Rattus obiensis, Fabre & Miguez & Holden & Fitriana & Semiadi & Musser & Helgen, 2023

Rattus obiensis View in CoL sp. nov.

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Figs 6h View Figure 6 , 7h View Figure 7 , 8h View Figure 8 , 9h View Figure 9 , 10h View Figure 10 , 13d,e View Figure 13 , 14d,e View Figure 14 , 15c,f View Figure 15 , 16c View Figure 16 , 20 View Figure 20

Holotype. The holotype (in the Museum Zoologicum Bogoriense, Cibinong MZB 38231 ) is a juvenile female ( Fig. 20 View Figure 20 ) (study skin, cleaned skull, postcranial skeleton, and tissue sample in ethanol) collected by P.-H. Fabre on 27 November 2013 with a live rat-trap baited with coconut and peanut butter. The dentition is fully erupted, the sutures on the skull are not fully closed, and the woolly coat is in immature pelage GoogleMaps . Paratype. A paratype ( MZB 38232 ) was also collected by P.-H. Fabre on 28 November 2013. It is a juvenile female (study skin, cleaned skull, postcranial skeleton, and tissue sample in ethanol) GoogleMaps .

Type locality. The type locality, in the southwest of the island of Obi (North Maluku Province, Indonesia), is on Gunung Sere above the villages of Tapaya and Wayaloar . The holotype was collected along a ridge at 970 m asl, near a campsite at 1.624°S 127.709°E, 870 m asl. The trapping site sits in disturbed secondary forest that was logged less than 20–25 years ago, as explained by the local community (Pak Sabar, personal communication).

Etymology. This species is named after the island of Obi, where the type locality is situated.

Distribution. Rattus obiensis is endemic to Obi Island, Maluku, Indonesia and has only been recorded at the type locality. The species may be more abundant at higher altitudes, as we did not catch it during 8 nights with 200 rat traps at a lower altitude camp (40–70 m) in the northern part of Obi Island; it was also not encountered by Tim Flannery during mammal surveys at low elevations in Obi and Bisa in January 1990 ( Flannery, 1995).

Diagnosis. As we only have two immature specimens, we focus our diagnosis on a selected set of external and cranio-mandibular characters stable across both adults and juveniles of Rattus . These consist especially of occlusal features of the molars. The molars of our specimens are fully erupted and very distinctive compared to other Moluccan Rattus . Even though we only have immature specimens, it is clear that, when grown, this new species would be a smaller animal than R. morotaiensis and R. halmaheraensis , the other members of the Rattus morotaiensis group.

Rattus obiensis is a small rat with broad spiny hairs, characterized by the following features: (1) a long dark tail, which is longer than the head and body length (TL/ HB = 128–130%; see also Table 2 View Table 2 ; Fig. 15 View Figure 15 and Fig. 20 View Figure 20 ); (2) long hind feet in proportion to the head and body length; (3) the palatal bridge extends slightly beyond M3 ( Fig. 16 View Figure 16 ); (4) a broad zygomatic arch that curves posteriorly outwards ( Fig. 4 View Figure 4 ); (5) the zygomatic plate is reduced and the rostrum is short and narrow; (6) in ventral view the squamosal root of the zygomatic arch does not overlap the level of the tympanic bulla; (7) in ventral view the zygomatic root of the zygomatic arch slightly overlaps at the level of the first upper molar; (8) the eustachian tube is slightly developed; (9) the short incisive foramina reach the first upper molar posteriorly; (10) the upper incisors are orthodont in configuration, with orange enamel faces, and have a distinct notch; (11) the incisor blade is narrow, less than or equal to its longest basal width; (12) a well developed posterior cingulum is present on M1 ( Fig. 13 View Figure 13 ); (13) cusp t3 is present on M2 (and variably present on M3); (14) on M1, cusps t1 and t4 are situated well posterior to the first (cusps t2 and t3) and second laminae (cusps t5 and t6), respectively; (15) large, peg-shaped anterolabial and anterolingual cuspids, subequal in size, are present on m1 ( Fig. 14 View Figure 14 ); (16) a poorly developed anterolabial cusplet is present on m1; (17) an anterolabial cuspid is present on m2 and m3; (18) posterolabial cusplets are present on all lower molars; (19) the posterolabial cusplet on m3 is distinct and produces a labial notch ( Fig. 14d–e View Figure 14 ; white arrows); (20) the posterior cingulum is present and well developed on m1 and m2; (21) crenulated enamel is present but relatively poorly developed. The mammae formula is as yet unknown. Our morphological results indicate that this species is closely related to R. morotaiensis and especially R. halmaheraensis but is well differentiated genetically.

