Rattus radityaniae, Dwijayanti & Maryanto & Achmadi & Wiantoro & Dharmayanthi & Kurnianingsih & Maharadatunkamsi, 2025

Rattus radityaniae sp. nov.

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( Table 1 View Table 1 ; Figure 4 View Figure 4 , 5 View Figure 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   GoogleMaps , Melak district   GoogleMaps , 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.

Paratypes ( Listed in Appendix 1)

All paratypes are from Borneo with locality detailed as follows: MZB 29118 , adult male, collected 22 May 2006 ( 00 o 18’13.02”S, 115 o 50’06.09”E) GoogleMaps ; MZB 29123 , adult female, collected 24 May 2006 ( 00 o 18’07.04”S, 115 o 50’04.08”E); GoogleMaps MZB 29121 , adult female, collected 23 May 2006; GoogleMaps MZB 29124 , adult male, collected 24 May 2006 ( 00 o 18’07.02”S, 115 o 50’03.08”E). GoogleMaps 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   GoogleMaps , Melak district   GoogleMaps , regency of Kutai Barat   GoogleMaps , East Kalimantan at secondary regrowth forest in the border between medium brown clay forest and swamp forest ( 00 o 22’30.05”S, 115 o 30’42.08”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 View 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 View 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.

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. tiomanicus and R. exulans , which ranges from light gray to white, and from R. argentiventer , which typically exhibits pale brown to brownish-gray tones. R. baluensis , in contrast, has grayish-brown ventral fur. Among commensal species, R. norvegicus possesses grayish-white underparts, whereas R. tanezumi generally has white or pale gray ventral fur ( Schwarz & Schwarz 1967; Corbet & Hill 1992).

Rattus radityaniae sp. nov. shares several morphological traits with R. tiomanicus , R. exulans , and R. argentiventer , but can be distinguished by a unique combination of measurements (see Table 1 View Table 1 ). It has a relatively shorter body size, with a head and body length (HBL) ranging from 107.86 to 132.09 mm. This measurement is smaller than those of R. tiomanicus (136.00–174.00 mm), R. argentiventer (139.50–199.00 mm), and R. baluensis ( 122–179 mm), but slightly larger than R. exulans (101.93– 139.00 mm).

In contrast, the tail of R. radityaniae sp. nov. is proportionally longer, ranging from 106.99% to 137.00% of its HBL. This is notably longer than the tail proportions of R. tiomanicus (79.31–112.50%), R. exulans (88.49–114.78%), and R. argentiventer (82.91–97.13%), although shorter than that of R. baluensis (102.96–159.84%).

When compared with common commensal species, R. norvegicus and R. tanezumi , R. radityaniae sp. nov. has a smaller body size (HBLs of 163–265 mm and 122–219 mm, respectively) ( Payne & Francis 1985; Phillipps & Phillipps 2016). It also differs in tail proportions: R. norvegicus has a distinctly shorter tail (79–97% of HBL), whereas R. tanezumi shows some overlap (95–120%) with R. radityaniae sp. nov. ( Payne & Francis 1985; Phillipps & Phillipps 2016).

Each dentary of R. radityaniae sp. nov. appears small and sturdy relative to the skull size. The examination of the upper incisors in Rattus radityaniae sp. nov. revealed a distinct morphology characterized by large, long, and strongly curved opisthodont incisors that mostly projected backward. The lower incisors are moderately long and curved, their cutting tips slightly curved, similar in shape to R. tiomanicus , R. argentiventer , and R. exulans . The incisors have dark orange color on the anterior surface of the uppers and pale orange on the lower. Meanwhile, R. baluensis has darker orange in both the upper and lower incisors ( Musser 1986). In comparison, R. exulans has opisthodont upper incisors that are smaller, narrower, and shorter. Rattus baluensis has opisthodont upper incisors but less projected forward ( Musser 1986) compared to R. radityaniae sp. nov. In contrast, R. argentiventer features curved, long, and proodont upper incisors, while R. tiomanicus has straighter, shorter, wider, and proodont incisors ( Figure 4 View Figure 4 ).

