the Mammal Diversity Database (MDD) in 2018, which then recognized ∼20% more mammal species (+1,251 and –172 species) compared to the previous authoritative compendium, the Mammal Species of the World, third edition (MSW3; Wilson and Reeder 2005; Burgin et al. 2018). Subsequent versions of the MDD have continued to track changes in speciesand higher-rank mammal taxonomy as reported in peer-reviewed literature, providing an up-to-date biodiversity resource available online at https://mammaldiversity.org, with past versions archived on Zenodo (MDD 2018, 2019, 2020a,b, 2021a,b,c,d,e, 2022a,b,c,d, 2023, 2024a,b,c). Rapid online taxonomic updates on the MDD contrast with the previous reliance on the periodic publication of taxonomic compendia such as the 3 MSW editions, 9 volumes of the Handbook of the Mammals of the World series (HMW), and the 2-volume Illustrated Checklist of the Mammals of the World (CMW; Honacki et al. 1982; Wilson and Reeder 1993, 2005; Wilson and Mittermeier 2009, 2011, 2014, 2015, 2018, 2019; Mittermeier et al. 2013; Wilson et al. 2016, 2017; Burgin et al. 2020). Although these compendia provided detailed snapshots of mammalian taxonomy and nomenclature based on published literature and expert opinions, their irregular publication intervals delayed the integration of taxonomic changes and new species descriptions into global biodiversity infrastructure. For example, conservation organizations like the International Union for the Conservation of Nature (IUCN) and community science platforms like iNaturalist rely upon authoritative mammal taxonomies in their decision-making processes (Di Cecco et al. 2021; Cazalis et al. 2022). Establishing the MDD helped creating a mammalian taxonomic, nomenclatural, and geographic resource that is comparable to that of other taxonand biome-specific online databases: for example, World Registry of Marine Species (WoRMS—Vandepitte et al. 2018); Reptile Database; AmphibiaWeb (Uetz et al. 2021); World Flora Online (Borsch et al. 2020); World Checklist of Vascular Plants (Govaerts et al. 2021). Geographic ranges of species underlie diverse biological questions, both applied (e.g., where is this species found?) and theoretical (e.g., why are species unevenly distributed?), and are a direct way of communicating the taxonomic meaning of species names. For example, the name Peromyscus maniculatus (Wagner, 1845) was long applied to deermouse populations across most of North America, but 2 studies in 2019 argued that this species only occurs east of the Mississippi River, with western populations now comprised of Peromyscus sonoriensis (Le Conte, 1853); Peromyscus labecula Elliot, 1903; and Peromyscus gambelii (Baird, 1857) (Bradley et al. 2019; Greenbaum et al. 2019). Because no consistent morphological characters are known to differentiate eastern P. maniculatus sensu stricto, from the 3 newly recognized western species, descriptions of the geographic range of each species are essential for communicating this new taxonomic arrangement (Sterner et al. 2020a). In general, geographic ranges can be communicated categorically (often via political boundaries; e.g., “Panhandle of Alaska and across N Canada” as used in MSW editions) or using range-map drawings (i.e., geospatial polygons as used in HMW, CMW, and other compendia). Both types of range information coarsely approximate the true distribution of a species and thus are only accurate at coarse spatial scales, usually ≥100 km resolution (Hurlbert and Jetz 2007; see also recent exchanges on this topic by Arbogast and Kerhoulas (2024) and Marsh et al. (2024)). In 2008, the IUCN completed the landmark Global Mammal Assessment (GMA), which made range maps available for ∼95% of the species recognized in MSW3 (5,489 species; Schipper et al. 2008). The IUCN Red List subsequently provided the sole source of digital range maps for mammals until Marsh et al. (2022) digitized range maps aligned to 3 alternative taxonomies: HMW (6,253 species); CMW (6,431 species); and MDD version 1.2 (6,362 species; note that each taxonomy also contains unmapped species that are extinct or data deficient—see tables later in the text Tables 1 and 2). Comparison among these datasets documents how the geographic concepts of species-rank mammal taxa have changed over relatively short time periods, mostly from taxonomic changes, in contrast to actual range shifts over longer periods (but see Pacifici et al. 2020). Because geographic knowledge is expected to keep evolving, the MDD taxonomy now tracks categorical changes in the country, continent, and biogeographic realm distributions of accepted species as a core aspect of taxonomic curation. Taxonomic curation of the MDD also hinges on tracking associated nomenclatural data from historical publications. Nomenclature and taxonomy intersect at the type specimen, which bears the name of the taxon under study (Pyle and Michel 2008). Type specimens provide a stable baseline for subsequent taxonomic revisions, given that species names are always linked to name-bearing type specimens (see Box 1). The requirement to designate type specimens when defining scientific names, which became common practice in the late 19th century, has created consistency for biological nomenclature. For all animal taxa, the rules of nomenclature are governed by the International Commission on Zoological Nomenclature (ICZN), which created the ICZN Code (ICZN 1999) to standardize how zoological taxonomists create and designate scientific names. Despite historical changes in how researchers define species and other taxa, this system, initially proposed by Linnaeus (1758) and now specified by the ICZN Code, has allowed for the consistent use of scientific names across centuries of publications. However, despite the critical role of nomenclature in taxonomic curation, it is often difficult for Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1085 researchers to access data (typically in natural history collections) including species synonyms, named subspecies, name authority citations, type localities, type specimen identifier(s) and physical location(s), and original description texts. This is especially true for scientific names that were described in older literature (e.g., before 1900), in journals that have no online repository, in languages other than English, or using type specimens in museums without online catalogs—collectively constituting a type of biodiversity “dark data” whose absence hinders integrative research efforts (Heidorn 2008; Upham et al. 2021). These nomenclatural details are crucial for conducting taxonomic revisions because a key step in the process of defining taxonomic units is identifying whether there is an existing available specific epithet for a potentially valid new species or whether a new name is needed. The MDD, therefore, jointly tracks the associated taxonomic, nomenclatural, and geographic data that pertain to all mammal scientific names, aiming to make this often difficult-to-access information more accessible and thereby accelerating the process by which integrative taxonomic research can be conducted globally. In this article, we introduce version 2.0 of the MDD (hereafter MDD2), a major release that, for the first time, includes a curated list of species-rank synonyms (including name combinations) for global mammals (Fig. 1; Table 1). We also summarize changes to the public interface and contents of the taxonomic listing during the 7 yrs since MDD version 1.0 was released. We analyze trends that emerged from 3 new features of MDD2: (i) species distributional data; (ii) species-rank synonyms through time; and (iii) associated nomenclatural metadata. We also describe recent collaborations that have greatly improved the quality of MDD data involving the taxonomic and nomenclatural databases “the Hesperomys Project” (hereafter “Hesperomys”; https://hesperomys.com) and “Bats of the World” (hereafter “Batnames”; https://batnames.org). These collaborations highlight the importance of data sharing and the need to continue building open forums for communicating biodiversity information globally, which can dramatically streamline taxonomic research efforts. Finally, we describe recent and upcoming changes to the MDD governance structure to invite greater editorial oversight from globally distributed groups of taxonomic experts. The MDD aims to continue providing an authoritative, open-access, and community-driven resource for advancing mammalogical research and conservation worldwide. Fig. 1. A graphical history of mammalian species-group name descriptions since the start of zoological nomenclature on 1 January 1758, incorporating all 28,382 available names (including preoccupied, replacement, and suppressed names) and 6,759 names currently recognized as valid species in MDD2. The histogram summarizes the number of available names (light gray) and valid species names (dark gray) described each year from 1 January 1758 to 15 August 2024 (a ∼267 yr period). The lines in the bottom panel summarize the 10-yr running means (starting 1768) for the proportion of names initially described at the ranks of species (blue) and subspecies (orange), as well as the proportions of names initially described as species and then lumped into another species (pink) or initially described as subspecies, forms, or varieties and then split into their own species (turquoise). Since all 4 lines were smoothed by taking 10-yr running means, each point on these lines represents a mean generated from the previous 10 yrs of data. See Supplementary Data SD9 for the raw data and running means used to create the line graph in this figure. Total recognized mammal species estimates are included from major compendia since Corbet and Hill (1980; red points), including all taxonomic compendia listed in Table 1 other than IUCN totals (which do not represent the actually recognized mammals at the time of versioning). A linear regression line, equation, and R2 value of these data are included on the graph. Six additional pre-1980s species estimates are also included for comparison (blue points), which are from resources that do not include full species lists (4,400 species in Storer (1951); 4,748, 4,732, and 4722 species in Walker et al. (1964, 1968, 1975); 4,237 species in Morris (1965); and 4,060 species in Anderson and Jones (1967)). Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1086 | Burgin et al. Table 1. Taxon counts for major mammalian taxonomic compendia since 1980 (see Fig. 1 for graphical representation of species totals for each, except IUCN totals), including the 3 editions of Corbet and Hill’s (C&H) A World List of Mammalian Species; editions 4 through 6 of Walker’s Mammals of the World (WMW); Anderson and Jones’ Orders and Families of Recent Mammals of the World (A&J); Duff and Lawson’s Mammals of the World: A Checklist (D&L); 3 editions of the Mammal Species of the World (MSW); Illustrated Checklist of the Mammals of the World (CMW); International Union for the Conservation of Nature (IUCN) 2008 and 2024-1 listings; and 2 major versions of the Mammal Diversity Database (MDD1 and MDD2). Taxa C&H1 1980 MSW1 1982 WMW4 1983 A&J 1984 C&H2 1986 C&H3 1991 WMW5 1991 MSW2 1993 Species 4,005b4,170 4,151 4,199 4,231 4,336 4,444 4,629c Wild extant NA NA NA NA NA NA NA NA Recently extinct NA NA NA NA NA NA NA NA Domestic NA NA NA NA NA NA NA NA Genera 1,014 1,033 1,017 1,057 1,055 1,066 1,116 1,135 Families 129 135 133 131 132 131 135 132 Orders 21 20 20 21 20 21 21 26 Taxa WMW6 1999 D&L 2004 MSW3 2005 IUCN 2008aMDD1 2018 CMW 2020 IUCN 2024 MDD2 2024 Species 4,809 5,115 5,416 5,489 6,495 6,554 5,983 6,753 Wild extant NA NA 5,338 5,410d6,382 6,451 5,899d6,629 Recently extinct NA NA 75 79 96 103 84 113 Domestic NA NA 3 0e17 20 0e17 Genera 1,192 1,117 1,230 1,241 1,316 1,343 1,308 1,353 Families 146 128 153 156 166 167 164 167 Orders 28 27 29 27 27 27 27 27 aIUCN 2008 totals based on Schipper et al. (2008). bLists 4,007 species, but we identified 2 duplicates (what is now Brucepattersonius iheringi is listed under both Microxus and Oxymycterus, and Prosciurillus leucomus is listed under both Prosciurillus and Callosciurus). cTotal provided in Solari and Baker (2007) and used in Burgin et al. (2018) was found to be incorrect while counting the MSW2 species total and we here provide a corrected total. dIncludes species classified as Extinct in the Wild and does not include humans. eThe IUCN does not assess domestic species. Methods Version 1.0 of the MDD (MDD1) was released in 2018 with a taxonomic cutoff date of 15 August 2017 (Burgin et al. 2018). Support from the American Society of Mammalogists (ASM) and VertLife Terrestrial NSF project (DEB: 1441737) enabled the creation of an online database with the aim of providing a continuously updated compendium of extant and recently extinct mammal species and higher-rank taxa. Since then, the MDD database and public web interface have been continually hosted at https://mammaldiversity.org with species-specific pages that include comments and citations relating to taxonomic and nomenclatural changes that have occurred since the MSW3 cutoff of 2004 (Wilson and Reeder 2005). Additional content of the MDD1 release included valid species names, authority author and year, species higher-rank taxonomy, biogeographic realm distribution, extinct/extant status, and taxonomy notes. Subsequent versions of the MDD added IUCN conservation status of species, English common names, and type localities (starting with version 1.2). Coarse synonym listings also began curation with version 1.2, consisting of all unique available and unavailable species-rank names included under each species and their respective authorities (previously called “nominalNames”), as did type specimens and geographic categories of country and offshore territory distributions. The present publication describes the release of MDD2, which focuses on summarizing the global patterns that arise from the more recent fields added to MDD2, including data pertaining to nomenclatural availability, type localities, type specimens, original description citations and links, and original name combinations when available for each species and synonym. A cutoff date of 15 August 2024 was set for including taxonomic, nomenclatural, and geographic distribution changes in MDD2. During the 7 yr since MDD1 was released, 16 incremental releases of the MDD (Table 2) have been published as CSV files with associated metadata and are archived on Zenodo (see Supplementary Data SD3 for complete species lists of all previous MDD versions). Since MDD version 1.3.1, a separate “diff” CSV file has listed major changes between the current and previous versions. Starting with MDD2, a synonym-specific CSV file is included that lists every species-rank scientific name applicable to extant and recently extinct mammal species, including all associated nomenclatural data for each name. Collaborations with other biodiversity databases have been critical for expanding and improving the curation