Description of a New Species of Rattus (Rodentia: Muridae) from East Borneo, Indonesia

© 2025 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. https://zoobank.org/urn:lsid:zoobank.org:pub:03FCF42C-F7E6-4452-83E7-EC4BCE1B8760 Description of a New Species of Rattus (Rodentia: Muridae) from East Borneo, Indonesia ENDAH DWIJAYANTI1*, IBNU MARYANTO1, ANANG SETIAWAN ACHMADI1, SIGIT WIANTORO1, ANIK BUDHI DHARMAYANTHI1, KURNIANINGSIH2, MAHARADATUNKAMSI1 1 Museum Zoologicum Bogoriense, Research Center for Biosystematics and Evolution, National Research and Innovation Agency (BRIN), Jl. Raya Jakarta Bogor Km. 46, Cibinong, Bogor 16911 Indonesia; E-mail: [email protected]; https://orcid.org/0000-0001-7005-5275; E-mail: [email protected]; https://orcid.org/0000-0003-1519-9393; E-mail: [email protected]; https://orcid.org/0000-0002-0784-9870; E-mail: [email protected]; https://orcid.org/0000-0001-6169-5772; E-mail: [email protected]; https://orcid.org/0000-0003-4635-6099; E-mail: [email protected]; https://orcid.org/0000-0001-5044-4353 2 Directorate of Scientific Collection Management, National Research and Innovation Agency (BRIN), Jl. Raya Jakarta Bogor Km. 46, Cibinong, Bogor 16911 Indonesia; E-mail: [email protected]; https://orcid.org/0009-0005-2597-8114 * Corresponding author. E-mail: [email protected] Received 3 November 2024 │ Accepted by V. Pešić: 17 June 2025 │ Published online 1 July 2025. Abstract We describe a new species of Rattus from Borneo that is distinguished from four other Bornean Rattus by the following characters: (1) body size smaller than R. baluensis, R. tiomanicus and R. argentiventer, but larger than R. exulans; (2) tail longer than head and body, distinguishing it from R. tiomanicus, R. argentiventer, and R. exulans, which have tail length equal to or slightly shorter than head and body length; but it shorter than R. baluensis (3) fresh specimens had creamy to yellowish ventral color; (4) compact, thick and heavy skull; (5) skull is much smaller than R. baluensis, R. tiomanicus and R. argentiventer, but larger than R. exulans; (6) wider and shorter nasal, and wider zygomatic arch than R. tiomanicus; (7) inter-orbital breadth slightly wider, braincase more rounded and inflated than R. tiomanicus; (8) tympanic bullae large relative to skull size; (9) incisive foramina wide relative to length, with posterior margins located just anterior to first molar; (10) mammae formula of female specimens has 4 pairs of teats (2 pectoral + 2 abdominal); and (11) palatal bridge is wide and constricting anteriorly. Key words Borneo, morphology, new species, Rattus. Introduction Borneo, one of the largest islands, is home to remarkable mammalian diversity and high levels of endemism, with a total of 280 mammal species, 57 of which (20.4%) are endemic (Maryanto et al. 2019). Ecologica Montenegrina 88: 49-62 (2025) This journal is available online at: www.biotaxa.org/em https://dx.doi.org/10.37828/em.2025.88.5 NEW SPECIES OF RATTUS FROM BORNEO 50 This diversity is largely composed of rodents and bats. The Murinae subfamily in Borneo includes 24 species, six of which (25.0%) are endemic. Despite numerous studies on mammals in Borneo, many regions