Description and comparison with immature Rattus halmaheraensis . Within the genus Rattus , R. obiensis is a distinctive lineage in its molecular phylogenetic divergence ( Table 1 View Table 1 and Fig. 2 View Table 1 View Figure 2 ), as well as in terms of body proportions and cranio-mandibular and dental characters. We captured two immature animals during our fieldwork on Obi Island but were unable to obtain any adults. Given the difficulty of accessing these islands and the significant human activity, we describe this new species here on the basis of these two specimens.Among semi-arboreal Rattus species, R. obiensis has the longest tail, which is 128–130% of head-body length ( Table 2 View Table 2 ). The tail is covered with short squarish scales that are much smaller than those found on R. halmaheraensis of similar age. Because they are immature, the fur of these specimens is greyish with a woolly undercoat, but is beginning to show some adult features, including flat spiny hairs and some longer brownish guard hairs. On the dorsum the hairs are soft, buff and grey. The guard hairs are bicoloured, with bases that are whitish grey and tips that are golden brown, buff, or greyish brown as in R. halmaheraensis of the same age. Some of the guard hairs are thickened and ivory coloured. The dorsal fur is darkest, becoming paler along the mid-line of the body, where the dorsum meets the venter, and here the guard hairs are whitish brown or yellowish brown. The cheeks are greyish. The belly is whitish grey, and one of the two specimens has a white pectoral patch. On the mid-belly, yellowish or golden brown hairs mix with the grey hairs. The undersides of the legs are whitish grey. Compared to immature specimens of R. halmaheraensis , R. obiensis is much smaller, has a paler coat, and has a longer tail with more hairs at the tip (though not so developed as the pencillate tail tip of R. feileri ; Fig. 12 View Figure 12 ). In addition, its coat is woollier, and its belly is covered with grey fur, a characteristic not observed even in very young individuals of R. halmaheraensis . Young R. halmaheraensis usually have less woolly and darker dorsal fur, with a paler white belly.

The skull of R. obiensis has a short and narrow rostrum with a narrow lacrimal region ( Fig. 16 View Figure 16 ). Rattus obiensis has a similar dorsal cranial shape in lateral view compared to R. halmaheraensis but is significantly smaller in size compared to juveniles or subadults of that species. Compared to R. halmaheraensis juveniles, it has a proportionally larger braincase, a narrower orbit, and a narrower interparietal region that is closely attached to the margin of the nuchal crest. The upper incisors are orthodont and both upper incisors have a prominent notch, seen in both specimens. The upper incisors are very narrow compared to R. halmaheraensis of similar age. In lateral view, the zygomatic arch sits at the level of the upper molar row and its squamosal root lies well in front of the post-glenoid process. The incisive foramina are proportionally larger than in immature specimens of R. halmaheraensis . The upper molars are proportionally smaller than in R. halmaheraensis . The tympanic bullae and braincase are proportionally similar to those of R. halmaheraensis , but R. obiensis has a distinctly larger and longer eustachian tube. The mandible of R. obiensis is wider and higher, with a narrower but longer angular process than in R. halmaheraensis of similar age.

The upper molars are very small in R. obiensis and have a distinctive laminar pattern compared to all other Moluccan and Indo-Pacific Rattus ( Figs 13–14 View Figure 13 View Figure 14 ). The laminae on the upper molars are oblique, with cusp t1 situated well behind cusps t2 + t3 on the first lamina, and cusp t4 situated well behind cusps t5 + t6 on the second lamina. Thus, the third, posterior lamina (cusps t8 and t9) appears anteriorly surrounded by the second lamina (cusps t4, t5 and t6). A similar pattern is observed on M2. This configuration is distinctive compared to all Maluku, Sulawesi, and Australo-Papuan Rattus ( Musser & Holden, 1991; Taylor & Horner, 1973; Taylor et al., 1982). Compared to juveniles of R. halmaheraensis , R. obiensis has narrower laminae. In contrast to the condition in R. morotaiensis and R. halmaheraensis , R. obiensis has less developed enamel crenulation, similar to the extent of crenulation in some other Rattus species (e.g., R. leucopus ). On M1, the posterior cingulum is well developed, and cusps t8 and t9 are proportionally reduced, compared to R. halmaheraensis . Cusp t3 is present on M2 in both specimens, but variable on M3 in the two available specimens ( Fig. 13d–e View Figure 13 ). The lower molars are very small, with a cusp pattern very similar to R. halmaheraensis in several key aspects. First, m1 has large peg-like anterolabial and posterolabial cuspids. Second, m2 has an anterolabial cuspid and posterolabial cusplets. Third, m3 has a well differentiated posterolabial cusplet, a diagnostic feature of both R. obiensis and R. halmaheraensis compared to R. morotaiensis and other Rattus from the region. Anterolabial cuspids on m3, present in R. morotaiensis but only in a minority of specimens of R. halmaheraensis , are present in both specimens of R. obiensis .