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

Rattus halmaheraensis sp. nov.

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Figs 6g View Figure 6 , 7g View Figure 7 , 8g View Figure 8 , 9g View Figure 9 , 10g View Figure 10 , 12a View Figure 12 , 13a–c View Figure 13 , 14a–c View Figure 14 , 15b,e View Figure 15 , 16a,b View Figure 16 , 19 View Figure 19

Holotype. The holotype (in the Australian Museum, Sydney, AM M.23652) is a young adult male collected on the island of Halmahera on 1 May 1991 by Tim Flannery near Goal, Sahu Timur, West Halmahera Regency (North Maluku Province, Indonesia). The skin, skull, and jaws are intact and in good condition . Paratypes. Three specimens from Halmahera collected in 1991 by an Australian Museum field crew headed by T. Flannery ( AM M.26614 , female, body in fluid; AM M.26615 , male, skin, and skull; AM M.26965 , female, skin, and skull) .

Type locality. The type locality, in the northwest of the island of Halmahera Island (North Maluku Province, Indonesia), close to Goal locality (1.2115°N 127.56007°E). This trapping site was situated along the edge of primary forest.

Referred specimens. Specimens previously attributed to Rattus morotaiensis from the island of Bacan ( Flannery, 1995) are here referred to R. halmaheraensis , including AM M.23653 (male, skin and skull), AM M.23720 (female, body in fluid with skull extracted), AM M.26616 (male, body in fluid), AM M.26617 (female, body in fluid), and AM M.27011 (male, skin and skull). A previously overlooked specimen in the Australian Museum, from Ternate (AM M.23655, female, skin and skull, from Ayr Tege Tege, Ternate, collected 2 January 1991 by T. Flannery), is also referred to R. halmaheraensis . A team from MZB recently collected a large series of R. halmaheraensis on Halmahera ( Fig. 1 View Figure 1 and Appendix 1). Six specimens from the island of Moti (MZB 33573–7) are more tentatively referred here to R. sp. cf. halmaheraensis but may represent an additional undescribed species (see phylogenetic results, Table 1 View Table 1 and Fig. 2 View Table 1 View Figure 2 ).

Etymology. We name this species after the island of Halmahera, where the type locality is situated.

Distribution. Rattus halmaheraensis is widespread on Halmahera ( Fig. 1 View Figure 1 ), occurring at altitudes from sea level to 1000 m. Populations morphologically similar to R. halmaheraensis have been documented from the adjacent islands of Bacan and Ternate, and are tentatively referred to here as R. halmaheraensis , but these have not yet been included in our molecular comparisons. Another allied population on Moti is known from a few specimens that are genetically ( Fig. 2 View Table 1 View Figure 2 ) and morphologically distinct from Halmaheran samples of R. halmaheraensis , indicating the need for further taxonomic study (Anang Achmadi, personal communication).