remain under-surveyed, as evidenced by recent discoveries of new records and the description of new species and subspecies. For instance, Achmadi et al. (2012) reviewed murid rodents of the genus Maxomys from Borneo, describing a new species, Maxomys tajuddinii A. S. Achmadi, Maryanto, & Maharadatunkamsi, 2012, with the type locality in Empakuq village (1°19'8.11"S, 120°6'8"E), Melak District, Kutai, East Kalimantan. Six species of Rattus have been recorded from Borneo comprising five widespread species, including R. argentiventer (Robinson & Kloss 1916), R. exulans (Peale 1849), R. norvegicus (Berkenhout 1769), R. tanezumi (Temminck 1844), and R. tiomanicus (Miller 1900) (Maryanto 2003; Maryanto et al. 2019), and one endemic species R. baluensis (Thomas 1894) from Mount Kinabalu (Musser 1986; Thomson et al. 2018). During our East Kalimantan expedition 2006, we collected six Rattus specimens with distinct characteristics that differentiated them from any other known murine species in Borneo. Given the unique features of this previously undescribed Rattus, we compared these specimens with Rattus argentiventer, R. tiomanicus, R. exulans, and R. baluensis as these species showed the closest similarities. The differences between them reveal the new species recognition of Rattus from Borneo. Material and Methods A total of 68 adult specimens were examined, consisting of Rattus sp. nov. (Borneo, n=6), R. tiomanicus (Borneo, n=12 ), R. argentiventer (Borneo, n=14), R. exulans (Borneo, n=12), and R. baluensis (Borneo, n=24) (Figure 1). Adult specimens were recognized for having completely fused basicranial sutures. Materials used in this study are collections from Museum Zoologicum Bogoriense (MZB) BRIN Indonesia, USNM National Museum of Natural History – Smithsonian Institution, and the field collection of Universiti Malaysia Sarawak (UNIMAS). All specimens examined were listed in Appendix 1. We excluded R. tanezumi and R. norvegicus from our morphometric analysis, as these commensal species are readily distinguishable from other Rattus spp. in Borneo based on differences in body size, tail length relative to head-body length, number of teats, habitat preferences, and fur coloration. However, we included a comparative assessment of their diagnostic characteristics with Rattus sp. nov. in the Results and Discussion sections. Figure 1. Map of Borneo showing specimens’ localities of this study. DWIJAYANTI ET AL. Ecologica Montenegrina, 88, 2025, 49-61 51 Twenty-three cranial and dental characters, as well as four body characters, were measured (in mm) using a digital calliper to the nearest 0.01 mm. Measurements and dentary nomenclature followed Musser (1970 & 1986), and Musser & Newcomb (1983). Comparisons of dorsal and ventral pelage coloration were made using a combination of fresh and preserved specimens, supplemented by information from relevant literature. Measurements of external characters were recorded by collectors (IM and ASA) in the field following the description in Maryanto (2003). All specimens were fixed in formalin 10% in the field and were transferred to 70% ethanol at Museum Zoologicum Bogoriense. The skull characters measurements were as follows: greatest skull length (GSL), zygomatic breadth (ZB), braincase height (HB), width of zygomatic plate (ZP), orbital breadth (IOB), diastema length (D), nasal length (NL), braincase width (BW), palatal length (PL), tooth row length (TR), incisive foramina length (IFL), bulla height (BH), bulla length (BL), bulla width (BBL), mesopterygoid fossa width (MFW), nasal breadth (NB), height of ramus angular process (RAP), the distance between the crowns of upper molar 1 to molar1 (M1M1), distance upper molar 2 to molar 2 (M2M2), the distance between the crowns of the upper third molars (M3M3), width of upper molar 1 (M1W), width of upper molar 2 (M2W), and width of upper molar 3 (M3W). External character measurements were as follows: head and body length (HBL), ear length (E), hindfoot length without the claw (HF), and tail length (TL). Sexual dimorphism for all characters was examined using analysis of variance. Canonical Discriminant Analysis (CDA) was performed to determine species clustering and to identify the key characters in separating the species. Skull and external body characters were analyzed separately. As long as R. argentiventer, R. tiomanicus, R. exulans, and R. baluensis are used to confirm this undescribed Rattus sp. nov. To avoid overfitting due to too many characters used in CDA, a subset of characters was selected based on the values of Wilk’s lambda. Computations for all analyses were conducted using the software PAST (Hammer et al. 2001). The Samples location (Figure 1) was made with GeoMapApp (www.geomapapp.org) (Ryan et al. 2009). Results and Discussion Statistical Analyses Mean, standard deviation, minimum and maximum values, and sample size (n) for all characters are presented in Table 1. None of these characters showed a sexual dimorphism. Therefore, all skull and external characters were used in the following analyses. Table 1. Male and female measurements of the skull, dentary and external characters, those non-significant sex effects of Rattus spp. (in mm) examined in this study. For abbreviations of the characters, please see the text. Rattus sp. nov. Rattus tiomanicus Rattus argentiventer Rattus exulans Rattus baluensis Measurements Skull GSL 35.56 ± 1.05 (34.24–37.20) 6 37.75 ± 0.95 (36.41–39.23) 12 40.85 ± 1.69 (38.08–44.05) 14 31.00 ± 1.88 (28.93–34.72) 14 39.77 ± 2.61 (34.51–43.22) 13 ZB 17.53 ± 0.51 (16.78–18.06) 6 18.34 ± 0.50 (17.65–19.10) 12 19.68 ± 1.06 (17.62–21.03) 14 14.53 ± 0.63 (13.32–15.75) 14 18.99 ± 1.19 (16.13–20.67) 13 HB 10.44 ± 0.67 (9.40–11.32) 6 11.35 ± 0.56 (10.32–12.43) 12 11.76 ± 0.42 (11.00–12.34) 14 9.40 ± 0.58 (8.56–10.80) 14 11.67 ± 0.62 (10.47–12.53) 13 ZP 3.82 ± 0.21 (3.46–4.02) 6 3.97 ± 0.33 (3.51–4.75) 12 4.80 ± 0.42 (4.19–5.60) 14 3.14 ± 0.35 (2.70–3.81) 14 3.90 ± 0.46 (2.97–4.74) 15 IOB 5.80 ± 0.25 (5.52–6.17) 6 6.08 ± 0.26 (5.69–6.41) 12 5.78 ± 0.25 (5.32–6.22) 14 4.84 ± 0.27 (4.47–5.25) 14 6.04 ± 0.31 (5.47–6.70) 15 D 9.59 ± 0.39 (9.17–10.18) 6 10.55 ± 0.57 (9.95–11.70) 12 11.87 ± 0.50 (11.06–13.00) 14 7.75 ± 0.48 (6.97–8.50) 14 10.54 ± 1.14 (8.32–12.08) 15 NL 11.99 ± 1.30 (10.31–13.85) 6 12.78 ± 0.57 (11.71–13.68) 12 14.40 ± 1.07 (12.27–16.40) 14 10.72 ± 0.95 (9.70–12.35) 14 13.80 ± 