Mus nitidulus

Shimada, Tomofumi, Aplin, Kenneth P., Jenkins, Paulina & Suzuki, Hitoshi, 2007, Rediscovery of Mus nitidulus Blyth (Rodentia: Muridae), an endemic murine rodent of the central basin of Myanmar, Zootaxa 1498, pp. 45-68 : 56-60

Redescription of Mus nitidulus and comparison with related species

Two of the newly collected mice were compared directly with specimens attributed to M. nitidulus by Thomas (1919) and to M. cervicolor nitidulus by Ellerman (1961). Specimens of M. nitidulus were also compared with the type series of Mus cervicolor popaeus ( Thomas, 1919) and with genotyped examples or type specimens of various other Mus species. This account of M. nitidulus is based primarily on the newly collected specimens, with additional reference as required to the historical series held in the Natural History Museum, London.

Mus nitidulus is a medium-sized mouse without any particularly striking features ( Fig. 1 ). Adults weigh around 15–18 gm and have a hindfoot length of 15–17 mm (Table 7). The tail in adults measures around 70– 75% of the combined length of head and body. The dorsal pelage is very soft, with fine, translucent and flexible spines that are not obvious to the touch, and is a pale greyish-brown colour, with pale grey bases and long brown tips to the contour hairs. The ventral fur is pale grey with cream tipping, while the upper chest, throat and insides of the fore-limbs have a richer cream wash that contrasts with the duller abdominal region. The cheek and sides of the snout are white and there is a distinct gingery eye ring. The ears are relatively large and TABLE 7. External measurements of Mus nitidulus and M. cervicolor popaeus from Myanmar. Specimens listed as M. nitidulus ? and M. c. popaeus ? are only tentatively assigned on morphological criteria. Under locality, MD = Mandalay Division, SD = Sangaing Division; SS = Shan State, YD = Yangon Division. Reproductive age (Immimmature; Ad—adult) as assessed from evidence of reproductive maturity (enlarged teats in females; enlarged scrotal sac in males).

BM(NH)14.7.19.196 Mt Popa, MD Ad 92 80 17 13 - BM(NH)14.7.19.197 Mt Popa, MD Ad 94 79 17 13 - BM(NH)14.7.19.199 Mt Popa, MD Ad 100 75.5 17 13 - BM(NH)14.7.19.200 Mt Popa, MD Ad 99 78.5 17 13 - BM(NH)18.7.1.6 Mt Popa, MD Ad 85 72.5 17 13 - CM30516 Popamyo Village, Mt Popa, MD Ad 80 60 15 11 13 CM30517 Popamyo Village, Mt Popa, MD Imm 45 40 12 10 3.5 CM30518 Popamyo Village, Mt Popa, MD Ad 67 53 15 12 10 CM30515 Lashio, SS Ad 75 54 15 11 14.5

M. cervicolor popaeus ?

18.7.1.1 Kindat, Lower Chindwin River, SD Ad 67 66 17.5 12 - quite hairy, especially on the outer surface. The mystacial vibrissae extend just past the base of the ears when folded back alongside the head. The upper rows of vibrissae are black, while the lower rows are either white or unpigmented.

The upper surface of the hindfoot in most specimens is off white with pale skin and predominantly cream hairs; however, in some specimens scattered dark scales and brown hairs impart a mottled effect. The plantar pads are relatively large and the skin between the pads is smooth but supports scattered, short hairs. The subdigital lamellae are complete except for the basal two or three which are divided centrally in most specimens. The total number of lamellae on the central digit of the pes is usually 6, less commonly 7. Lamellar hairs are lacking on both the manus and pes. The tail varies from strongly to weakly bicoloured (i.e. pigmented above and below but still darker on the upper surface) and it is moderately hairy. Scale hairs usually number three per scale and are mostly 2 – 3 scales long. In adults the basal one-third of the tail has 20–26 scale rows per centimetre. Females have ten teats including one pectoral pair, two postaxillary pairs and two inguinal pairs.