of data included in MDD2. In particular, integration of nomenclatural data from Hesperomys (Zijlstra 2024)—an online database cataloging the nomenclature of living and extinct animals, with a focus on mammals—has greatly strengthened the MDD. These data were primarily integrated into MDD2 species-rank synonym data, resulting in expanded coverage of type localities, type specimens, and other nomenclatural data detailed below. Coordination with Batnames (Simmons and Cirranello 2024) has also been critical to standardize the taxonomic arrangement of the Order Chiroptera so that, starting in January 2024 with MDD v1.12 and Batnames v1.5, all MDD species and higher-rank taxa of bats are matching between these databases (however, subspecies of bats are currently only listed in Batnames). A further collaboration with Yale University researchers enabled the digitization of geographic range maps as polygon shapefiles for all mammal species, as matched to the taxonomies of MDD v1.2, HMW series, and CMW (see Marsh et al. 2022 and map downloads updated to MDD v1.4 via the “mddmaps” R package; Robles-Fernández 2024). Finally, efforts to coordinate with the ASM Mammal Images Library have aligned taxonomic names and concepts used in the expert-identified images to each new release of the MDD (available for download at https://www. mammalogy.org/image-library). Taxonomic, nomenclatural, and geographic data in the MDD will continue to be updated with new publications in forthcoming database releases. Continued changes are expected regarding species taxonomic status, nomenclatural information, geographic distribution, and common name(s)—given the steady rate of new taxonomic evidence being gathered and integrated relative to older publications. The MDD is a living database, and feedback from mammalogists Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1087 regarding errors, missing information, and potential updates is openly welcome. The MDD currently accepts public feedback in 2 ways: (i) emails to [email protected]; and (ii) GitHub issues filed via the MDD About page: https://github.com/mammaldiversity/mammaldiversity.github.io/issues. Feedback is reviewed and acted upon regularly by the MDD team. Taxonomic decision making. A fundamental goal of the MDD has been to maintain an updated global taxonomic checklist of currently recognized extant and recently extinct mammal species. To do so, the MDD team has continuously tracked mammal taxonomic literature with the aid of Google Scholar email alerts, periodic examination of journal webpages, and users directly sending publications. The MDD curation team— including all authors of this paper—consists of student research assistants and web developers (paid by the ASM), along with volunteers and consultants. This group is collectively advised by the ASM Biodiversity Committee (https://www.mammalsociety.org/committees/biodiversity). Taxonomic curation of the MDD has emphasized changes since the publication of MSW3 in 2005. Most taxonomic changes incorporated into the MDD have, thus far, closely followed the most recent empirical evidence given in peer-reviewed articles, which each represent the current best hypothesis of species boundaries. By consistently following the most recent evidence from peer-reviewed literature, taxonomy in the MDD is designed to collectively reflect the most up-to-date taxonomic perspective of the global mammalogical community and thereby minimize subjective biases coming from the MDD team. Nevertheless, occasionally controversial issues in the peerreviewed literature have required the MDD team to make subjective decisions regarding the most justified current taxonomy. In cases where there is insufficient evidence to support a taxonomic change or if multiple conflicting taxonomic arrangements are published in rapid succession, the MDD team provides justifications and citations on the relevant per-species pages, with summaries given on the About page at https://mammaldiversity.org. In cases of ongoing controversy, a “flag” designation is noted in the CSV file. Most subjective taxonomic decisions have been made internally by the MDD team. However, beginning with v1.10 (December 2022), the MDD piloted a protocol for publishing taxonomic opinion “white papers” on Zenodo in collaboration with members of the Global Bat Taxonomy Working Group, which is part of the IUCN SSC Bat Specialist Group and aligned with Batnames. Three subjective decisions were issued: (i) on the species status of Myotis evotis in relation to Myotis keenii (Lausen et al. 2019, 2021; Morales et al. 2021) by Upham et al. (2022b); (ii) on recognizing Aeorestes, Dasypterus, and Lasiurus as either subgenera or genera (Baird et al. 2015, 2017, 2021; Ziegler et al. 2016; Novaes et al. 2018; Teta 2019) by Francis et al. (2023); and (iii) on whether the 5 Myotis lucifugus subspecies should be recognized as subspecies or species (Morales and Carstens 2018) by Francis and Simmons (2022). This third white paper was issued without MDD involvement but is still followed in the MDD taxonomy. The goal of formally issuing subjective decisions is to transparently evaluate the available taxonomic evidence, present a detailed justification for choosing 1 taxonomic arrangement over another, and then make recommendations for gathering evidence to reach more definitive conclusions. The approach of self-publishing on Zenodo was chosen to avoid further arguments in the peer-reviewed literature and to incentivize stakeholders to gather new data to support their claims. Two other major subjective decisions have been made by the MDD team. The first was to exclude almost all of the new taxonomic arrangements proposed in Groves and Grubb’s Ungulate Taxonomy (Groves and Grubb 2011; Groves et al. 2011; Burgin et al. 2020)—a change that was made in MDD v1.2. Groves and Grubb (2011) recommended a large number of taxonomic changes within Perissodactyla and terrestrial Artiodactyla that were largely based on qualitative Table 2. Taxon counts for all 18 versions of the MDD from the initial release of MDD1 in 2018 to the present release of MDD2. Taxa MDD1.0 February 2018 MDD1.1 March 2019 MDD1.2 September 2020 MDD1.3 December 2020 MDD1.3.1 January 2021 MDD1.4 April 2021 MDD1.5 June 2021 MDD1.6 August 2021 MDD1.7 November 2021 Species 6,495 6,527 6,485 6,513 6,513 6,533 6,554 6,557 6,567 Wild extant 6,382 6,410 6,363 6,391 6,391 6,411 6,432 6,435 6,447 Recently extinct 96 100 103 103 103 103 103 103 101 Domestic 17 17 19 19 19 19 19 19 19 Diff: de novo NA +22 +98 +14 0+13 +11 +5 +13 Diff: Split NA +33 +154 +15 0+18 +10 +3 +6 Diff: Lump NA −31 −285a−4 0−10 0−5 −8 Genera 1,316 1,323 1,331 1,330 1,330 1,332 1,335 1,341 1,343 Families 166 168 167 167 167 167 167 167 167 Taxa MDD1.8 February 2022 MDD1.9 April 2022 MDD1.9.1 June 2022 MDD1.10 December 2022 MDD1.11 April 2023 MDD1.12 January 2024 MDD1.12.1 January 2024 MDD1.13 July 2024 MDD2.0 August 2024 Species 6,591 6,596 6,596 6,615 6,649 6,718 6,718 6,753 6759 Wild extant 6,471 6,476 6,476 6,494 6,526 6,594 6,594 6,623 6629 Recently extinct 101 101 101 101 105 107 107 113 113 Domestic 19 19 19 20 18 17 17 17 17 Diff: de novo +21 +8 0+22 +15 +37 0+24 +2 (296)b Diff: Split +6 +7 0+26 +49 +36 0+25 +4 (321)b Diff: Lump −3 −10 0−30 −29 −8 0−12 0 (357)b Genera 1,342 1,342 1,342 1,347 1,345 1,351 1,351 1,353 1353 Families 167 167 167 167 167 167 167 167 167 Species-level differences (“diffs”) between versions are noted with the terms “de novo” (additions that coined a new species epithet), “split” (additions that applied an existing species epithet to a newly recognized taxon), and “lump” (removals that combined 2 or more existing species-rank taxa). Totals of split species do not include names erroneously excluded from earlier versions, and total lumped species do not include names removed due to being considered nomina dubia or unavailable. See Supplementary Data SD4 for all MDD version “diff” files and Supplementary Data SD5 for the “diff” file between MDD1 and MDD2 specifically. aMost of the ungulate revisions from Groves and Grubb (2011) were removed except when corroborated by later publications. bTotal differences between MDD1 and MDD2 in parentheses. Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1088 | Burgin et al. Box 1. Definitions for taxonomic and nomenclatural terms relevant to this publication. Definitions Box: Taxonomy—the science of naming, defining, and classifying organisms into taxonomic entities, dealing with both species- (Alpha Taxonomy; species, subspecies) and higher-rank taxonomic groups (Beta Taxonomy; genus, family, order, etc); taxonomic changes result in changes to biological concepts, by lumping taxa together (Lump), splitting a taxon into multiple taxa (Split), descriptions of new names for previously undescribed taxa (De Novo), or moving taxa between different higher taxa (e.g., species moving between genera). Taxon/Taxa—a set of organisms together forming a single defined biological grouping at any taxonomic rank, with each ranking nested within the rank above; Species-rank Taxa include species, subspecies, and formerly used infrasubspecific rank names (e.g., forms, varieties); Higher-rank Taxa include all taxonomic rankings above the species-rank (class to subgenus applicable for the MDD). Nomenclature—the process of assigning scientific names to taxa based on a set of standardized rules in which each species has a 2-part name, starting with a genus and ending in a specific epithet (Binomial Nomenclature); nomenclatural changes result in changes to the name used for a taxon that do not change its taxonomic concept; zoological nomenclature for species-, genus-, and family-rank names are governed by the rules published in the ICZN Code (ICZN 1999), written by the International Commission on Zoological Nomenclature (ICZN). Authority—the surnames of the author(s) of the original description of a scientific name followed by the year that name was described; for species-rank names, parentheses are added around the entire authority when a specific epithet was described in a different genus than it currently resides; the year listed in the authority should be the year the name was published according to the ICZN Code (although the year of description does not always match what is printed on the publication itself). Name-bearing Type—a biological entity that is permanently attached to a scientific name, acting as a reference to identify which taxon a scientific name belongs to; for species-rank names, the type is usually a single animal—some part of which or its entirety is usually preserved as a museum specimen (Holotype)—or 2 or more animals preserved as museum specimens (Syntypes); syntypes can be later restricted to a single type specimen within the series as needed (Lectotype); when the original type material for a species-rank name has been lost, destroyed, or not designated in the first place, 1 or more specimens should be designated as a Neotype; family-rank names have a Type Genus and genus-rank names have a Type Species rather than physical biological material. Type Locality—the geographic locality from which the type material for a species-rank name originated, which may be later restricted or redefined if necessary. Valid Name/Taxon—a scientific name that represents the correct name for a biological taxon as defined by the rules in the ICZN Code and recognized by a particular authority; usually but not always the oldest available name. Synonym—a scientific name that has been identified as representing a specific taxon, but is not the valid name for that taxon; synonyms can be names initially meant to represent a new taxon (both Available and Unavailable Names for nomenclatural use, as defined below), or may represent later use of those names (Name Combinations or Spelling Variants; see definitions of these terms below). Available Name—a scientific name that meets the standards for nomenclatural availability as defined by the ICZN Code; some names may be available for nomenclatural use but are not assignable to a single species, such as a Nomen Dubium/Nomina Dubia (a name completely unidentifiable to a valid taxon), Species Inquirenda/ae (a name not currently identified to a valid taxon, but with the potential to be), or names based on a Composite (type material from multiple individuals of different valid species) or Hybrid type material (type specimen of interspecific hybrid origin). Unavailable Name—a scientific name that does not meet the standards for nomenclatural availability according to the ICZN Code, often resulting from the name being improperly published (Unpublished Name) or published without a taxonomic description (Nomen Nudum/Nomina Nuda), among other reasons. Preoccupied Name—an otherwise available scientific name that matches the spelling of an older name within the same taxonomic rank, making it unavailable for nomenclatural use; in species-rank names, the preoccupying name can either be originally described in the same genus (Primary Homonym), or subsequently moved into that genus (Secondary Homonym); when the name for a currently recognized species is preoccupied, a Nomen Novum/Nomina Nova (new name) should be described to replace the preoccupied name. Name Combination—the unique combination of a genus and specific epithet (or subspecific epithet) to form the scientific name of a then-valid species or subspecies (and forms or varieties in older literature); name combinations are not type-bearing synonyms, rather representing the history of taxonomic change associated with a name moving between genera, species, and subspecies. Spelling Variant—a difference in spelling from the original spelling of a scientific name, which can be incorrect in the original publication (Incorrect Original Spelling) or later publications (Incorrect Subsequent Spelling), or a purposeful change considered to be unjustified (Unjustified Emendation) or justified (Justified Emendation) according to the ICZN Code; justified spelling changes are usually those of specific epithets with adjective etymological roots, which change to match the etymological gender of the genus, or to Latinized names with non-standard characters (e.g., ñ, ö), although there are other rare exceptions allowing for justified emendations to the original spelling of a name. morphological characteristics with small sample sizes. These changes have been controversial within the mammalogical community, and many specialists have subsequently reverted to the taxonomy presented in MSW3 (e.g., Holbrook 2013; Gutiérrez and Garbino 2018). The exception was for the MDD to retain changes that have been subsequently validated by peer-reviewed research (e.g., Nanger gazelles; Lorenzen et al. 2008; Siegismund et al. 2013). Second, the MDD excluded a number of recently suggested taxonomic changes Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1089 and new species, subspecies, and generic names described by Raymond Hoser, who has prolifically named species in a self-published journal, generally using data from