1.64 (10.04–15.40) 15 BW 14.69 ± 0.3 (14.31–15.06) 6 14.64 ± 0.47 (13.68–15.35) 12 16.73 ± 0.55 (15.40–17.39) 12 13.17 ± 0.59 (12.37–14.20) 14 16.51 ± 0.69 (14.75–17.48) 13 ..continued on the next page NEW SPECIES OF RATTUS FROM BORNEO 52 Table 1 PL 17.15 ± 0.74 (16.21–18.12) 6 18.31 ± 0.76 (17.35–19.71) 12 19.60 ± 0.85 (18.32–21.07) 10 13.81 ± 0.90 (12.77–16.00) 14 7.98 ± 0.52 (6.95–8.79) 15 TR 5.99 ± 0.19 (5.79–6.28) 6 6.24 ± 0.22 (5.92–6.78) 12 7.01 ± 0.16 (6.83–7.38) 14 5.13 ± 0.38 (4.76–6.20) 14 6.94 ± 0.20 (6.64–7.34) 15 IFL 6.07 ± 0.21 (5.71–6.28) 6 6.79 ± 0.37 (6.24–7.33) 12 7.45 ± 0.53 (6.30–8.02) 14 5.73 ± 0.55 (4.97–7.00) 14 7.54 ± 0.76 (5.94–8.49) 15 BL 6.89 ± 0.30 (6.32–7.13) 6 6.71 ± 0.29 (6.25–7.31) 12 7.64 ± 0.32 (7.10–8.20) 14 5.96 ± 0.54 (5.38–7.44) 14 6.69 ± 0.26 (6.14–7.08) 14 MSW 2.43 ± 0.19 (2.18–2.70) 6 2.75 ± 0.20 (2.41–3.13) 12 2.35 ± 0.22 (2.00–2.90) 14 2.05 ± 0.21 (1.72–2.50) 14 2.76 ± 0.21 (2.32–3.08) 13 M1M1 3.87 ± 0.23 (3.59–4.26) 6 4.11 ± 0.29 (3.42–4.43) 12 4.06 ± 0.20 (3.74–4.38) 12 3.17 ± 0.36 (2.32–3.75) 14 4.16 ± 0.40 (3.36–4.84) 15 M2M2 4.23 ± 0.29 (3.87–4.71) 6 4.72 ± 0.29 (4.14–5.03) 12 4.40 ± 0.32 (3.89–5.19) 12 3.48 ± 0.43 (2.44–3.95) 14 4.48 ± 0.43 (3.56–4.88) 15 M3M3 4.65 ± 0.32 (4.33–5.05) 6 5.05 ± 0.33 (4.36–5.55) 12 4.93 ± 0.40 (4.36–5.76) 12 3.86 ± 0.30 (3.36–4.35) 14 4.97 ± 0.44 (4.00–5.47) 15 M1W 1.79 ± 0.10 (1.65–1.91) 6 1.80 ± 0.09 (1.65–1.99) 12 2.20 ± 0.09 (2.00–2.31) 14 1.56 ± 0.11 (1.47–1.90) 14 2.04 ± 0.06 (1.92–2.13) 15 M2W 1.71 ± 0.06 (1.65–1.80) 6 1.71 ± 0.19 (1.23–1.94) 12 2.06 ± 0.06 (1.98–2.16) 11 1.54 ± 0.17 (1.40–1.94) 14 1.97 ± 0.07 (1.87–2.10) 15 M3W 1.35 ± 0.18 (1.00–1.51) 6 1.39 ± 0.09 (1.25–1.54) 12 1.60 ± 0.14 (1.26–1.80) 11 1.14 ± 0.15 (1.00–1.59) 14 1.55 ± 0.05 (1.43–1.60) 15 RAP 10.96 ± 0.76 (10.17–11.99) 6 11.17 ± 0.49 (10.47–11.94) 12 13.11 ± 0.74 (12.01–14.27) 14 8.47 ± 0.89 (6.90–10.00) 14 9.61 ± 1.06 (7.18–10.84) 15 NB 4.58 ± 0.22 (4.36–4.89) 6 4.78 ± 0.24 (4.33–5.18) 12 4.90 ± 0.39 (4.30–5.40) 14 4.00 ± 0.45 (3.20–5.00) 14 4.02 ± 0.25 (3.42–4.29) 15 BH 6.39 ± 0.21 (6.17–6.66) 6 5.95 ± 0.22 (5.45–6.22) 12 6.97 ± 0.20 (6.61–7.24) 14 5.66 ± 0.49 (4.97–6.55) 14 6.46 ± 0.31 (5.68–6.89) 14 BBL 3.93 ± 0.06 (3.83–3.99) 6 3.88 ± 0.30 (3.40–4.35) 12 4.40 ± 0.33 (3.85–4.89) 14 3.95 ± 0.39 (3.37–4.69) 14 3.97 ± 0.30 (3.66–4.64) 14 Body HBL 120.65 ± 9.98 (107.86–132.09) 6 156.88 ± 10.98 (136.00–174.00) 12 173.95 ± 17.37 (139.50–199.00) 10 114.01 ± 11.02 (101.93–139.00) 12 152.43 ± 16.63 (122.00–179.00) 21 T 143.65 ± 5.78 (133.00–148.40) 6 148.07 ± 7.78 (138.00–158.00) 12 153.95 ± 10.13 (135.50–165.00) 10 114.32 ± 8.19 (101.17–130.85) 12 183.95 ± 22.01 (139.00–220.00) 21 HF 27.87 ± 1.57 (25.95–30.43) 6 29.11 ± 1.91 (24.15–31.79) 12 35.40 ± 2.37 (32.00–38.00) 10 23.51 ± 1.58 (21.10–25.60) 12 29.00 ± 5.07 (22.00–36.00) 21 E 17.44 ± 0.91 (16.58–18.35) 4 18.00 ± 2.18 (12.62–20.77) 12 20.15 ± 0.88 (19.00–21.00) 10 16.29 ± 2.52 (9.60–18.85) 11 19.37 ± 1.86 (16.00–22.00) 19 Canonical Discriminant Analysis (CDA) of the skull revealed a clearly different cluster among species. A total of four canonical scores explain 100% of the total variation. The variation explained and eigenvalue (in parentheses) by