Diagnosis. Rattus halmaheraensis is medium-sized rat, smaller than R. morotaiensis , with a spiny coat that is dark reddish-brown ( Fig. 15 View Figure 15 ). This species is characterized by: (1) a dark brown tail longer than head and body length (TL/ HB = 110–136%; Table 2 View Table 2 ), sparsely haired and slightly tufted at the tip ( Fig. 12 View Figure 12 ); (2) a distinctive spiny coat speckled with large flat spiny guard hairs; (3) a long hind foot relative to the length of the head and body; (4) the bony palate extends a moderate distance behind M3 to form a narrow shelf; (5) the postorbital and temporal ridges are moderately developed; (6) the rostrum is narrow and its ventral side is characterized by a depression of the premaxillary bone, visible in both ventral and lateral views; (7) the zygomatic plate is moderately wide; (8) in lateral profile, the skull is arched between the nasal and occipital; (9) in ventral view, the squamosal root of the zygomatic arch is positioned at the level of the tympanic bulla; (10) in ventral view, the maxillary root of the zygomatic arch is positioned anterior to, or at, the first upper molar; (11) the incisive foramina are long and reach the anterior edge of M1 ( Fig. 7 View Figure 7 ); (12) the mammae formula is 1 pectoral + 1 post-axillary + 0 abdominal + 3 inguinal (1+1+0+3=10); (13) the angular process does not extend beyond the posterior part of the articular condyle; (14) the incisor blade is very narrow, less than or equal to its longest basal width; (15) the posterior cingulum is weakly developed or absent on M1 ( Figs 9 View Figure 9 and 13 View Figure 13 ); (16) cusp t3 is usually present on the second upper molar (in 85% of available specimens); (17) cusp t1 of M1 is located just behind the level of cusps t2 and t3 and is well separated from the lamina in young specimens; (18) there are large peg-like anterolabial and anterolingual cuspids, subequal in size, on m1 ( Fig. 10 View Figure 10 ); (19) anterolabial and posterolabial cusplets are always present on m1; (20) the anterolabial cusplet on m1 is as large as the anterolabial cuspid and often accompanied by a second, tiny cusplet (alc2, Fig. 14b View Figure 14 ); (21) a posterolingual cusplet is present in several specimens (pli, Fig. 14a View Figure 14 ); (22) anterolabial cuspid and posterolabial cusplets are always present on m2 and m3 (23) the posterolabial cusplets on m3 produce a distinct labial notch ( Fig. 14a–c View Figure 14 , white arrows); (24) a wide cingular margin is present on m2; (25) crenulated enamel is present on all molars. Our molecular phylogenetic results, as well as those published by Thomson et al. (2018), indicate that this species is related to R. morotaiensis and R. obiensis sp. nov., but is well differentiated genetically as well as morphologically.

Description. Rattus halmaheraensis is medium-sized, with spiny fur, dorsally grizzled olive-brown mottled with reddish patches, and a long tail, 110–136% of head and body length ( Table 2 View Table 2 and Fig. 15 View Figure 15 ). It is smaller in body and cranial size than R. morotaiensis . Body mass can reach approximately 250 grams. On the dorsum, the wide (0.02 mm) and long spines (10 to 16 mm long, compared to more than 20 mm long on the rump in R. morotaiensis ) are pale ivory or olivegrey from base to tip, and dark brown or blackish for the distal third. There are long, soft guard hairs between these spines, which are usually bicoloured, grey proximally and reddish distally. These guard hairs are usually only slightly longer than the spines, but some can reach as long as 40 mm on the rump of the animal. The spines are channelled and convex on the underside, forming a pointed and inverted groove. The dorsum of juveniles may be slightly spiny, but they usually have a softer coat with some thin inflated spines, and the youngest individuals (e.g., MZB 33551) have moulting grey hairs. Adults and juveniles usually have a whitish belly, throat, and undersides of the legs and chin, often with some orange or rust colouring on the throat. White spines and guard hairs are usually shorter on the belly and on the undersides of the legs. Juvenile coats are usually softer with thinner white hairs, but always whitish on the belly and hind legs. The colour of the upperparts varies in the large series of specimens from Halmahera at the MZB, with young animals being darker, and older animals being paler and also having a broad brownish tinge on the throat. Mystacial, superciliary and submental, genal and interramal vibrissae adorn the head. Most of the mystacial vibrissae are dark brown or blackish with an unpigmented distal end that varies in length. The ears are of medium size (9–11% of the head body length) with a dark brown or brownish tip and a buff or pale grey base. From the base to the tip, a few very short and thin buffy or silvery hairs cover the outer ear. The dorsal surfaces of the front and hind feet, including the proximal part of the digits, are covered with very short buff or brown hairs. The distal ends of the digits on the front and hind feet are covered with short silvery hairs. The nails are cream-coloured, each covered with silvery hairs that are more abundant on the hind feet. The palmar and plantar surfaces are pinkish or whitish-brown, unpigmented and hairless. The manus has two large and prominent metacarpal pads and three smaller interdigital pads. Both the interdigital and metacarpal pads are connected. Digital pads are also well developed on the digits of the fore and hind feet. On the hind feet, four interdigital pads are moderately developed. The two central interdigital pads are in close contact and both are connected to large lateral interdigital pads. The hypothenar is broad, as is the thenar. The thenar pads are long and have a broad comma shape with a distal wider base. Ulnar vibrissae are visible, mostly unpigmented but somewhat darker in three specimens. The tail is dark brown, with large square tail scales, with 8–9 scales per centimetre (juvenile tails have 9–11 scales per centimetre), and three hairs per scale, each slightly longer than a scale. A small tuft of dark hairs is present at the tip of the tail, but this is not as strongly developed as in R. feileri ( Fig. 12 View Figure 12 ). Females usually have 10 functional teats with 1 pectoral, 1 post-axillary and 3 inguinal pairs.