other publications as justification. These changes were previously restricted to reptiles, amphibians, and a few spiders and fish, but 46 mammalian names were described between 2018 and the MDD2 cutoff date. Based on the suggestions of Wüster et al. (2021), the MDD considers these names as examples of “taxonomic vandalism” that should be ignored other than being listed in the synonym dataset (given the nomenclatural status of “rejected_by_fiat” to signify that the broader taxonomic community rejects these names, not the ICZN Code). Species pages and distributional data. Each species in the MDD has a separate webpage that includes the accepted scientific name; authority author(s) and year; higher-rank taxonomic arrangement; primary and alternative English common names; original description citation and link to original description article; type locality, type specimen catalog number, and link to the type specimen in a natural history collection; list of species-rank synonyms and associated nomenclatural data; country-level distribution and accompanying map of listed countries; taxonomic notes and associated citations; IUCN Red List status (see Conservation status section below); whether the species is extant or extinct, wild or domesticated, or has a “flagged” designation denoting an ongoing taxonomic controversy; and an associated permanent hyperlink (permalink) to access the species page directly. Nomenclatural data presented on species pages are sourced from the synonym CSV file, and those data are detailed under the “Nomenclatural metadata” section below. Higher-rank taxa. The MDD also tracks the higher-rank taxonomy of living and recently extinct mammal species, including the ranks of class, subclass, infraclass, magnorder, superorder, order, suborder, infraorder, parvorder, superfamily, family, subfamily, tribe, genus, and subgenus. All currently recognized species on the MDD have been included within at least an order, family, and genus, with intermediary higher and lower ranks assigned when suggested in the literature. For now, taxonomic changes at the family and genus ranks are noted in the taxonomic notes for species affected by those changes, while changes at the order rank are noted in the “About” page on the MDD website (e.g., use of the order names Eulipotyphla and Artiodactyla instead of Lipotyphla and Cetartiodactyla; Asher and Helgen 2010). In cases where an intermediate rank is used (e.g., subfamily, tribe) and there are lower-rank taxa that are not confidently placed within that intermediate rank, the term incertae sedis is used. For example, within the subfamily Sigmodontinae, most genera are grouped within tribes, but the genus Abrawayaomys has yet to be placed into any recognized sigmodontine tribe (Ventura et al. 2013), so the tribal affiliation for that genus is incertae sedis. Common names. The primary English common names (also called vernacular names) for all species were initially sourced from the HMW and CMW series, which employed a standardized naming convention. Because common names are not formally regulated for mammals, there is some leeway for modifications. However, the MDD aims to maintain consistent naming conventions. The primary common name reflects the most widely used or uncontroversial English common name for a species while avoiding repeated common names or “names-withinnames” (e.g., “Common Hippopotamus” rather than “Hippopotamus” used for Hippopotamus amphibius to avoid potential confusion with the “Pygmy Hippopotamus,” which also includes the term “Hippopotamus”). For species not included in the HMW or CMW series or that required a common name change due to taxonomic revisions, the MDD team generates a new or revised common name (e.g., when the Nine-banded Armadillo, Dasypus novemcinctus, was split into multiple species, the MDD used “Mexican Long-nosed Armadillo” as the common name for D. mexicanus and “Southern Long-nosed Armadillo” for D. novemcinctus sensu stricto; Barthe et al. 2025). Common names suggested by the species name authority or taxonomic change publications are prioritized, but if no common name has been suggested or if the suggested name conflicts with the primary common name of another accepted species, then a new common name based on diagnostic features, geographic distribution, or scientific name etymology of the species is generated. The MDD also maintains consistent common name components within each genus when appropriate (e.g., “deermouse” for all Peromyscus species) and capitalizes all words of the common name as proper nouns (e.g., Aztec Deermouse for P. aztecus). The “other common names” field includes other known common names that have been used in English either widely or locally or used for subspecies within that species, as initially sourced from the HMW and CMW series, with additional names added as they were found while searching the literature. The MDD also lists names from other languages if they are widely used, but this list is not exhaustive. The MDD team expects to continue adding and revising common names regularly as they are found or updated. Conservation status. IUCN Red List statuses (Least Concern [LC], Near Threatened [NT], Vulnerable [VU], Endangered [EN], Critically Endangered [CR], Extinct in the Wild [EW], Extinct [EX], Data Deficient [DD], Not Evaluated [NE]) are listed for each species, as currently sourced from version 2024-1 of the IUCN Red List (IUCN 2024a). If a species is recognized under a different scientific name by the IUCN than what is used in the MDD (e.g., different genus, spelling differences), the scientific name used by the IUCN is given in parentheses following the status in the “iucnStatus” field (e.g., “VU (as Onychogalea fraenata)” for O. frenata). Additional fields with binary coding are included for whether a species is extant or recently extinct and wild or domesticated. All species that are believed to have been extant after the year 1500 CE, according to evidence published in the peer-reviewed literature, are included in the MDD. The representatives of 17 mammal species that were domesticated by humans during prehistoric times are considered separate species from their ancestors in the MDD, all but 2 of which (Bos primigenius and the unknown ancestor of Camelus dromedarius) are extant. Recognition of these domestic forms as distinct species by the MDD contradicts much of the evolutionary and taxonomic research on these taxa, which has shown that the wild and domestic forms should be considered a single species in most cases (e.g., Driscoll et al. 2007; Groenen 2016; Jackson et al. 2019, 2021). However, the MDD chooses to recognize these specific domestic forms as distinct species to avoid confusion regarding which forms are being discussed in various political and conservation situations. Recognizing domestic species also emphasizes the wild forms as an entity to be separately considered and protected. Thus, the MDD acknowledges that recognizing these 17 domestic forms as species is a deviation from the species recognition standards outlined in the Taxonomic decision making section above and follows the naming conventions for domestic species and their wild ancestors outlined by Gentry et al. (2004). Geographic distributions. The MDD started tracking native continental and country (including offshore territories) distributions of all currently valid and wild mammal species starting with v1.2. As of MDD2, 4 scales of geographic distribution are documented: (i) countries and offshore territories; (ii) country subregions; (iii) continents; and (iv) biogeographic realms. Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1090 | Burgin et al. Regional distributions of all terrestrial species are defined by individual organisms physically existing within the boundaries of the defined region. Aquatic marine or freshwater species (all cetaceans, sirenians, pinnipeds, and marine mustelids) are defined as existing either along the shoreline of a given region or in an inland water body within or bordering the defined region; the 1 exception is for biogeographic realm distributions, where aquatic species are only included if they are found in inland water bodies within the biogeographic realm (marine distributed species are listed as “Marine” in that distribution field). Species distributional data were initially sourced from the IUCN, HMW, and CMW and supplemented with records published in peer-reviewed literature and regional taxonomic and geographic compendia. Recently introduced and extinct populations are not included in the distribution of living mammals, although the former distribution is listed for globally extinct species, as well as ancient introductions (before 1500 CE) of extant species (e.g., Iberian populations of Genetta genetta represent ancient human-introduced populations and are included in the MDD; Delibes et al. 2019). All distributional listings are generally supported by a confirmed record of the species living there (i.e., a museum specimen or photographic evidence), although if a species is suspected to occur in a region but there is no evidence to confirm its presence a question mark is listed after the region name. A coarse countryand offshore territory-level map is also included on the webpage for each species—this should not be confused with a more fine-grained “expert range map” for the species, which are currently not hosted directly by the MDD (but see the R package by Robles-Fernández 2024). Region naming conventions generally follow the International Organization for Standardization (ISO), with occasional exceptions to match abbreviated naming conventions or for regions not covered by the ISO (see Supplementary Data SD7 for a list of regions used in the MDD). The list of countries and offshore territories includes all United Nations member and observer states, as well as relevant offshore territories geographically separated from their governing country (e.g., French Guiana is listed separately from France). Country subregions include all states, provinces, territories, and other applicable subregions for larger countries. As of MDD2, country subregions of species have only been implemented for the United States (all 50 states and the District of Columbia). However, country subregions have been fully implemented in tracking the higher geography of type localities on the synonym sheet (see “Nomenclatural metadata” section below). MDD species distributions are coded for 7 continents with the following definitions: North America (North and Central America to the border between Panama and Colombia, Greenland, Bermuda, and all of the Greater and Lesser Antilles except Aruba, Bonaire, Curaçao, and Trinidad and Tobago); South America (continent south of the border between Panama and Colombia, including Aruba, Bonaire, Curaçao, Trinidad and Tobago, Galapagos Islands, and Falkland Islands); Europe (Eurasia west of the Ural mountains and river, north of the Caucasus mountains, and north and west of the Black and Caspian seas, including the Azores and all the islands in the Mediterranean except Cyprus); Asia (Eurasia east of the Ural mountains and river, east of the Bosporus and Dardanelles and including Cyprus, south of the Caucasus mountains, and south and east of the Black and Caspian seas, east of the Suez Canal, and west of Weber’s Line, including Christmas Island); Africa (continental Africa west of the Suez Canal, including Madagascar, Madeira, Canary Islands, Ascension, Tristan da Cunha, and Saint Helen, amongst other offshore islands); Oceania (continental Australia, and Melanesia, Polynesia, and Micronesia east of Weber’s Line, including Hawai’i); and Antarctica (continental Antarctica and all Subantarctic islands, including South Georgia and the South Sandwich Islands). The MDD considers 8 terrestrial biogeographic realms, as defined by Olson et al. (2001): Nearctic, Neotropical, Palearctic, Afrotropical, Indomalayan, Australasian, Oceanian, and Antarctic; because these are terrestrial biogeographic realms, marine distributed species are listed as “Marine.” In some cases, the MDD deviates from Olson et al. (2001) when a small portion of a biogeographic realm is geographically separate from the primary area of the realm, such as including southern Florida in the Nearctic rather than Neotropical realm and southern Iran in the Palearctic rather than Afrotropical realm. Nomenclatural metadata. MDD2 includes a newly curated list of species-rank synonyms—created in collaboration with Hesperomys (Zijlstra 2024)—for every mammal species in the MDD, totaling 50,230 synonyms. The full synonym list is available on Zenodo and as Supplementary Data SD2. The synonym list includes available and unavailable names; spelling variants and name combinations; and names that are associated with currently valid species, subspecies, synonyms, nomina dubia, or species inquirendae. Nomenclatural availability of each name was determined based on application of the ICZN Code (ICZN 1999) as well as executive decisions made by the ICZN on the protection or rejection of specific names when applicable. The MDD follows the ICZN Code to inform issues of the nomenclatural availability and spelling of scientific names, which has led to changes in the spelling of specific epithets on the MDD that occasionally differ from the published literature, typically involving changes in gender agreement. These cases are usually noted in the taxonomic notes for species if these changes affect the spelling of a valid species name, and all such spelling changes between MDD1 and MDD2 are listed in Supplementary Data SD5. Literature search Nomenclatural information included in both the MDD and Hesperomys was collected by manually and programmatically searching the primary taxonomic literature, Mammalian Species accounts, and secondary global (e.g., MSW, HMW, CMW) and regional taxonomic compendia (e.g., Gardner 2008; Jackson and Groves 2015; Patton et al. 2015). Baseline synonym information was sourced from MSW3, which listed the specific epithet and authority for the synonyms of each species (Wilson and Reeder 2005). These searches spanned multiple online publication repositories, including but not limited to Google Scholar (https://scholar.google.com), the Biodiversity Heritage Library (https://www.biodiversitylibrary.org), HathiTrust (https://www.hathitrust.org), the Internet Archive (https://archive.org), and Gallica (https://gallica.bnf.fr). The primary goal of this ongoing search is to verify the existence, nomenclatural status, and associated data of every name applied to an extant or recently extinct mammal species. In cases where the MDD team has found an earlier citation for a name, differences in spelling or authority, or differing type localities from those listed in recent literature, this information is updated between the MDD and Hesperomys synonym datasets, along with the valid species if applicable. These efforts have also located names that were not included in recent taxonomic compendia (e.g., MSW3), which are added to the MDD-Hesperomys synonym dataset. Each name has been associated with its original description citation, including the original name combination, page of first use, type locality, and type specimen when available. Much of the initial nomenclatural data and citations were sourced from the major compendia mentioned above, but these sources often provided abbreviated citations that can be difficult to decipher and track, which required additional data acquisition directly from the original references. Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1097 recognized in the IUCN taxonomy (IUCN 2024a; see Fig. 6). The IUCN currently recognizes 5,983 mammal species, 5,799 of which are shared with MDD2 while 184 are considered by MDD2 to be synonyms of other species. Of the total species recognized in MDD2, 19.2% (1,295 species) are threatened with extinction (VU, EN, CR, and EW IUCN statuses) and 11.5% (773 species) are classified as Data Deficient (DD). Thus, fully 25.5% (1,716 species) of MDD2-accepted mammal species are either listed as DD or are NE relative to the IUCN Red List (collectively termed “understudied species” later in this section; Figs 4E and 6). It should be noted that the total threatened species proportions listed above (and those later in this section) are calculated based on MDD2 totals, which include 943 wild NE species; however, the total proportion of threatened species would increase to 22.4% (1,339 species) and DD species to 14.0% (837 species) if considering only those MDD2 species listed on the IUCN and the IUCN species total of 5,983. MDD2 considers 113 species to be recently extinct and documents the extinction of 25 entire genera and 7 entire families since the year 1500. In contrast, the IUCN lists 84 recently extinct species, 79 of which are shared with MDD2, leaving 32 recently extinct species listed in MDD2 that are NE by the IUCN, and 5 recently extinct species in the IUCN not recognized by MDD2 due to taxonomic revisions (e.g., Cryptonanus ignitus is now a synonym of C. chacoensis; Teta and Díaz-Nieto 2019). Furthermore, 2 species considered extinct in MDD2 are currently considered extant by the IUCN (Dasycercus cristicauda as NT; Tonatia saurophila as LC) despite taxonomic revisions elevating the extant populations to valid species under different names and restricting the original name to extinct populations (Basantes et al. 2020; Newman-Martin et al. 2023). The orders with the largest number of threatened extant species are Primates (335 species; 64.2% of total MDD2 species), Rodentia (313; 11.4%), Chiroptera (217; 14.6%), and Artiodactyla (122; 32.9%). Of orders with over 100 total species, Primates have the highest proportion of threatened species, followed by Artiodactyla, Diprotodontia (51; 32.1%), and Carnivora (75; 23.5%). All living species of Pholidota (8), Proboscidea (3), and Sirenia (4, plus 1 extinct) are considered threatened by the IUCN. The families with the most threatened species are Muridae (120; 13.8%), Cercopithecidae (114; 69.1%), Cricetidae (83; 9.5%), and Pteropodidae (73; 35.6%). Of families with over 100 species, Cercopithecidae, Pteropodidae, Bovidae (50; 32.1%), and Rhinolophidae (18; 15.8%) have the largest proportion of threatened species. There are 18 families where every species in the family is considered threatened by the IUCN, the most speciose of which are Fig. 5. Mammal species richness across the 50 states of the United States, including both terrestrial mammals and marine mammals found along the coast of coastal states. State-level species totals are included in Supplementary Data SD7. Fig. 6. Proportion of the 6,742 wild mammal species (not including domestic species) recognized in MDD2 categorized by IUCN Red List category (version 2024-1): 54.2% (3651 species) are considered “Low Risk” (Least Concern [LE] or Near Threatened [NT]); 25.5% (1716 species) are considered “Understudied” (Data Deficient [DD] and Not Evaluated [NE]); 19.2% (1295 species) are considered “Threatened” (Vulnerable [VU], Endangered [EN], Critically Endangered [CR], or Extinct in the Wild [EW—not visible]); and 1.2% (80 species; note that some MDD2 extinct species are NE on the IUCN) are considered globally Extinct (EX) on the IUCN. Total species per category are listed next to the category abbreviation in the figure legend while percent of total MDD2 species per column appears above each column. Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1098 | Burgin et al. Lepilemuridae (25), Lemuridae (21), Hylobatidae (20), and Indriidae (19); note that all 4 of these families are within Primates, 3 being Madagascar endemic lemur families. Another 18 families have 50% or more of their species considered threatened on the IUCN. Considering the most understudied species per clade relative to IUCN status, the orders with the highest proportion of DD species are Scandentia (5 species; 21.7% of total MDD2 species), Macroscelidea (4; 20%), and Cingulata (5; 20%)—while Rodentia (371; 13.5%), Chiroptera (215; 14.4%), and Eulipotyphla (105; 17.5%) have the highest raw numbers of DD species. The orders with the highest proportion of MDD2-accepted species that are NE by the IUCN are Microbiotheria (1; 50%), Pilosa (7; 41.2%), and Didelphimorphia (39; 30.9%); whereas Rodentia (458; 16.7%), Chiroptera (184; 12.4%), and Eulipotyphla (113; 18.9%) have the highest raw numbers of NE species. Combining these DD and NE categories, the orders with the most understudied species are Rodentia (829; 30.2%), Chiroptera (399; 26.9%), and Eulipotyphla (218; 36.4%). Only 8 of 27 orders and 72 of 167 families of mammals are currently “completely evaluated” by the IUCN, meaning that all MDD2-accepted species in that family are ranked in categories other than DD or NE. Across terrestrial biogeographic realms, the Neotropical realm has the most NE species (378; 20.6% of total MDD2 species in that region), followed by the Palearctic (196; 17.2%), Indomalayan (135; 12.4%), Afrotropical (122; 8.1%), Nearctic (94; 13.1%), Australasian (63; 7.3%), and Oceanian (1; 4.8%). Similarly, the number of total understudied species is highest in the Neotropical (627; 34.1%), followed by the Afrotropical (306; 20.3%), Palearctic (293; 25.6%), Indomalayan (287; 26.3%), Australasian (159; 18.5%), Nearctic (107; 14.9%), and Oceanian (2; 9.5%). The most species threatened with extinction are located in the Afrotropical (348; 23.1%), followed by the Indomalayan (283; 26.0%), Neotropical (237; 12.9%), Australasian (214; 24.9%), Palearctic (127; 11.1%), Nearctic (92; 12.9%), and Oceanian (10; 47.6%). The biogeographic realms with the most recently extinct species in MDD2 are the Neotropical (46, including 37 in the Caribbean) and the Australasian (39, including 33 in Australia), with all other regions including 8 or less recently extinct species—see consistent reports of disproportionate Caribbean (Cooke et al. 2017) and Australian (Woinarsk et al. 2015) extinctions. Similar figures are found when grouping by continents rather than biogeographic realms, with the exception that Asia has the overall most understudied species (549; 26.1%) and most species threatened with extinction (432; 20.5%), followed by South America (519; 32.7%) or Africa (363; 22.4%) for those categories, respectively. Considering individual countries and offshore territories, the most understudied species (Fig. 4E) are in Brazil (174 species; 22.7% of MDD2 total for that country), Indonesia (171; 22.1%), China (168; 25.0%), Peru (136; 23.9%), Colombia (121; 23.3%), Ecuador (117; 26.7%), Mexico (113; 19.5%), Argentina (99; 24.9%), Vietnam (73; 21.2%), and Democratic Republic of the Congo (72; 15.2%). Ecuador, China, and Argentina contain the highest proportion of understudied species relative to the MDD2 total for that country. The greatest concentrations of species threatened with extinction (Fig. 4D) are found in Indonesia (211; 27.2%), Madagascar (131; 51.6%), Brazil (94; 12.2%), Mexico (88; 15.2%), India (82; 19.4%), China (77; 11.5%), Malaysia (77; 22.0%), Australia (68; 17.5%), Thailand (63; 18.9%), and Colombia (57; 11.0%). The highest proportion of threatened species relative to the MDD2 total is found in Madagascar, Solomon Islands (24; 30.4%), and Mauritius (7; 29.2%). The most recently extinct species recognized in MDD2 are Australia (33 species), Dominican Republic (14), Haiti (14), Cuba (8), and Madagascar (6), further highlighting the recent extinctions in Australia and the Caribbean. A total of 44 countries and offshore territories have had at least 1 species go extinct that was partly or wholly distributed within its borders since 1500 CE. Nomenclatural metadata. MDD2 contains a curated dataset of 50,230 species-rank names that are applicable to all currently recognized mammal species (including nomina dubia and species inquirendae that were meant to apply to extant or recently extinct mammals when described). These data consist of 28,382 available names (includes all names initially described as available and those subsequently made unavailable due to being preoccupied or suppressed); 2,127 unavailable names (including all names used with the intent of being available for taxonomic purposes but that are unavailable according to the ICZN Code); 3,445 spelling variants; 16,182 name combinations; and 435 instances of other subsequent name use (Table 4; Supplementary Data SD8). There are 23,973 more species-rank names listed in MDD2/Hesperomys compared to MSW3, which listed 26,257 such names. This difference is primarily due to the inclusion of more unavailable names, spelling variants, and name combinations (i.e., names not usable for taxonomic purposes), although a number of available names described subsequent to, or omitted from, MSW3 have been added to the MDD2/Hesperomys synonym list as well. Authority authors are included for all names and valid species in MDD2, but authority year is missing for 8 names (all are synonyms of Equus species, 7 of which have been found subsequent to the MDD2 cutoff date and will be included in later versions). For nearly all synonymous names and every valid species, MDD2 includes original name combination, taxonomic rank, and some degree of citation information (99.8% of all names). Citation information has been fully verified for 88.6% of names (91.0% of valid species), and the page number on which the name first appears or is described is included for 88.3% of names (and all but 3 valid species). For 75.1% of names (86.1% of valid species) a hyperlink to the full text of the original description was located, and for 51.2% of names (68.3% of valid species) direct links were included to the page that name was first mentioned or described on the BHL. Type localities were also recorded for almost all applicable names (99.6% of originally available names and all valid species) and have been verified in the original description for 66.5% of available names and 78.2% of valid species. MDD2 also includes coordinates of the type localities in decimal degrees for 9.8% of available names and 27.4% of valid species (including almost all names described after the year 2000; Fig. 3). Type specimen locations were gathered for 73.3% of originally available names (87.9% of valid species), 46.4% of which (53.5% of valid species) are linked to the specimen database entry in an online collections catalog. Most species descriptions designate a single type specimen (the holotype), which MDD2 records for 18,396 names. However, there are also 1,111 names based on multiple type specimens (syntypes) and 1,092 for which a lectotype has been selected from among a series of syntypes. There are 144 cases where the original type material has been lost or was nonexistent in the first place, resulting in a neotype being designated. There are also 289 names in MDD2 recorded as having nonexistent type specimens, which will require future neotype designation. We estimate that MDD2 coverage is nearly complete for type specimens housed in North America, where almost all major institutions have published type catalogs. Most of the names for which we have not yet recorded type specimen information likely have no surviving types or were based on types housed in a European collection. Historical taxonomy and nomenclature. Species-rank available names were described from 1758 to 2023 at an average rate of 106.2 names per year (Fig. 1; raw data and running means available in Supplementary Data SD9). However, when divided Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1099 into roughly 50-yr time bins, the following average rates of yearly naming emerge: 32.7 (1758 to 1799), 95.0 (1800 to 1849), 117.8 (1850 to 1899), 248.4 (1900 to 1949), 53.1 (1950 to 1999), and 48.5 (2000 to 2023). The time bins with the lowest naming rates were 1758 to 1799 and 2000 to 2023, although this latter time bin is about half as long, while the highest-rate periods were 1900 to 1949 and 1850 to 1899. From 1758 to 1800, there were 19 yrs where fewer than 10 available names were described and 4 yrs where none were described (1759, 1762, 1763, 1764; although 1762 has unavailable names). Sustained rates of species-rank name description were not reached until 1819, after which every year had >30 available name descriptions until 1973. Yearly peaks associated with single publications occurred in 1827 (206 names), 1842 (287 names), 1860 (255 names), and 1867 (238 names). During the 1890s, most years had >200 described names, peaking at 366 in 1898. This trend continued into the first decades of the 20th century, leading to 1913 as the year with the most documented new name descriptions across the ∼267 yrs of taxonomic history (529 available names described). The mid-20th century experienced a sustained decrease in naming rates, with only 3 yrs since 1943 having more than 100 available name descriptions (1946 with 120, 1955 with 127, and 1956 with 108). Only 24 names were described in 1973, making it the year with the least name descriptions since the early 19th century (only 12 of which are currently valid species). The proportion of names that are currently recognized as valid species in MDD2 increased from the 1970s to the 21st century as rates of subspecies name description decreased. The year 2021 has the most names described during the 21st century (69 names) and 2001 had the least (33 names). There have been 28 species-rank names described in 2024 prior to the MDD2 cutoff date of 15 August (including 24 currently valid species; increasing to 42 valid species during the writing of this manuscript). Considering the history of taxonomic rank use over time, most species-rank names described before the 20th century were described as species rather than subspecies, forms, or varieties (Fig. 1; forms and varieties before 1900 were often equivalent to the subspecies rank and are treated equivalently here). Over 70% of names were described initially as species until the time bin of 1895 to 1904, after which it steadily fell until a resurgence in the 21st century. While species were the primary