the first, second, third, and fourth canonical axes were 83.90% (117.77), 14.10% (19.83), 1.90% (2.72), and 0.10% (0.06), respectively. All individuals of Rattus sp. nov., R. tiomanicus, R. argentiventer, R. exulans, and R. baluensis were 100% correctly classified (Tables 2 and 3). The combination of the two-dimensional plot of canonical scores 1 vs. 2, 1 vs. 3, and 2 vs. 3 shows a clear separation between species (Figure 2). Canonical Discriminant Analysis (CDA) on the body of all individuals correctly grouped 91.4% into their respective species. All individuals of Rattus sp. nov. and R. exulans were correctly clustered into their groups. However, there were some overlaps between individuals of other Rattus spp. examined. One individual (8.3%) of R. tiomanicus clustered into the group of Rattus sp. nov., one individual of R. argentiventer (10%) clustered into the group of R. tiomanicus, and three individuals of R. baluensis (15.79%) clustered into Rattus sp. nov. CDA on body character extracted four canonical scores, which combined explained 100% of the variance, comprising 62.62% (eigenvalue 3.82), 32.15% (1.96), 4.76% (0.29), and 0.46% (0.03), respectively (Figure 3, Table 2). A combination of canonical score 1 vs. 2 and canonical score 1 vs. 3 DWIJAYANTI ET AL. Ecologica Montenegrina, 88, 2025, 49-61 53 largely separated Rattus sp. nov. from R. exulans, R. argentiventer, R. tiomanicus and R. baluensis. Canonical score 1 separated Rattus sp. nov. from R. exulans. It partially separated Rattus sp. nov. from R. baluensis and R. argentiventer. Canonical score 2 completely separated Rattus sp. nov. from R. argentiventer. The canonical score 3 showed overlap between species (see Figure 3, Table 3). Figure 2. Scatter plot of canonical scores 1 vs 2 (A) and canonical scores 1 vs 3 (B) reduced skull character based on CDA. Table 2. Standardized and unstandardized (in brackets) CDA using a reduced set of five skull characters and four body characters. Character Canonical Score 1 2 3 Skull PL -1.428 (-2.089) 0.269 (0.393) 0.046 (0.067) TR 0.662 (3.219) 0.337 (1.640) 0.255 (1.238) M2M2 0.758 (2.247) 0.181 (0.537) 0.974 (2.888) BH 0.246 (0.792) 0.202 (0.653) -0.953 (-3.073) M1W 0.238 (3.248) 0.688 (9.399) -0.211 (-2.885) Constant (-10.129) (-40.106) (-3.588) Body HBL -0.165 (-0.012) 1.163 (0.081) -0.808 (-0.057) T 1.007 (0.069) -0.728 (-0.050) 0.113 (0.008) HF 0.465 (0.135) -0.120 (-0.035) 1.232 (0.358) E 0.090 (0.048) 0.187 (0.099) -0.222 (-0.118) Constant (-13.703) (-5.041) (-1.069) NEW SPECIES OF RATTUS FROM BORNEO 54 Figure 3. Scatter plot of canonical scores 1 vs 2 (A) and canonical scores 1 vs 3 (B) body character based on CDA. Table 3. Predicted group memberships using CDA on skull and body characters. No Species n Predicted Group Membership (percentage) Total n 1 2 3 4 5 Skulla 1 Rattus sp. nov. 6 (100%) 0 (0%) 0 (0%) 0 (0%) 0 (0%) 6 2 Rattus tiomanicus 0 (0%) 12 (100%) 0 (0%) 0 (0%) 0 (0%) 12 3 Rattus argentiventer 0 (0%) 0 (0%) 10 (100%) 0 (0%) 0 (0%) 10 4 Rattus exulans 0 (0%) 0 (0%) 0 (0%) 12 (100%) 0 (0%) 12 5 Rattus baluensis 0 (0%) 0 (0%) 0 (0%) 0 (0%) 14 (100%) 14 Bodyb 1 Rattus sp. nov. 6 (100%) 0 (0%) 0 (0%) 0 (0%) 0 (0%) 6 2 Rattus