The molars of this new species are narrow, brachyodont, and low-crowned with relatively smooth enamel surfaces without grooves. The cusps of all the molars are situated closely adjacent to one another. The first upper molar consists of two anterior rows of cusps, each in the form of a broad and large chevron, somewhat triangle-shaped posterior surface. The anterior chevron of the first upper molar comprises cusps t 1, t 2, and t 3, coalesced one to each other so that their margins are obliterated. The second and third upper molars have a large cusp of t 1, a single chevron coalesced with the posterior mass of cusps t 8 and t 9. The third upper molar is small, with a large oblong cusp t 1 and chunky posterior surface, which consists primarily of cusp t 8 merged to cusp t 9, forming a distorted triangle-shaped chewing surface. The occlusal pattern of the third molar is basically a compacted form of that characterizing the second molar; a conspicuous cusp t 1, no cusp t 3, and a small posterior oblong chunk. A cusp of t 7 is difficult to detect in all upper molars, and it may not be present in this new species. The occlusal surfaces of the first to third lower molars coalesced one to each other so that their margins are not clear, especially for adults. The upper and lower molars of this new species are smaller, narrower, and shorter relative to the skull size compared to R. tiomanicus , R. argentiventer and R. baluensis but larger than R. exulans .

This new species is also readily distinguished from all other Rattus species examined in this study by its relatively large tympanic bullae compared to skull size. When considering the GSL, R. radityaniae sp. nov. has a bulla length that is greater than that of R. argentiventer (17.87–20.39% vs. 18.03–19.43%), R. tiomanicus (16.93–18.63%), and R. baluensis (15.63–18.22%), but shorter than that of R. exulans (18.03–24.44%). The bullar height as a percentage of the GSL is relatively higher in R. radityaniae sp. nov. (17.48–18.64%), exceeding the average heights of R. tiomanicus (14.64–16.89%), R. argentiventer (16.17–17.76%), R. exulans (17.00–21.88%), and R. baluensis (14.85–17.98%). Additionally, R. radityaniae sp. nov. features a wider bulla relative to GSL (10.54–11.65%) compared to R. tiomanicus (9.03–11.95%), R. argentiventer (8.97–11.99%), narrower than R. exulans (10.43– 15.01%), but overlapping with R. baluensis (8.49–12.62%).

The skull of R. Radityaniae sp. nov. is notably compact and much smaller than those of R. tiomanicus , R. argentiventer , and R. baluensis , with greatest skull lengths (GSL in mm) of 34.24–37.20 vs. 36.41– 39.23, 38.08–44.05, and 34.51–43.22, respectively, but longer than that of R. exulans ( 28.93–34.72 mm). It has a wider and shorter nasal region, as well as a broader zygomatic arch compared to R. tiomanicus . The inter-orbital breadth of R. radityaniae sp. nov. is slightly wider, with a more curved and inflated braincase than that of R. tiomanicus . Compared to R. radityaniae sp. nov., R. baluensis has the tapering type of skull, with a more flattened braincase.

Differences in cranial and external morphology among Rattus radityaniae sp. nov., R. tiomanicus , R. argentiventer , R. baluensis , and R. exulans are clearly evident in bivariate plots of skull and body characters. Relative to greatest skull length (GSL), R. radityaniae sp. nov. exhibits a longer palatal length than R. baluensis and R. exulans , but shorter than R. tiomanicus and R. argentiventer ( Fig. 7A View Figure 7 ). Additionally, the combination of palatal length (PL), bulla height (BH), and maxillary tooth row length (TR) distinguishes R. radityaniae sp. nov. from the other species ( Figs. 7B, 7C View Figure 7 ). Based on tail length relative to head-body length (HBL), R. radityaniae sp. nov. has a slightly shorter tail than R. tiomanicus , R. argentiventer , and R. baluensis , but longer than R. exulans ( Fig. 7D View Figure 7 ).

Etymology

The new species is named after Fitri Afina Radityani, Ibnu Maryanto’s daughter, in recognition of her understanding when her father was frequently away from home doing expeditions.