taxonomic unit used for name descriptions during the 18th and 19th centuries, there were significant jumps in Table 4. Summary of nomenclatural metadata released in MDD2 separated into nomenclatural status and validity status. Status Total names Authority author Authority year Names since 2000 Original combination Original rank Authority citation Verified authority citation Nomenclature status Total available names 28,382 28,382 28,374 1,191 28,297 28,297 28,316 23,493 Nomina nova 925 925 925 13 925 925 925 791 Preoccupied names 1,100 1,100 1,100 51,095 1,095 1,093 812 Suppressed names 39 39 39 039 39 39 29 Total unavailable names 2,127 2,127 2,127 122 2,119 2,119 2,119 1,696 Spelling variants 3,445 3,445 3,445 718 3,437 3,437 3,439 3,114 Name combinations 16,182 16,182 16,182 8,466 16,182 16,182 16,182 16,135 Other subsequent usages 435 435 435 2434 434 433 386 Validity status Synonymous names 42,880 42,880 42,872 9,494 42,783 42,783 42,798 37,821 Valid species 6,759 6,759 6,759 899 6,759 6,759 6,759 6,150 Species inquirendae 94 94 94 094 94 94 88 Nomina dubia 475 475 475 29 470 470 475 414 Composite type material 10 10 10 010 10 10 7 Hybrid type material 12 12 12 112 12 12 12 Total Total names 50,230 50,230 50,222 10,423 50,128 50,128 50,148 44,492 Status Authority Page Authority Link Authority Page Link Type Locality Original Type Locality Type Coordinates Type Specimen Type Specimen Link Nomenclature status Total available names 28,024 21,823 18,483 28,259 18,877 2,771 20,811 13,179 Nomina nova 913 721 615 913 691 31 498 242 Preoccupied names 1,071 763 679 1,086 719 19 390 178 Suppressed names 39 29 29 36 25 0 8 1 Total unavailable names 2,076 1,480 1,249 1,243 720 28 196 54 Spelling variants 3,412 2,572 1,502 245 85 1 9 5 Name combinations 10,728 11,714 4,301 100 61 010 4 Other subsequent usages 431 363 319 226 127 0 6 3 Validity status Synonymous names 37,001 31,393 20,679 22,886 14,247 943 14,977 9,600 Valid species 6,756 5,820 4,619 6,759 5,282 1,852 5,941 3,617 Species inquirendae 93 80 63 79 71 118 3 Nomina dubia 467 393 328 327 255 492 25 Composite type material 97610 6 0 3 0 Hybrid type material 12 10 8 6 6 0 1 0 Total Total names 44,338 37,703 25,703 30,067 19,867 2,800 21,032 13,245 Totals for available names include preoccupied names, nomina nova, and suppressed names (all of which are also summarized separately). Totals for unavailable names include all nomenclatural status conditions that are not some form of subsequent use, while name combinations, spelling variants, and other miscellaneous subsequent uses are summarized separately (see Supplementary Data SD8 for a full list of nomenclatural and validity statuses and finer scale numeric summaries). Multiple nomenclatural labels can be applied to a single name, while only 1 validity status can be applied to a name, so column totals summarize the validity statuses. Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1100 | Burgin et al. subspecific name descriptions (generally as forms or varieties) in 1788 (61.4% of available names described that year were subspecies), 1792 (62.0%), 1801 (73.9%), and 1860 (69.8%). The early 20th century marked an increase in the prevalence of polytypic species (species with recognized subspecies), with >50% of all names described as subspecies from 1922 to 1973, peaking in 1948 at 89.0%. Following this period, the prevalence of subspecies name descriptions began to decrease, briefly dropping below 50% in 1974 before rising. In 1999, subspecies descriptions consistently dropped to below 50% of all yearly name descriptions, decreasing to only 6.4% in 2014 and have continued to be <10% of yearly names described (Fig. 1, lower panel). The majority of all available mammal names are considered synonyms of valid species on MDD2 (i.e., 21,254 names; 74.9% of available names). Of those, 17,906 names (63.1%) were initially described at the species rank while 10,367 (36.5%) were initially described as subspecies, forms, or varieties. Furthermore, 11,508 names (40.6%) described as species have been subsequently lumped, while 698 names (2.5%) described as subspecies have been subsequently split (i.e., elevated to species). There are 6,055 names (21.3%) described as species that are recognized as valid species in MDD2, although it should be noted that many of these names were treated as synonyms or subspecies before being later revalidated. Most names that were described as species and since lumped (i.e., now synonymous names in MDD2) were published before 1920, with the 10-yr running mean of this statistic consistently remaining over 50% from 1785 to 1920 (Fig. 1, lower panel). Fewer than 25% of the available species-rank names described per year have been subsequently lumped from 1927 to 2023 (other than 1976 and 2001). In contrast, the late 20th century had the highest rates of elevating taxa initially described as subspecies to species, with a 10-yr running mean of ∼1% to 4% from 1886 to 1958 compared to ∼5% to 9% from 1959 to 2001. Few years before 1886 and after 2009 include names described as subspecies that are now recognized as species, with most yearly rates ranging from ∼0% to 1% during these time periods. It should be noted that these rates are based on the taxonomic rank at which each name was originally described and whether it is now considered a synonym or valid species in MDD2. Thus, these values do not capture when each name was split or lumped or whether a name was split or lumped multiple times throughout its taxonomic history. Across terrestrial biogeographic realms, the Palearctic has the most available names with type localities in that region (20.5% of total available names), followed by the Afrotropical (19.7%), Neotropical (19.4%), Indomalayan (15.4%), Nearctic (10.9%), Australasian (7.4%), Oceanian (0.2%), and Antarctic (<0.1%) (Fig. 2C; Table 3). The continent with the most available names is Asia (27.1%), followed by Africa (22.3%), North America (18.4%), South America (14.3%), Europe (9.7%), Oceania (6.3%), and Antarctica (<0.1%) (Fig. 2D; Table 3). The 10 countries with the most available name type localities are the United States (2,552 names), Indonesia (1,732), Mexico (1,301), Brazil (1,088), China (1,084), Russia (975), South Africa (916), Australia (895), India (689), and Kenya (523; Fig. 4C). The type localities of new species-rank names described since 2000 have a global distribution, but there is a strong latitudinal gradient, with a larger proportion of names described from tropical than temperate regions (Fig. 3). Species-rank name type localities are concentrated throughout high-elevation regions (e.g., Andes, East African and Ethiopian highlands, and Himalayas), islands (e.g., Madagascar, the Philippines, and Sulawesi), and in biodiversity hotspots (e.g., Atlantic Forest). Few newly described names from the 21st century have type localities in northern North America, Europe, and northern Asia, and most that do are either subspecific names or names now regarded as synonyms. The countries with the most type localities of valid species described during the 21st century are Brazil (98 names), Madagascar (78), Indonesia (57), China (53), and Peru (52). Website and database usage. The MDD website located at https://mammaldiversity.org increasingly serves a global audience of users. Over a recent 17-mo period, it was visited by 1,200 to 2,900 users/mo for a total of 25,991 unique users that were located in 170 different countries (Google Analytics: 19 May 2023 to 19 October 2024). The countries with the highest overall usage during this period were the United States (34.3% of users), Brazil (6.7%), Mexico (6.2%), and China (5.4%), which topped the 12 countries with over 500 users and the 50 countries with over 50 users (see Supplementary Data SD11 for complete usage data). Global usage rates rose from an average of ∼450 weekly users to ∼700 between months 1 to 10 and 11 to 17 of tracking. A large spike in MDD website traffic on 15 to 17 July 2024 involved 1,326 weekly users, likely stemming from the MDD v1.13 taxonomy release on 13 July 2024. Archived versions of the MDD taxonomy on Zenodo (https://doi. org/10.5281/zenodo.4139722) have garnered 33,208 page views and 17,159 downloads over a 4-yr period (September 2020 to -October 2024). Discussion The Mammal Diversity Database version 2.0 (MDD2) is a digital compendium of global mammal taxonomic, nomenclatural, and geographic information that is more complete and accessible than prior efforts, providing a steppingstone to greater knowledge about mammalian biology. MDD2 provides an online overview of research studies authored by mammalogists worldwide over the past 267 yrs (1758 to 2024), jointly illuminating the latest advances in taxonomic understanding of this charismatic vertebrate clade. The release of MDD1 7 yrs ago marked a shift from decadal printed volumes to semi-annual online versions (Burgin et al. 2018). However, continued advances to the MDD have required improved strategies to find, extract, and link relevant information from the disparate sources needed to assemble real-time updates to global mammal taxonomy. Because much of the information used to update the MDD is in rare archives, offline sources, or behind paywalls, manual data curation by a dedicated team of students and volunteers has been crucial in collating these data and linking their contained biodiversity knowledge. Documenting 267 yrs of change: historical perspectives on mammalian taxonomy. The curatorial efforts of MDD2, Hesperomys, and Batnames have uncovered extensive information relevant to the history of studying mammal taxa—especially type localities, type specimens, original description citations, and hyperlinks to those sources. These data remain important for modern taxonomic revisions when considering the identity and status of previously published names, but also paint a broad trajectory of mammalogical study spanning from the dawn of Linnaean taxonomy to the present. The MDD2 dataset allows for novel explorations of how the recognized taxonomy of mammal species and higher taxa has changed through time, which is briefly contextualized here. The history of mammalian taxonomic study shows an arc of changing biodiversity knowledge from 1758 to 2024 (Fig. 1). This history is here broken into 4 major eras: (i) 1758 to 1880, when many large, widespread, and charismatic mammals were described sporadically in monographs; (ii) 1881 to 1939, the peak of descriptive taxonomy associated with greater subspecies use and journal-based, taxonor region-specific publications; (iii) 1940 to 1999, a period of decreasing taxonomic descriptions, peak subspecies use, and transition toward revisionary taxonomy; and (iv) 2000 to present, the current period of technology-driven research advances in integrative revisionary mammalian taxonomy. We discuss each of these Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1101 historical eras in the following sections, leading to the current era, which is discussed in a separate section (see Mammalian systematics in the modern era). 1758 to 1880 The late 18th century featured the first publications to describe binomial names for many charismatic mammal species (e.g., ungulates, carnivores, cetaceans; particularly in Europe), setting the foundation for the development of modern taxonomic and nomenclatural practices. Binomial nomenclature and taxonomy for mammals and other animals starts with C. Linnaeus in Sweden (1758), whose Systema Naturae (10th edition) still holds the title of proposing the largest number of currently valid mammal species in a single publication. These species were initially placed in 8 orders (only Primates is still in use today) and 39 genera (all of which are still in use except Simia, which is suppressed for nomenclatural use; ICZN 1929). During this period, there was little standardization in the use of taxonomic units other than orders, genera, and species, with many genera having species compositions equivalent to modern mammal families, suborders, or orders (e.g., Vespertilio was used for all bats by Linnaeus). The use of families as a taxonomic group between orders and genera would not be applied to mammals until Tableau Élémentaire de l'Histoire Naturelle des Animaux (Cuvier’s 1797) and would not be given the consistent suffix “-idae” in mammals until J.E. Gray (1821). There are considerably fewer species-rank names described from this period compared to the 19th and 20th centuries, although there are 2 notable single-year spikes in species descriptions associated with single publications: Linnaeus (1758) with 208 names (152 currently valid species) and R. Kerr in Britain (Kerr 1792) with 202 names (27 currently valid species). Other foundational authors and publications describing mammal species during the late 18th century include P.S. Pallas in Russia (1766, 1767 to 1774), and J.C.P. Erxleben (1777), J.F. Gmelin (1788), and J.C.D. von Schreber in Germany (1774 to 1844; some sections published posthumously). Most of these publications were monographs adding to or revising previously published global animal or mammal taxonomies rather than journal articles describing individual species or clades. During this period, many species were renamed or described multiple times across publications, resulting in many synonymous names for single species. Some names were even described from the same type material or just to replace previously published names (e.g., Mandrillus sphinx having 6 available names in the 1700s alone) with those of the author. Species-rank names described during this period were often based on illustrations or firsthand accounts of living or recently deceased animals without reference to voucher specimens, but museum specimens would become the preferred reference material for biological descriptions later in the 19th and early 20th century (i.e., museum-based type material). Early in the 19th century, taxonomic works were mostly published in Europe, involving prolific authors such as É. and I. Geoffroy Saint-Hilaire, G. and F. Cuvier, and A.G. Desmarest in France; J.E. Gray and G.R. Waterhouse in Britain; L. Fitzinger in Austria; and J.A. Wagner and W.C.H. Peters in Germany. However, through the 19th century, some researchers of European heritage working elsewhere started publishing taxonomic literature, including S.F. Baird, C.S. Rafinesque, and E. Coues in the United States; A. Smith in South Africa; and E. Blyth and B.H. Hodgson in India. Although publications through the 19th century were moving away from large monographs and toward journal articles, some monographic works still contributed to major individual peaks in taxonomic activity during 4 major single-year peaks in species-rank name descriptions from 1800 to 1890: those of R.P. Lesson in France (Lesson 1827, 1842) and Fitzinger (1860, 1867; although most of Fitzinger’s names were domestic breeds proposed as forms or varieties). The 19th century also saw significant philosophical advances in evolutionary theory, which drove further advances in the understanding of mammal biodiversity. The higher classification of mammals began to take shape with H.M.D. de Blainville, who used reproductive biology and skull and skeletal features to group mammals into distinct subclasses: monotremes (Ornithodelphes), marsupials (Didelphes), and placentals (Monodelphes; de Blainville 1816, 1864). These works paralleled the evolutionary ideologies expressed in C. Darwin’s Origin of Species (Darwin 1859) through the emphasis of skeletal similarities and adaptive alterations among related taxa, which were further emphasized by other contemporary works focused on the classification of mammals and other animals based on tooth morphology (Giebel 1855), brain structure (Bonaparte 1840), and reproductive biology (Owen 1868). The modern terms for these 3 extant mammal clades were established by T.N. Gill (1872), who initially proposed Prototheria for monotremes and Eutheria for marsupials and placentals; and T.H. Huxley (1881), who then termed Metatheria for the marsupials (albeit not initially intended as a scientific name), leaving placental mammals in Eutheria. The terms Prototheria, Metatheria, and Eutheria have varied in definition both historically and in modern use, but are now usually given a broader cladistic definition including the living and related stem-group fossil relatives (e.g., de Queiroz and Gauthier 1992) of: (i) monotremes (now often included under the clade Australosphenida, treating Prototheria as a defunct paraphyletic clade name); (ii) marsupials (often called Marsupialia for the crown clade); and (iii) placentals (often called Placentalia for the crown clade). Prototheria, Metatheria, and Eutheria are retained in the MDD to match the most common use of the names of extant mammals for the time being. A more detailed account of the early history of mammalian taxonomic research is available in Burke (1968). 