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

Rattus taliabuensis sp. nov.

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Figs 6d View Figure 6 , 7d View Figure 7 , 8d View Figure 8 , 9d View Figure 9 , 10d View Figure 10 , 11c,f View Figure 11

Holotype: The holotype is an adult (scrotal) male collected by the commercial collector J. J. Menden on 27 September 1938 on “ Insel Taliaboe Molukken ” (Taliabu Island, Maluku) and labelled SNSD 119968 (in Staatliche Naturhistorische Sammlungen Dresden, Museum für Tierkunde). Method of collection and exact locality are not known. The skin is in good condition, with the tail slightly split across its length due to skin preparation. The skull is intact apart from a broken basioccipital and tympanic bulla. The holotype is the only known specimen .

Type locality. Sula Islands, Taliabu Island. The label indicates a 300 m altitude for the type locality.

Etymology. Rattus taliabuensis is named after its geographical provenance, from Taliabu in the Sula Archipelago, Maluku, off eastern Sulawesi.

Distribution. Known only from the type locality, Taliabu ( Fig. 1 View Figure 1 ), Maluku, Indonesia.

Diagnosis. Rattus taliabuensis is a large-bodied rat with a spiny coat that is dark reddish-brown on the upperparts (brown with reddish-brown guard hairs on the back) and lighter reddish-brown on the underparts ( Fig. 11 View Figure 11 ). This rat has a short tail compared to its head and body length (TL/ HB = 70%; see also Table 2 View Table 2 ), with large tail scales. This species is distinguished from all other species of Rattus by the following set of characters: (1) a dark tail, shorter than its head-body length ( Table 2 View Table 2 ); (2) short hind feet relative to head-body size; (3) the incisor enamel is orange and the upper incisors are opisthodont in conformation; (4) the incisor blade is moderately narrow and its size is less than or equal to its longest basal width; (5) the zygomatic plate is wide and the rostrum is long and narrow; (6) laterally, the skull is almost flat between the nasal tips and the occiput; (7) the posterior palatine foramina are at the level of the anterior part of M3; (8) the palatal bridge extends beyond M3 to form a broad bony shelf, as in most of the other Rattus species considered here; (9) the eustachian tube is large (long and wide, Fig. 7 View Figure 7 ); (10) the post-glenoid cavity is not fused with the middle lacerate foramen; (11) the posterior margins of the long and wide incisive foramina reach the levels of the first upper molars; (12) the posterior cingulum is absent on M1 and M2 ( Fig. 9 View Figure 9 ); (13) cusp t3 is present on M2 but not on M3; (14) t1 bis is absent on M1; (15) cusp t1 of the first upper molar is at the same level as cusps t2 and t3; (16) there is no anterocentral cusplet on m1 ( Fig. 10 View Figure 10 ); (17) anterolabial and posterolabial cusplets are present m1; (18) anterolabial cuspid and posterolabial cusplet are present on m2; (19) only an antero-labial cusplet is present on m3. The mammae formula is unknown, the only known specimen being an adult male.

Description and comparison with Rattus feliceus and Rattus elaphinus : Rattus taliabuensis is a large, dark rufous rat with a short tail measuring 70% of the length of the head and body ( Table 2 View Table 2 and Fig. 11 View Figure 11 ). The fur covering the upper parts is rufous, with longer guard hairs on the rump. The coat is very spiny, with some long spiny guard hairs prominent on the rump (20–30 mm) and shorter ones on the head, neck, and shoulders (6–20 mm). There are long reddish guard hairs with dark tips, which are more sparsely distributed in the antero-dorsal region. The front of the body is paler and more rufous than the back, which is darker. The belly is lighter with a few medium sized, hard spines that vary from cream rufous, orange rufous to dark rufous. There is a darker area at the base of the scrotum and between the shoulders. Despite some similar external proportions between R. feliceus and R. taliabuensis , their colour pattern is very different, with R. feliceus having both a distinctive white belly and a darker colouration on the upperparts. The overall orange-red belly colouration of R. taliabuensis is quite distinct from all Maluku rats. Like several other species of Indo-Pacific Rattus , R. taliabuensis is covered with very spiny hairs, although the spines are not as thick as those of R. feliceus and R. morotaiensis . These spines have a thin base and are white in colour with dark brown or orange tips. The guard hairs between these spines have a grey base with an orange or dark brown tip. Similar to the morphology of Halmaheramys bokimekot and H. wallacei ( Fabre et al., 2018: 192, fig. 2C), this species has some long guard hairs that extend onto the rump fur. In terms of external proportions, R. taliabuensis is characterized by a shorter tail (70% of head body length) and a shorter hind foot length than any other Moluccan species ( Table 2 View Table 2 and Fig. 11 View Figure 11 ). Its short tail is very characteristic due to the presence of large squarish scales (6–7 scale rows per centimetre measured near the base of the tail), all of which contain very short hairs half the length of a tail scale. These square-shaped tail scales are very rare in murids from the region and are only found in Halmaheramys bokimekot and H. wallacei , albeit with spiny hairs instead. The forefeet of R. taliabuensis exhibit the normal Rattus morphology, with three interdigital pads and thenar and hypothenar pads. The morphology of the forefeet is also distinctive with two large thenar and hypothenar pads and a central interdigital pad larger than both lateral and medial pads. The dorsal part of the hand is almost bare, with very small scales covered with tiny rufous hairs. The toes are strong and short, with short and narrow claws. The claws on the forefeet are small and almost devoid of fine hair. The chunky, short toes on the forefeet contrast with the thinner and longer toes of R. elaphinus from Taliabu. The hind feet of R. taliabuensis are broad with a moderately long thenar pad and a small hypothenar pad. The pale dorsal part of the hind foot contrasts well with its darker ventral side. There are more silvery hairs covering the dorsal part of the hind feet, as well as a small tuft of silvery hairs on the edge of the claws. This rat has very long reddish mystacial vibrissae (50–75 mm) extending beyond the posterior part of the ears. A few superciliary (30–45 mm) and 1–2 genal (25–30 mm) whiskers are also present, and these are moderately long compared to the mystacial whiskers. The small ears are dark brown and covered with tiny silvery hairs.