The skull of R. halmaheraensis is smaller than in R. morotaiensis ( Figs 6–8 View Figure 6 View Figure 7 View Figure 8 ). It has a short and narrow rostrum with a weakly developed lacrimal groove. The frontal and postorbital ridges are present in adults but not in juveniles ( Fig. 16 View Figure 16 and Fig. 19 View Figure 19 ); these ridges are less pronounced overall, and less developed in immature specimens, than in R. morotaiensis ( Figs 6 View Figure 6 , 18 View Figure 18 , 19 View Figure 19 ). In lateral profile the top of the skull curves from nasal to occipital, a distinctive feature compared to R. morotaiensis . In R. halmaheraensis the braincase is smaller with a more rounded shape and an antero-posteriorly reduced interparietal bone than in R. morotaiensis . A distinctive feature of R. halmaheraensis is its very narrow rostrum, with a diagnostic premaxillary constriction, most visible from the ventral side ( Fig. 7 View Figure 7 ). In ventral view, the incisive foramina of R. halmaheraensis are longer than in R. morotaiensis , slightly overlapping the anterior surface of the first upper molars ( Fig. 7 View Figure 7 ). The palatal bridge in R. halmaheraensis does not extend as far behind the third molars as in R. morotaiensis . In R. halmaheraensis , the maxillary root of the zygomatic arch reaches the first upper molar, and the squamosal root of the zygomatic arch reaches the level of the tympanic bulla. The upper incisors of R. halmaheraensis are more gracile and less opisthodont than in R. morotaiensis .

The dentition of R. halmaheraensis is very distinctive. The upper and lower molar rows are proportionally smaller relative to the skull size compared to R. morotaiensis ( Figs 7 View Figure 7 , 13–14 View Figure 13 View Figure 14 ). The strongly crenulated enamel, molar cusp patterns, and shape of the laminae are similar in size to R. morotaiensis , though the antero-posterior decrease from M1 to M3 is more pronounced in R. halmaheraensis ( Figs 13–14 View Figure 13 View Figure 14 ). Cusp t3 is usually present on M2 (85% of specimens) and M3 (75% of specimens). The most distinctive upper molar features of R. halmaheraensis are (1) a smaller cusp t1 on M1, placed more ventrolaterally to the t2+t3 lamina compared to R. morotaiensis , and (2) cusps t2 and t3 of M1, which are fused into a distinctive, straight lamina compared to the more tuberculate lamina of R. morotaiensis ( Fig. 13 View Figure 13 ). The anterolabial cuspid and posterolabial cusplet are always present on m1 and m2 and also on m3. Unlike R. morotaiensis , this posterolabial cusplet forms a distinct notch on the hypoconid of m3 ( Fig. 14 View Figure 14 ; white arrows). The most distinctive feature of the m1 of R. halmaheraensis is an anterolabial cusplet which is as large as the anterolabial cuspid and often followed by a second tiny cusplet (alc2, Fig. 14b View Figure 14 ). An unusual posterolingual cusplet is also present in several specimens (pli, Fig. 14a View Figure 14 ). All specimens show strongly crenulated enamel ridging, which is unique to R. halmaheraensis and R. morotaiensis within Rattus .