Externally, M. nitidulus is very similar in size and appearance to specimens of M. cervicolor from localities in the central region of Myanmar (including the type series of M. cervicolor popaeus ) and it is no surprise that the two were so long confused. The dorsal pelage of M. cervicolor popaeus is a richer brown with dark grey bases and shorter brown tips, and has a ‘crisper’ texture, produced by slightly better developed spines. The ventral fur also has richer and more even hues in M. cervicolor popaeus , with more obvious cream to buff tipping on the belly and no differentiation of the upper chest, throat and insides of the fore-limbs from the lower venter. The sides of the snout are darker in M. cervicolor popaeus than in M. nitidulus and the eye-ring observed in M. nitidulus is lacking, though some gingery fur is evident behind and below the eye. The upper surface of the hindfoot is unpigmented and clothed in pure white hairs in M. cervicolor popaeus , the plantar pads are smaller, the skin between the pads is granular rather than smooth and supports more numerous hairs, and the subdigital lamellae also subtend hairs along their anterior margins. The tail of M. cervicolor popaeus is moderately bicoloured with tail scales pale grey dorsally and white ventrally. The ears of M. cervicolor popaeus are slightly smaller and less hairy than those of M. nitidulus .

Mus fragilicauda from Thailand ( Auffray et al. 2003) and Laos ( Suzuki et al. 2004) also closely resembles each of M. nitidulus and M. cervicolor popaeus in external appearance. However, it differs from both in the presence of more obvious spines in both the dorsal and ventral pelage (although the fur remains soft to the touch). It resembles M. nitidulus in having paler fur on the cheek and lower part of snout, but there is no obvious eye-ring in M. fragilicauda . The upper surface of the foot is pigmented and bears scattered dark hairs in the Lao specimens, but appear to be unpigmented and clothed in pure white hairs in the one Thai specimen examined by us (MNHN-CG1999.87).

Mus booduga and M. terricolor are both much smaller mice, with adult bodyweights up to 11 g in M. terricolor and 14 g in M. booduga ( Rao 1992) . Both species have tails that usually approximate the combined length of head and body ( Ellerman 1961). The ventral fur is usually pure white in M. booduga , contrasting with the grey-based hairs of M. nitidulus and M. terricolor .

The cranium of M. nitidulus is illustrated in dorsal and ventral views in Figure 6 , along with examples of M. cervicolor popaeus , M. fragilicauda , M. terricolor and M. booduga . Selected cranial measurements are given in Table 8 for each of M. nitidulus and M. cervicolor popaeus . Cranial shape changes considerably with age in all of these mice. However, if two similar aged specimens (as judged from tooth wear) of M. nitidulus and M. cervicolor are compared, the rostrum of M. nitidulus appears longer and narrower than that of M. cervicolor , and the zygomatic arches narrower and more rounded in outline in M. nitidulus , and broader and more angular in M. cervicolor . The zygomatic plate in M. nitidulus is folded back against the rostrum, compared with a more flaring plate in Mus cervicolor , and it bears a sharp ridge instead of a rounded process for attachment of the superficial masseter tendon. The difference in zygomatic plate morphology is also reflected in the shape of the zygomatic notch—deep and relatively narrow in M. nitidulus and more broadly u-shaped in M. cervicolor . The pterygoid fossa in M. nitidulus is narrower than in M. cervicolor (in which the lateral margins of the fossa are bowed outwards) and the mesopterygoid fossa is slightly broader. The anterior face of M 1 in adult M. nitidulus is positioned behind the posterior margin of the zygomatic plate, whereas it is forward of this point in similar aged M. cervicolor . The upper incisors are slightly proodont in M. nitidulus and more strictly orthodont in M. cervicolor .

The cranium of M. fragilicauda is very similar to that of M. nitidulus in general outline and proportions. The rostrum is slightly shorter in M. fragilicauda , the pterygoid fossa is slightly broader and the mesopterygoid fossa is narrower. More significantly, the zygomatic plate is more strongly flared in M. fragilicauda and the superficial masseteric process is rounded rather than ridge-like. The configuration of the zygomatic plate is more similar in M. fragilicauda and M. cervicolor but this is probably due to sharing of primitive characteristics, as the superficial masseteric process is also knob-like in M. caroli and M. cookii and in the majority of other murines.