tiomanicus 1 (8.3%) 11 (91.67%) 0 (0%) 0 (0%) 0 (0%) 12 3 Rattus argentiventer 0 (0%) 1 (10%) 9 (90%) 0 (0%) 0 (0%) 10 4 Rattus exulans 0 (0%) 0 (0%) 0 (0%) 11 (100%) 0 (0%) 11 5 Rattus baluensis 3 (15.8%) 0 (0%) 0 (0%) 0 (0%) 16 (84.2%) 19 a. 100% of original grouped cases correctly classified b. 91.4% of original grouped cases correctly classified Systematics Overview Rattus radityaniae sp. nov. https://zoobank.org/urn:lsid:zoobank.org:act:5C584DF1-CE21-4A7A-9DAD-09EC5D69673C (Table 1; Figure 4, 5, and 6) Holotype Museum Zoologicum Bogoriense MZB 29119 (Field No. MW 298), adult female, weight 68 grams, head body length 128.48 mm, tail length 148.40 mm, ear length 18.35 mm, hind foot length 30.43 mm; skull removed, carcass fixed in 10 percent formalin and preserved in 70 percent ethanol. Skin dried and stuffed after fixing in formalin. Collected by Maryanto and Achmadi on 22 May 2006. Type locality Intu Lingau village, Melak district, Kutai Barat regency, East Kalimantan, Borneo, Indonesia (00°18'07.04"S, 115°50'04.08"E). The habitat was a selective logged dipterocarp forest on waterlogged, periodically inundated sandy clay. DWIJAYANTI ET AL. Ecologica Montenegrina, 88, 2025, 49-61 55 Paratypes (Listed in Appendix 1) All paratypes are from Borneo with locality detailed as follows: MZB 29118, adult male, collected 22 May 2006 (00o18’13.02”S, 115o50’06.09”E); MZB 29123, adult female, collected 24 May 2006 (00o18’07.04”S, 115o50’04.08”E); MZB 29121, adult female, collected 23 May 2006; MZB 29124, adult male, collected 24 May 2006 (00o18’07.02”S, 115o50’03.08”E). All of these specimens were collected in selectively logged dipterocarp forests on waterlogged, periodically inundated sandy clay in Intu Lingau village, Melak district, regency of Kutai Barat, East Kalimantan; and MZB 29117, young adult male, collected 29 May 2006 from Dempar village, Melak district, regency of Kutai Barat, East Kalimantan at secondary regrowth forest in the border between medium brown clay forest and swamp forest (00o22’30.05”S, 115o30’42.08”E). All specimens were collected by Ibnu Maryanto and Anang S. Achmadi. All specimens were initially fixed in 10% formalin. Skins were later removed and preserved dry, and skulls were cleaned by hand. Habitat The holotype was collected from the western Mahakam River, East Kalimantan, Borneo, in the area of Empakuq and Dempar, from a selectively logged dipterocarp forest on brown clay and logged forest on waterlogged sandy clay, which was periodically inundated. Four paratypes of R. radityaniae sp. nov. were collected from Empakuq and one from a regrowth forest on medium brown clay at Dempar village. So far, there is no record of R.radityaniae sp. nov. from other places in Borneo, but it probably occurs more broadly. Other murinae species that occurred in the same area were R. tanezumi, Maxomys tajuddinii, M. whiteheadi (O. Thomas, 1894), Niviventer cremoriventer (G. S. Miller, 1900), and Sundamys muelleri (Jentink, 1880). Diagnosis Rattus radityaniae sp. nov. has a medium body size, with a head and body length ranging from 107.86 to 132.09 mm, a tail length of 133.00 to 148.40 mm, a hind foot length of 25.95 to 30.43 mm, an ear length of 