1881 to 1939 Prior to the 1890s, taxonomic and nomenclatural practices were unstandardized, leading to differential naming systems and confusion regarding a given species proper name, especially as many authors knowingly renamed already-named species. The lack of structure spurred the need for standardized nomenclatural and taxonomic rules for zoological taxonomy, which culminated in the publication of the International Rules of Zoological Nomenclature in 1905 (Blanchard et al. 1905) that would eventually develop into the first edition of the ICZN Code (ICZN 1961). These documents established fundamental nomenclatural rules that are still used today including the principle of priority for scientific names, the use of physical type material and type localities to define species-rank names, and the rule that names must be published according to the ICZN Code to be considered available. Thus, this period is further characterized by the appearance of more seriously cataloged museum type specimens and more precisely defined type localities (rather than broadly defined type localities at the country or continent level). This standardization contributed to the rapid increase in mammalian taxonomic activity during the late 19th and early 20th century, which was also characterized by an increasing prevalence in the use of the subspecies rank (replacing forms and varieties). Mammal-focused expeditions also occurred more frequently, generally associated with colonial exploration, resulting in specimens being sent to museums in Europe and North America (rarely in the country of collection; e.g., see Mohammed et al. 2022). These trends continued into the first decades of the 20th century, expanding to researchers of European heritage in Germany (e.g., P. Matschie), the United States (e.g., C.H. Merriam, J.A. Allen, W.H. Osgood, G.S. Miller), Russia (e.g., S.I. Ognev), China (e.g., P.M. Heude), and South Africa (e.g., A. Roberts). Additionally, some of the first researchers of non-European heritage started Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1102 | Burgin et al. publishing mammal names, such as Yang Zhongjian (then known as C.C. Young) from China and K. Kishida and T. Mori from Japan (Japanese was also the first non-European language known to feature a mammal name description). However, this rapid increase is most prominently associated with O. Thomas and other researchers at the British Museum of Natural History. Thomas, who initially wanted to study echinoderms, was the most prolific mammalian taxonomist in history, describing ∼2,900 genusand species-rank names from 1880 to 1929 (Hill 1990) including 2,725 available species-rank names (9.6% of available names), 937 of which are currently valid species in MDD2. By the 1920s, new taxonomic descriptions usually appeared in articles focused on individual taxa, regions, or expeditions published in journals instead of monographic books. A single journal, the Annals and Magazine of Natural History, published the descriptions of 3,434 available species-rank mammal names from 1841 to 1959, 12.1% of available names in MDD2 (including 990 currently valid species). Other prolific journals during and after the late 19th century include the Proceedings of the Biological Society of Washington with 1,500 available species-rank names from 1886 to 2021 (more consistently before the 1970s) and the Proceedings of the Zoological Society of London with 1,483 available species-rank names from 1831 to 1962. Additionally, the American Society of Mammalogists and the associated Journal of Mammalogy (including 616 available species-rank names) was founded in 1919, establishing a nexus for focused mammalogical research in the United States and eventually the world more broadly. A large proportion of the names proposed as species during this time period are now treated as subspecies or synonyms (Fig. 1, upper panel). A prime example of the frequency with which now-synonymous names were proposed during this period is Merriam’s (1918) treatment of the Brown Bear (Ursus arctos) as ∼74 distinct species in North America alone, as opposed to the currently defined single Holarctic species. Researchers primarily described mammal species based on relatively few specimens sent to museums from widely separated localities. Additionally, species were often described based on subtle morphological distinctions, and some of the traits used for species diagnoses at this time are now known to vary substantially within species relative to environmental factors (e.g., pelage coloration). These dynamics contributed to the description of many names initially proposed as species or subspecies that are now recognized as a single species with intraspecific variation (e.g., over 200 available synonymous names referable to Thomomys bottae, as defined in MDD2). Furthermore, most of the names described during this period were not actively cataloged in global-scale compendia (other than E.-L. Trouessart’s series cataloging living and fossil mammals earlier in the period; Trouessart 1897, 1899, 1905). In retrospect, this practice of not cataloging all described mammal names has made it difficult to estimate how many mammal species were actively recognized across different time periods, particularly so in the early 20th century. Thus, subsequent efforts were needed to both catalogue and review the influx of new names, ushering in a shift from descriptive to revisionary taxonomy in the next era in mammalian biodiversity research. 1940 to 1999 Descriptive taxonomic activity began decreasing after the 1920s, and especially into the late 1930s and 1940s following the Great Depression and leading into World War II. During this period, fewer names were being described, and even fewer are now considered valid species (e.g., only 3 currently valid species were described in 1961). In contrast to the previous period, proportionately more new names described during the midto late 20th century were described as subspecies, and many names described earlier as species were joined together into polytypic species with many subspecies. This decrease in species-rank name descriptions likely stems from multiple factors, including global political instability that shifted research funding away from systematics (e.g., World War II, the Cold War). There were also significant advances in the philosophy of species concepts, evolutionary theory, availability of specimens, and molecular methods during this period that contributed to a shift from descriptive to revisionary taxonomy. To organize the large number of names coined in previous decades, several major regional compendia were published such as Allen (1939) for Africa, Ellerman and Morrison-Scott (1951) for Eurasia, Miller and Kellogg (1955) for North America, and Cabrera (1958, 1961) for South America, among others. Although the taxonomic information in these treatises is now outdated, they remain invaluable as sources of bibliographic and geographic information for mammal names historically described in these regions (these have been significant resources in collecting synonyms and their metadata for Hesperomys and the MDD). Higher-level mammal taxonomy also began to take shape at this time. The publication of The Principles of Classification and a Classification of Mammals (Simpson 1945), which is the foundation of modern mammalian ordinal and familial arrangements, is based on a more complete consideration of the deep-time evolutionary history of both living and extinct mammals. Collectively, these compendia helped organize mammal taxonomy following the prior peak in descriptive taxonomy, contributing to the creation of global compendia like Walker et al. (1964; and 6 subsequent editions), Morris (1965), and Anderson and Jones (1967, 1984), which provided some of the first estimates for total extant mammal species richness. Subsequently, the 3 editions of A World List of Mammalian Species (C&H1, C&H2, C&H3; Corbet and Hill 1980, 1986, 1991) and 3 editions of Mammal Species of the World (MSW1, MSW2, MSW3; Honacki et al. 1982; Wilson and Reeder 1993, 2005) provided the first complete taxonomic compendia of the modern era, each explicitly listing all contemporarily recognized extant and recently extinct mammal species. A testament to the rapid transition from descriptive to revisionary taxonomic research is illustrated by comparing estimated mammal species totals from across this period (Fig. 1, upper panel). The earliest known estimate following the descriptive peak is ∼15,000 extant mammal species in the first edition of General Zoology (Storer 1943). Although Storer does not mention how this count was determined, it can be treated as a rough estimate of the number of mammal names described as species and subspecies up to that point. We base this on the fact that Storer (1943) gave 25,000 as a combined estimate for bird species and subspecies but then reduced these numbers to 8,600 bird species and 4,400 mammal species in Storer’s (1951) second edition. Mammal species estimates further decreased to 4,237 species in Morris (1965), who based his total on various compendia plus recent primary literature that had synonymized names described in previous decades. Morris also expressed a resounding sentiment of mid-1960s mammal taxonomists: “As more and more specimens became available and as the concept of reproductively isolated species became accepted, so the tide was turned and the “splitters,” as they were called, were replaced by the modern “lumpers.” They set about the task of cleaning up the enormous mess and sank name after meaningless name into oblivion.” (Morris 1965:18). Estimates of total mammal species richness continued to decrease before reaching a low in 1980 when Corbet and Hill listed 4,005 recent mammal species (C&H1), being the first known publication since the peak of mammalian descriptive taxonomy to include a complete list Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1103 of every then-recognized valid mammal species. C&H1 marks the shift from predominantly lumping species back to predominantly splitting species, as total estimates of extant mammal species richness began increasing thereafter (Table 1, Fig. 1). The 20th century also saw transformative advancements in the theory and methods used to delimit species, which directly influenced mammalian systematics. Population genetics theory established the first quantitative framework for understanding the genetic variation within and between populations, providing a genetic basis for re-interpreting previously defined morphological variation (Charlesworth and Charlesworth 2017). The shift from phenetics to cladistics introduced a phylogenetic approach to analyzing shared-derived traits, offering a means of reconstructing taxonomic relationships based on evolutionary theory (Hennig 1966). Understanding of the evolutionary significance of genetic heritability helped inspire formal species concepts, such as the Biological Species Concept (BSC; Dobzhansky 1937; Mayr 1942) that emphasized reproductive isolation as the basis for species limits, and the Phylogenetic Species Concept (PSC; Cracraft 1983) that diagnosed the monophyly of lineages to define species. The development of the General Lineage Concept (GLC; de Queiroz 1998) near the end of this period advocated for the definition of species as independently evolving lineages along a continuum, which helped clarify operational issues with the BSC, PSC, and other species concepts. The application of species concepts was aided by new empirical methods, including karyology, protein electrophoresis, and eventually DNA sequencing. Molecular genetic sequencing and computational phylogenetic analysis revolutionized taxonomy in the late 20th century, resolving many longstanding questions in mammalian systematics and opening the doors to comparative genomics (O’Brien et al. 1999). The shift toward a phylogenetic and cladistic framework for taxonomy sparked the development of the Classification of Mammals Above the Species Level (McKenna and Bell 1997), the first phylogeny-informed higher-level mammal taxonomy and the last global taxonomic compendia to consider all clades of living and fossil mammals. Additionally, this period saw more taxonomically focused specimen collections, advancing knowledge of species distributions to aid in further revisionary studies (Dunnum et al. 2018; Bakker et al. 2020). Together, these innovations enabled the modern era of mammal systematics, in which the names and taxa described in previous decades would again be revised to create more uniformly meaningful units. Mammalian systematics in the modern era. The 44 yrs since the publication of C&H1 (Corbet and Hill 1980) have seen a nearly 70% increase in the number of recognized mammal species (Table 1). Similarly, total recognized species richness has increased by ∼46% in the 4 decades since MSW2 (Wilson and Reeder 1993; Cole et al. 1994), ∼25% since MSW3 (Wilson and Reeder 2005; Reeder et al. 2007), and ∼4% since the release of MDD1 (Burgin et al. 2018). The recent and rapid net increase in recognized mammal species is comparable to activity in other major