Dorsally, the skull of R. taliabuensis is longer than that of R. elaphinus , with a proportionally longer rostrum ( Fig. 6 View Figure 6 ). The postorbital ridge is well developed. However, compared to R. elaphinus and R. feliceus , the postorbital ridge is reduced from the middle of the parietal. The zygomatic notch is similar to R. feliceus and reduced compared to R. elaphinus . On the lateral side, the zygomatic plate is broad, with a zygomatic arch that hangs well above the level of the molar row, as in other Taliabu and Moluccan Rattus . The top of the skull is flat as in R. feliceus and R. elaphinus . As in R. feliceus , the tympanic bulla of R. taliabuensis is not swollen ( Fig. 8 View Figure 8 ), and the middle lacerate foramen is well separated from the front of the tympanic bulla and connected to the post-glenoid foramen. The incisive foramen is shorter in R. taliabuensis than in R. feliceus . The palate of R. taliabuensis does not extend as far back from M3 as in R. feliceus . The 2 most distinctive features of R. taliabuensis compared to R. feliceus and R. elaphinus are its wide and long eustachian tube and its reduced auditory bullae. The upper incisors are orange and opisthodont as in R. feliceus . The mandible of R. taliabuensis is also very similar to that of R. feliceus . Its angular process is broad and stocky, with a large anterior deep masseter ridge. The coronoid is broad and poorly developed (possibly broken due to poor cleaning), but otherwise similar to R. feliceus .

The molars of R. taliabuensis are narrower than those of R. feileri . Compared to R. elaphinus , they are slightly larger and morphologically similar ( Fig. 7 View Figure 7 ). There is a clear difference between the upper molars of R. taliabuensis and R. elaphinus . In fact, cusps t1 and t4 are less separated from their corresponding cusps t2+t3 and t5+t6, respectively ( Fig. 9 View Figure 9 ). These cusps t1 and t4 appear to be very small and well separated from their lamina in R. elaphinus , reminiscent of the molar morphology of R. hoffmanni ( Musser & Holden, 1991) , which is not the case in R. taliabuensis . Regarding the lower molars ( Fig. 10 View Figure 10 ), the morphology of R. taliabuensis includes a classical shape with a wide lamina, both anterolabial and posterolabial cusps are present and large on m1 and m2. In R. elaphinus they are present but smaller in proportion. The anterolabial cusplet of m3 is wider than in all observed specimens of R. elaphinus . If the global morphology of the skull shape of R. taliabuensis is close to that of R. feliceus , the shape of their teeth is also similar (see Figs 9–10 View Figure 9 View Figure 10 ), with the anterolabial cusplet being significantly larger in R. taliabuensis .

Ecology. Little is known about the ecology of Rattus taliabuensis . Morphological features suggest that it is probably terrestrial in lifestyle. In rats, a short tail, large body size and broad feet are usually associated with terrestrial habits. Given ongoing habitat disturbance on Taliabu due to logging, agriculture, and forest fires ( Rheindt, 2010), this species may be threatened or even possibly extinct, though much more survey work is required to more fully understand the extant mammal fauna of Taliabu. It probably co-occurs with Rattus feileri and Rattus elaphinus .