The cranium of M. terricolor differs considerably from that of M. nitidulus and the other, aforementioned taxa. In particular, the molar teeth, braincase and auditory bullae are all proportionally larger and the rostrum is relatively shorter. One point of special similarity to M. nitidulus is the presence in M. terricolor of a prominent masseteric ridge on the zygomatic plate. However, the body of the zygomatic plate in M. terricolor is flared laterally rather than folded against the rostrum as in M. nitidulus .

TABLE 8. Selected cranial measurements (mm) of M. nitidulus and M. cervicolor popaeus from Myanmar. Specimens listed as M. nitidulus ? and M. c. popaeus ? are only tentatively assigned on morphological criteria. Measurement abbreviations are: ONL Occipito-Nasal Length; ZW Zygomatic Breadth; IOB Interorbital Breadth; GBR Greatest Breadth of Rostrum; ZPL Zygomatic Plate Length; M1-3L Upper molar row crown length; APFL Anterior Palatal Foramen Length. Toothwear classes are: 1—No wear or slight wear on M3 alone; 2—Moderate wear on M3, slight wear on M2; 3— Moderate wear on M2, slight wear on M1; 4— Moderate wear on all molars; 5— Heavy wear on all molars.

The cranium of M. booduga is similar to that of M. terricolor but the braincase is even more inflated and the interorbital region, wider. The zygomatic plate bears a prominent masseteric ridge and is folded against the rostrum as in M. nitidulus .

The molar morphology of M. nitidulus is illustrated in Figure 7 , along with examples of M. cervicolor and M. fragilicauda . The two members of the M. booduga group are very similar but differ in the shape of cusp t1 on M1 (rounded in M. nitidulus , more elongate in M. fragilicauda ) and the relative size of upper and lower third molars (more reduced in M. fragilicauda ). The molar morphology of M. cervicolor differs more substantially and is consistent across all populations. The upper molars are narrower, due mainly to a reduction in the breadth of the central row of cusps (t2, t5, t8), and cusp t4 on M1-2 is more clearly separated from cusp t5. Cusp t1 of M 1 in M. cervicolor is elongate and thus differs from M. nitidulus but resembles the condition in M. fragilicauda . In the lower molars, M. cervicolor differs from M. nitidulus and M. fragilicauda in the shape of the posterior cingulum of M1-2 (broad and lenticular vs narrow and rounded). Wider comparisons of molar morphology in the genus Mus will be published elsewhere.

Figures

FIGURE 1. Adult male of Mus nitidulus photographed shortly after capture at Hmawbi Rice Farm, Yangon Division, Myanmar. This individual is the designated lectotype BM (NH) 2006.301.

FIGURE 6. Representative adult crania of five species of mice in dorsal (top row) and ventral (bottom row) views. Taxon and specimens details: M. n. = Mus nitidulus (CM 30496); M. c. = M. cervicolor subsp. (CM 30655; Vientiane, Laos); M. f. = M. fragilicauda (CM 30219; Lamam, Laos); M. t. = M. terricolor (CM 30656; Comilla, Bangladesh); M. b. = M. booduga (BMNH 37 a; Lectotype, S. Mahatra, India). The particularly close similarity in cranial proportions between M. nitidulus and M. fragilicauda is clearly seen, along with the general similarity to M. cervicolor.

FIGURE 7. Scanning electron micrographs of upper (top row) and lower (bottom row) molar rows of three species of mice. Taxon and specimens details: M. n. = Mus nitidulus (CM 30496); M. c. = M. cervicolor subsp. (CM 30655; Vientiane, Laos); M. f. = M. fragilicauda (CM 30219; Lamam, Laos). The close similarity in molar morphology between M. nitidulus and M. fragilicauda is clearly illustrated. Other abbreviations: M 3 = upper third molar; m 3 = lower third molar; pc = posterior cingulum; t 1 - t 6 = cusps t 1, t 3 etc. The central row of cusps on M 1 are cusps t 2, t 5 and t 8. The molar rows of each species are arbitrarily rescaled to approximately the same total lengths.