clades of life, for example, reptiles: +49% from 1995 to 2024 (Uetz 2016; Uetz et al. 2024); amphibians: +60% from 1985 to 2024 (AmphibiaWeb 2024); foraminifera: +21% from 1987 to 2020 (Hayward et al. 2020). Parallel taxonomic efforts have focused on cataloging hyperdiverse clades (e.g., vascular plants: ∼1.3 million names for ∼0.35 million accepted species; Borsch et al. 2020) or parsing among competing taxonomic perspectives (birds: 4 main lists differing by ∼20% or more; Conix et al. 2024). Thus, in this section we consider recent advances in mammal taxonomy relative to 2 more general advances in taxonomic research: (i) new kinds of data and increased data resolution; and (ii) more sophisticated methods of species delimitation. The shift to integrating molecular and morphological data to diagnose species limits in the late 20th and early 21st century spurred the resurgence of taxonomic study across many clades, a period often regarded as the “taxonomic renaissance” (Mallet and Willmott 2003; Sites and Marshall 2003; Miller 2007). Mammal research typifies activities of this renaissance in 2 ways: (i) increased splitting of existing species through taxonomic revisions (774 species split since MSW3); and (ii) proportionately more names described as new species rather than subspecies (83.4% of available names since 2000 are described as new species rather than subspecies, versus 36.1% from 1950 to 1999). Despite controversy over whether this shift constitutes “taxonomic inflation” (e.g., Zachos et al. 2013; Groves 2014), the recent changes to mammal taxonomy have been bidirectional, often involving both synonymization and new species recognition. For example, 226 species-rank names that were recognized as species in MSW3 have since been synonymized, or about 1 lump for every 6 newly recognized species from 2004 to 2024 (considering both de novo and split names). Similarly, there has been a net increase of 123 recognized genera since MSW3, typically to establish generic monophyly, which has also been bidirectional (+178 and –55 genera). There are 26 genera with all included species described after MSW3, indicating that they were added based on the discovery of divergent new species rather than refining the classification of known species. These generic revisions have changed the binomial names of at least 514 species that are recognized in both MDD2 and MSW3, a conservative estimate that does not consider generic changes to the many post-MSW3 species recognitions. Even the recognized species richness of groups such as Microcebus mouse lemurs, which rose from 2 to 25 species over 4 decades (Franz et al. 2020), was recently reduced to 19 species after an integrative taxonomic revision (van Elst et al. 2025—published after the MDD2 cutoff date). Thus, like the preceding time period discussed above, revisionary taxonomy is perhaps the most appropriate characterization of mammal taxonomic change in this modern era (albeit with more frequent splitting than lumping). Additional shifts from single-gene to genome-scale molecular analyses, and from linear and geometric morphometrics to computed tomographic (CT) scans, have revolutionized how species-rank units are delimited in mammals and other taxa. The increasing use of integrative taxonomic approaches—defined as delimiting taxa via independent lines of evidence (e.g., comparing phylogeographic, morphological, ecological, and behavioral data; Dayrat 2005; Padial et al. 2010)—coupled with more rigorous statistical methods for delimiting species (reviewed in Smith and Carstens 2022) has led to revised species richness estimates within many taxa (Jörger and Schrödl 2013; Fišer et al. 2018). Documenting previously cryptic evolutionary structure has increased the informativeness of the species unit for downstream questions (e.g., biogeography, conservation; Vences et al. 2024). However, some researchers have pushed back against this movement, arguing that the recognition of “less distinct” species with histories of hybridization is a type of “taxonomic anarchy” that hampers conservation (Garnett and Christidis 2017). In response, others have emphasized that science-based taxonomy that “follows the evidence” is essential for the integrity of conservation efforts (Raposo et al. 2017; Thomson et al. 2018). Additionally, the loss of intermediate populations through human-mediated habitat fragmentation may lead to the recognition of previously connected populations as distinct species, which some argue as contributing to artificial taxonomic inflation (e.g., Clavero et al. 2024). In contrast, others express that this phenomenon has been common throughout evolutionary history and should be considered in taxonomic inferences (“speciation-by-extinction”; Seeholzer and Brumfield 2023). We later discuss how this interplay among biodiversity informatic disciplines impacts decision making (see the Mammal taxonomy in conservation and biodiversity infrastructure and governance sections below). Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1104 | Burgin et al. Nevertheless, new data and tools have not quieted the debate regarding “how distinct” a given population should be to be recognized as a separate species, especially given the substantial histories of introgressive hybridization that are now being uncovered within some mammal clades. Throughout the 21st century, phylogenetic reconstructions based on 1 or a few genes have been used to justify species limits among mammals. However, recent evidence suggests that it may matter more which rather than how many characters are shared versus distinctive between putative species. Some mammal species appear capable of retaining phenotypic and ecological distinctiveness from related species even in the face of substantial introgressive hybridization (e.g., mustelids, felids, bats; Li et al. 2019; Colella et al. 2021; Foley et al. 2024). For example, comparative genomic evidence in cats (Bredemeyer et al. 2021) and house mice (Forejt et al. 2021) suggests that some regions on the X chromosome are “protected” from recombination, causing hybrid male offspring to be sterile if altered, thereby reinforcing the distinction among lineages. Consistent with this finding is the observation that 17% to 93% of the genomes of European Myotis bats apparently have introgressive origins (Foley et al. 2024), suggesting that many genomic regions can be freely exchanged as long as certain other regions are conserved, protecting the distinctiveness of a given species. Currently, broadening such conclusions into generalities is likely premature. Many mammal taxa lack sufficient geographic and taxonomic sampling to identify and interpret such hybridization histories (e.g., in the case of M. keenii and M. evotis; Lausen et al. 2019, 2021; Morales et al. 2021; Upham et al. 2022b). Sampling difficulties are often exacerbated by the costs of fieldwork, specimen curation, and genome sequencing, but newfound efficacy with the sequencing of genomes from preserved museum specimens promises to soon fill many populational gaps (e.g., Card et al. 2021; Raxworthy and Smith 2021; Benham and Bowie 2023). In a future where population genomic data are available for most mammal species, following the evidence about what genomically constitutes separate species may continue to surprise us. Overall, trends in modern taxonomic research appear to reflect a paradigm shift in how systematists define the species unit (reviewed in Padial and De la Riva 2021). Suggested reimaginations of the subspecies rank to avoid species inflation (Hillis 2020; Dufresnes et al. 2023) are balanced against the uneven history of subspecific classification that may render those categories ambiguous (Patton and Conroy 2017; Burbrink et al. 2022). For now, the MDD has refrained from formally recognizing subspecies in the taxonomy of mammals, but may start doing so in some groups (e.g., bats) where species-to-subspecies relationships have been extensively tracked and thus retain meaning (Simmons and Cirranello 2024). More broadly, it is remarkable that the once-vigorous philosophical debate surrounding the validity of different species concepts is now largely superseded by integrative taxonomic approaches. We acknowledge that the historical discourse surrounding this debate is important for understanding the current state of mammal systematics, but we defer to other sources that discuss it in depth (see Bradley and Baker 2001; Gippoliti and Groves 2012; Gippoliti et al. 2013, 2018; Zachos and Lovari 2013; Zachos et al. 2013, 2019; Zachos 2015, 2018a,b; Gippoliti 2019; Taylor et al. 2019). Contemporary mammal systematics now focuses on evaluating the number of independent lines of evidence that support or refute distinctiveness of a taxon (i.e., integrative taxonomy; Dayrat 2005; Padial and De la Riva 2021). To this end, recent efforts have sought to establish a “traffic-light system” to evaluate levels of taxonomic support for felid species (Kitchener et al. 2022), which parallels a proposed “scoring system” for bird species delimitation (Tobias et al. 2010, 2021; see the Governance section below where a proposal for an expanded grading system is discussed). This shift toward integrative taxonomy as opposed to choosing a single species concept underscores the importance of sharing digitized evidence in up-to-date taxonomic databases, especially in light of recent suggestions for applying machine learning approaches to automate integrative taxonomy (Karbstein et al. 2024). Patterns and shortfalls in global mammal biodiversity. Despite recent progress, considerable gaps remain in our collective knowledge of mammal species-rank taxonomy and geographic distributions—termed the Linnaean and Wallacean shortfalls, respectively (Hortal et al. 2015). These shortfalls pose hurdles to zoonotic disease studies and other biomedical research (Upham et al. 2021) as well as to the implementation of species conservation and management plans (Senior et al. 2024). This is particularly true for mammals given that they can serve as sentinels of change in threatened ecosystems (Moore 2008; Marneweck et al. 2022) or key nodes in pathogen-sharing networks (Plowright et al. 2021; Venkatesan 2023; Weber et al. 2023). Through the compilation and curation of up-to-date taxonomic, geographic, and nomenclatural data, the MDD aims to continue lessening these shortfalls for mammalogical research. This curatorial work is synergistic with 2 parallel efforts in the biodiversity research community: (i) at the specimen level, where holistic specimens (Cook et al. 2016) and their derived data are becoming linked to form “digitally extended specimens” (Lendemer et al. 2020; National Academies of Sciences, Engineering, and Medicine [NAS] 2020); and (ii) at the publication level, where related data from publications are being extracted and linked to form “digitally accessible knowledge” (Fawcett et al. 2022). The non-copyrightability of primary research findings, particularly of the parts of articles that describe how names are conceptually applied to taxa (i.e., taxonomic treatments; Agosti and Egloff 2009; Benichou et al. 2023), provides an explicit legal framework for continuing to integrate these kinds of liberated and linked biodiversity data into the MDD. Efforts to study the Linnaean shortfall typically estimate how many undescribed species likely exist and how long until the gap will be filled, using estimates for current rates of description based on authority years of species epithets. Using this approach in 2018, the MDD team estimated that recognized extant and recently extinct mammal richness will increase to 7,409 species by 2050 and to 9,009 by 2100 if observed trends in historical species descriptions were to continue (Burgin et al. 2018). That pace exceeded the previous estimate of 7,500 total species based on the same method (Reeder et al. 2007). However, those estimates were both based on partial histories of mammal species description that ignore taxonomic changes resulting from species splitting and lumping, instead focusing only on de novo species descriptions. Lessa et al. (2024) reviewed this problem in detail, concluding that “relevant estimates of the number of known and unknown species will only be achieved by accounting for the dynamic nature of the taxonomic process itself” (p. 1365). Indeed, if we reconsider the predictions of Burgin et al. (2018) using the same approach, the expectation for 418 valid de novo species descriptions from 2010 to 2019 compares to only 369 currently valid species that were actually described de novo (i.e., with an authority year during this interval). That decadal total compares to 315 during 2000 to 2009 (originally 341 species in Burgin et al. 2018, but reduced by synonymizations in MDD2). Thus, these data reinforce the perspective that rates of de novo species description are only part of the “discovery rates” story (Alroy 2002), with the other major component being rates of species splitting and lumping that result from revisionary taxonomic efforts. To more accurately assess the Linnean shortfall in mammals, we calculated the rate of new species recognition based on the estimated species totals reported in 14 authoritative taxonomic compendia over 44 yrs (1980 to 2024; Table 1, Fig. 1, upper panel). These totals each Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 Journal of Mammalogy, 2025, Vol, 106, Issue 5 | 1105 represent the net outcome of species splits, lumps, and de novo descriptions at a given time point. Ideally, these estimates would be calculated using year-by-year species split, lump, and de novo description totals to observe finer scale trends in species recognition, but these data are not yet available. Conducting a linear regression of these compendia-based data reveals an average rate of 64.9 species recognized per year, which is higher than the rate of 27.4 species/yr reported in Burgin et al. (2018) based on de novo descriptions alone. If this trend continues, we estimate that the total recognized mammal species richness will increase to ∼7,084 species by 2030 and ∼8,382 species by 2050. Both of these estimates are considerably higher than those given by Burgin et al. (2018) and Reeder et al. (2007), but are nonetheless realistic owing to the combined consideration of species splits, lumps, and de novo descriptions. The estimate of ∼11,625 species by 2100 seems more far-fetched given that this trend is likely to level off at some point, but the ∼70% increase over the current MDD2 total would be equivalent to the 1980 to 2024 increase since C&H1 was published. The potential for unidentified cryptic mammal species remains high, particularly within small-bodied and wide-ranging taxa. Parsons et al. (2022) used mitochondrial genetic data from a subset of 4,310 mammal species to find which covariates best predict unrecognized cryptic species, with body mass and range size ranking as the top predictors. A meta-analysis across metazoans similarly supported this conclusion (Cahill et al. 