16.58 to 18.35 mm, and a weight of 48 to 78 g (Table 1). Its hair is soft and dense, featuring sharp and long guard hairs typical of Rattus species, covering the upper parts. The dorsal hairs are grey at the base, transitioning to orange-brown in the middle, with black tips. The underparts of fresh specimens exhibit a creamy to yellowish pelage, with a sharp demarcation between the dorsal and ventral colors along the flank. In museum specimens, the belly fur tends to appear pale buff with grey bases. The tail is unicolored and dark, extending beyond the length of the head and body (TL/HBL ratio of 106.99% to 137.00%). The tail scales of Rattus radityaniae sp. nov. comprise 12–13 rings per cm at the mid-section. This new species has a mammae formula of four pairs (2 + 2 = 8), including two pectoral and two abdominal pairs of teats. An examination of the upper incisors in Rattus radityaniae sp. nov. revealed a distinct morphology characterized by large, long, and strongly curved opisthodont incisors. Description and Comparisons Descriptive statistics for Rattus radityaniae sp. nov., including skull and external body characters, as well as those of four other Rattus species examined in this study, are provided in Table 1. This new species is readily distinguished morphologically from all other Rattus species analyzed. Rattus radityaniae sp. nov. has a mammary formula of four pairs (2 + 2 = 8), differing from R. tiomanicus (2 + 3), R. baluensis (2 + 3) and R. argentiventer (3 + 3). Although R. exulans shares the same mammary formula as the new species (2 + 2), it has a significantly smaller body size (Suyanto 2006; Liat 2015; Nasir et al. 2023). Furthermore, the new species is definitely different from the commensal rats. Rattus norvegicus has six pairs (3 + 3 = 12) (Cardiff et al. 2018), whereas R. tanezumi has five pairs ( 2 + 3 = 10) (Wilson et al. 2017). The dorsal pelage of Rattus radityaniae sp. nov. closely resembles that of R. tiomanicus and R. argentiventer, particularly in general texture and coloration. However, it can be distinguished from R. exulans, which displays yellowish-brown to greyish-brown dorsal fur with prominent black guard hairs. It also differs from R. baluensis, which possesses long, dense, and soft hairs with a dark tawny colour, interspersed with elongated black guard hairs. NEW SPECIES OF RATTUS FROM BORNEO 56 Figure 4. Distal view of upper incisors of (A) R. radityaniae sp. nov., (B) R. tiomanicus, (C) R. argentiventer, (D) R. exulans, and (E) R. baluensis. DWIJAYANTI ET AL. Ecologica Montenegrina, 88, 2025, 49-61 57 Figure 5. View of the (A) skull and (B) external of Rattus radityaniae sp. nov. (Holotype MZB No. 29119 and paratype MZB No. 29123). In comparison to commensal species, R. radityaniae sp. nov. exhibits distinct dorsal coloration. Rattus tanezumi typically has a dorsal pelage ranging from greyish-brown to black, while R. norvegicus is characterized by coarse, dense, brown pelage (Schwarz & Schwarz 1967; Corbet & Hill 1992). The ventral pelage of R. radityaniae sp. nov. is creamy to yellowish in fresh specimens, gradually fading to yellowish-gray when preserved. This coloration differs from the ventral fur of R. A B