2024). Rodents, bats, and eulipotyphlans are predicted to be the largest sources of unrecognized mammal species despite the high rate of new species recognition in these clades. However, less speciose mammal taxa like Hyracoidea, Scandentia, Pilosa, Pholidota, Lagomorpha, and Perissodactyla that have received comparatively less taxonomic attention in the 21st century are also likely to contain cryptic species (Parsons et al. 2022). Within Pholidota, for example, a potentially new species was identified from DNA derived from tissue confiscated from the illegal pangolin trade (Gu et al. 2023), and another new species was also recently described (described as Manis indoburmanica, but Manis aurita Hodgson, 1836 may have nomenclatural priority; Wangmo et al. 2025). Similarly, a new species of Hyracoidea was described with evidence that further cryptic hyrax species exist (Dendrohyrax interfluvialis; Oates et al. 2022). Didelphimorphia stands out as having both a high degree of potential for cryptic species and extensive recent taxonomic research, with over 50 new species recognized since MSW3 (39.7% of valid Didelphimorphia species in MDD2). Conversely, the order Primates likely is predicted to contain considerably less remaining cryptic species, owing to focused and ongoing taxonomic research during the 21st century (161 new species since MSW3, 30.8% of valid Primates species in MDD2). We highlight that clade-specific taxonomic research remains imperative across the mammalian phylogeny, even in large-bodied groups like Perissodactyla and Artiodactyla that still mostly lack integrative taxonomic revisions. Geographically, recent progress in bridging the Linnaean shortfall for mammals has followed a latitudinal gradient, with more newly split species and de novo descriptions coming from tropical over temperate regions (Fig. 2, Table 3). The Neotropics and Indomalaya have particularly high proportions of newly recognized species since MSW3, including 9 of the top 10 countries (Figs 2A and 4B) and top 2 continents for new species (South America and Asia; Fig. 2B). Only around half of these newly recognized tropical species are splits, as opposed to the Nearctic/North America and Palearctic/Europe where splits form a higher proportion of new species recognitions since MSW3. These geographic concentrations of mammal taxonomic activity are consistent with previous estimates (Patterson 2000; Reeder et al. 2007; Burgin et al. 2018; Moura and Jetz 2021). In contrast, some geographic regions have fewer newly recognized species (and type localities) than expected given their latitude and species richness (Fig. 3). This is most evident in parts of Africa, Asia, and South America, such as West and Central Africa, the Amazon Basin, Central Asia, most of India, and some Indonesian islands (e.g., Java, Sumatra, Borneo), supporting calls for regional taxonomic work (e.g., Klopper et al. 2002; Chatterjee et al. 2020; Monadjem et al. 2024). These regions correspond to “biodiversity blindspots” according to a recent global assessment of digitized records in natural history collections (Ball et al. 2025), further highlighting the need for concerted specimen collection, digitization, and capacity-building efforts. Historically, much of the mammalogical research done in Global South countries was done by researchers from Europe and North America, leading to limited taxonomic research funding and access to museum specimens for Global South researchers (Salvador et al. 2022). Most specimens from tropical regions are housed in European and North American institutions (see figure 2 in Heberling et al. 2021). This imbalance between where mammals live and where their specimens are preserved helps to perpetuate geographic disparities in biodiversity knowledge and conservation outcomes (see Pritchard et al. 2022). However, there are signs that this imbalance is shifting. Global South countries are developing in-country biodiversity research capacity and infrastructure, addressing explicit goals for bridging the Linnaean and Wallacean shortfalls (Klopper et al. 2002; Prathapan and Rajan 2020). Neotropical countries like Brazil and Argentina stand out for their substantial zoological research capacity and international collaborations, helping establish the Neotropics as the top region for newly recognized mammal species (see Fig. 2 and Table 3; see also Dalapicolla and Percequillo 2020). Country-specific efforts to track species richness are critical to these goals. In Brazil, the establishment of a continuously updated mammal species list similar to the MDD provides a key local resource for decision makers (see Quintela et al. 2020; versioned releases on Zenodo at https://doi. org/10.5281/zenodo.5590556). In China, extensive systematic research throughout the 21st century has resulted in many newly recognized species (see Fig. 4B). Although substantial work remains to balance the inequities in taxonomic research globally, these in situ advances in Global South taxonomic research are promising steps toward filling the regional gaps in mammal knowledge. Mammal taxonomy in conservation and biodiversity infrastructure. Despite advances in the understanding of species-rank taxonomy and geographic distributions since MSW3, ∼25% of wild mammal species remain “understudied” relative to the IUCN Red List–either Not Evaluated (i.e., unclassified; ∼14%) or classified as Data Deficient (∼11%; Fig. 6). The IUCN Red List has assessed the conservation status of 5,983 mammal species as of version 2024-1, which is only 494 species more than were assessed in the 2008 IUCN Red List version (5,489 species; summarized in Schipper et al. 2008). The total number of IUCN-assessed species is also 759 fewer than recognized in MDD2 (excluding domestic species not assessed by the IUCN). This IUCNto-MDD2 gap is due to multiple factors, both infrastructural and sociological, that lead to delays in the integration of the latest research findings into the IUCN taxonomy. It should be noted that not all MDD2-IUCN differences are due to a lack of updated taxonomy since some clades have had more regularly updated taxonomic arrangements that differ from MDD2 based on taxonomic opinion, such as marine mammals and primates. Unfortunately, delayed IUCN assessments are not unique to mammals and are viewed as a top priority for improving conservation outcomes globally (reviewed in Bachman et al. 2019). This lag in taxonomic change integration is also not unique to the IUCN, reflecting other ongoing challenges in biodiversity data science related to the linking and dissemination of primary research findings (e.g., Lendemer et al. 2020; Upham et al. 2021; Benichou et al. 2023; Sterner et al. 2023). The MDD is working to make Downloaded from https://academic.oup.com/jmammal/article/106/5/1082/8253815 by guest on 17 October 2025 1106 | Burgin et al. progress in this domain via curation and regular versioning of the collective implications of recent taxonomic publications. However, there is potential for much closer integration of MDD efforts with downstream biodiversity resources like the IUCN Red List. Failing to do so risks unnoticed species declines or population extinctions, both for the ∼25% of mammal species that are understudied and the other ∼25% classified as threatened. Untracked taxonomic rearrangements also risk the misallocation of resources (e.g., Sylvilagus mansuetus is listed by the IUCN as Critically Endangered, but is not recognized in MDD2 due to synonymization with S. bachmani; Álvarez-Castañeda and Lorenzo 2016). Thus, greater MDD-to-IUCN integration carries potential for speeding up assessment processes and improving outcomes in mammal conservation. Conservation practices in mammals often prioritize named taxonomic units like species and subspecies as the primary units of conservation, as seen in global frameworks like the IUCN Red List and national frameworks like the Endangered Species Act in the United States and Species at Risk Act in Canada. However, this practice can overlook finer-scale intraspecific diversity (Des Roches et al. 2021) and taxonomically unrecognized species in understudied taxa (i.e., “dark extinctions”; Boehm and Cronk 2021), resulting in the loss of cryptic biodiversity (see review by Hending 2025). To address these issues, some researchers and managers have argued for formally recognizing intraspecific variation as subspecies (e.g., Dufresnes et al. 2023) or non-Linnaean conservation units (e.g., Evolutionarily Significant Units, ESUs; Casacci et al. 2014). However, others have pointed out that although some conservation organizations recognize subspecies and other infraspecific conservation units (e.g., the IUCN, NatureServe), the species rank has remained the primary “currency of conservation” in various mammal clades (Rylands and Mittermeier 2014; Gippoliti et al. 2018). Ideally, all units along the population-to-species continuum would receive some conservation protections regardless of rank, but the finite resources available for conservation and incompleteness of global biodiversity assessments have provided obstacles (Gippoliti and Amori 2007; Coates et al. 2018). As a result, conservation priorities sometimes guide taxonomic practices rather than vice versa, leading either to the splitting of species into multiple smaller units or the lumping of species to conserve those same units as subspecies or ESUs. The relationship between taxonomy and conservation is dynamic, and debate over the extent to which conservation concerns should influence the science of taxonomy is not settled. The recent taxonomic history of primates and terrestrial ungulates (Perissodactyla and non-cetacean Artiodactyla) best illustrates the impact that conservation can have on species recognition. Both groups have been at the center of splitting versus lumping debates (e.g., Rylands and Mittermeier 2014; Gippoliti et al. 2018) due to their inclusion of many traditionally charismatic species threatened with extinction (∼64% of primates and ∼40% of wild ungulates in MDD2 are threatened in the IUCN Red List). Both groups have received more frequent taxonomic updates in the IUCN than other mammals, with only ∼3% of primates and ∼8% of wild ungulates in MDD2 Not Evaluated by the IUCN. However, the similarities end there because these 2 groups have experienced contrasting taxonomic trajectories thus far in the 21st century. Primates species began being split with the publication of Primate Taxonomy (Groves 2001), which was controversial at the time but initiated a wave of taxonomic revisions that massively increased their recognized species richness—176 species (Napier and Napier 1967) versus 376 in MSW3, growing to 522 in MDD2 (Groves 2014). The publication of Ungulate Taxonomy (Groves and Grubb 2011), which split many ungulate species, served as a similar catalyst for taxonomic revision within ungulates. However, the outcome of that major shakeup in ungulate taxonomy did not have the same effect as in primates, since later authorities (including the MDD after version 1.2) have generally reverted to pre-2011 taxonomies and focused on making taxonomic changes based on more integrative taxonomic revisions (see “Taxonomic decision making” section of Methods for further details). For example, recognized Giraffa diversity has ranged from 1 to 11 species, but most workers now recognize 4 species based on integrative taxonomic approaches (Winter et al. 2018; Petzold and Hassanin 2020; Coimbra et al. 2021; Kargopoulos et al. 2024). It is clear that both primates and ungulates require further taxonomic research, but the non-uniformity of taxonomic ideologies being applied remains a hotly debated topic (e.g., Taylor et al. 2019; Zachos et al. 2019; Amori and Luiselli 2024). The MDD taxonomy increasingly serves as an arbiter of species recognition for conservation organizations at global (e.g., IUCN), national (e.g., NatureServe), and regional (e.g., state agencies) levels. Additionally, the MDD aims to serve as a foundational taxonomic connection between other biodiversity resources, such as species record aggregators (e.g., GBIF, Map of Life, iNaturalist), digitized museum archives (e.g., Arctos, Symbiota, iDigBio), and various other biological databases (e.g., GenBank, NCBI Virus, FuTRES, Morphosource, Global Biotic Interactions—GloBI). Efficient communication and application of biodiversity knowledge between conservation and biodiversity resources requires agreed-upon organismal units that can serve as foundational designations for downstream decisions made on those units. Most often these units are species (but specimens or individuals are also commonly used). The value of accurately communicating “species” categories highlights the basal importance of taxonomy and nomenclature in biodiversity infrastructure (Grieneisen et al. 2014; Sterner et al. 2020a). However, as discussed throughout this article, taxonomy and nomenclature are by no means static—the creation of species lists is plagued by gaps in biodiversity knowledge (i.e., the Linnean and Wallacean shortfalls), differing taxonomic opinions, failure to track and digitally integrate those changing taxonomies, and inconsistencies in how the ICZN Code is implemented (Garnett and Christidis 2017; Raposo et al. 2017, 2021; Franz and Sterner 2018; Upham et al. 2021). There are also other continuously updated clade-specific (e.g., Society of Marine Mammalogy marine mammal list; Committee on Taxonomy 2025) and geographic (e.g., the Brazilian mammal checklist; Quintela et al. 2020) taxonomic checklists that differ from MDD2 based on alternative taxonomic opinions and more specialized geographic knowledge, adding further bioinfrastructural complexity by offering different taxonomic perspectives and highlighting the need for communication between the MDD and other mammalian taxonomic databases. Nevertheless, the value of updated species lists to biodiversity science outweighs the pitfalls of maintaining them. The value of species-centric taxonomic databases like the MDD grows in proportion to the number of disparate sources of biodiversity knowledge that they can effectively integrate while maintaining coherency. Doing so improves the rigor of a wide range of studies in conservation, biodiversity, and biomedical sciences that rely on accurate taxonomy, facilitating work that might otherwise go undone. Governance Moving forward, the MDD is in the process of forming external taxonomic subcommittees that are scheduled to begin activity in early 2026. Subcommittee members will include a global representation of taxon-specific experts to help with ongoing processes of curating mammal taxonomic, geographic, and nomenclatural information, while also serving an advisory role to the ASM’s Biodiversity Committee in coordinating the MDD. Initial plans seek to divide these efforts across 15 to 20 subcommittees, each focusing on a clade or composite taxonomic group (e.g., Perissodactyla, Cricetidae, or Chiroptera). 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