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Forbes’s Tree Mouse

Chiruromys forbesi

French: Grand Chiruromys / German: Forbes-Breitkopf-Baummaus / Spanish: Raton arboricola de Forbes

Other common names: Forbes’s Chiruromys, Greater Tree Mouse

Taxonomy. Chiruromys forbest Thomas, 1888,

“Sogere, S. E. New Guinea; altitude 1750 feet [= 533 m]” (= Sogeri Plateau, Astrolabe Range, Central Province, Papua New Guinea) .

Although forbesi was proposed in the new genus Chiruromys, since 1896 it and related forms had been accommodated within Pogonomys on account of general external similarities and conformity of molar patterns. G. H. H.Tate, H. Rummler, and E. Laurie and |]. E. Hill, however, all recognized subgenera within Pogonomys, albeit with somewhat variable composition. E. Dennis and J. I. Menzies documented morphological and chromosomal features that identifiedtwo phyletic clusters within thebroadly defined Pogonomys and which supported reinstatement of Chiruromys for forbesi and related forms ( lamia, kagi, and vates). This action was subsequently supported by immunological and molecular-sequencing results showing deep divergence of the two genera, and no clear evidence of a special relationship between them. Riimmler considered all of the variation to be due to effects of preservation and fading. Tate and subsequent authors interpreted it as diversification among insular populations. Most recently, G. G.Musser and M. D. Carleton reported on unpublished morphometric studies that showed the Goodenough Island population to average larger in cranial dimensions than the other two, with the three insular samples as a group, averaging larger than those from the southeastern Papuan Peninsula in both cranial and dental dimensions. A notable feature of the variation is the shorter hindfoot of all Southeast Papuan specimens (30-32-7 mm in adults) compared with those from the islands (over 35 mm in adults), despite overall similar body sizes. There is no difference in basic chromosomal arrangement between the Fergusson and Goodenough populations. C. forbes: is probably sympatric with C. vates in the Astrolabe Range. A population of Chiruromys on Sudest Island, in the more remote Louisiade Islands, was referred to C. forbesi by Menzies and Dennis. This record was dismissed by T. F. Flannery in 1995 but substantiated in 2005 by Musser and Carleton, who mentioned American Museum of Natural History (New York) specimens of Chiruromys from Sudest Island, representing a distinctive but as yet unnamed species. Currently regarded as monotypic but in need of further assessment.

Distribution. New Guinea, including the Papuan Peninsula (Owen Stanley, Astrolabe, and Maneau ranges) and an outlying record at Markham River Valley, Morobe Province, and D’Entrecasteaux Is (Goodenough, Fergusson, and Normanby).

Descriptive notes. For populations on south-eastern Papuan Peninsula: head-body 137-150 mm, tail 211-235 mm, ear 12-17 mm, hindfoot 30-37 mm. No specific data are available for body weight. For populations in the D’Entrecasteaux Islands: head-body 134-161 mm, tail 210-222 mm, ear 17-22-2 mm, hindfoot 30-32-7 mm; weight 100-122 g. Species of Chiruromys are small to medium-sized arboreal mice with short, broad head,soft dense fur; eyes relatively large, more forward-facing than typical for Murinae; long, coarsely scaled tail has terminal, upward-facing prehensile pad; and molars with complex crown patterns, retaining many primitive murine features. Forbes’s Tree Mouse is the largest member of genus. Fur on upperparts is soft but somewhat shaggy, body hairs measuring 12-15 mm,always dark brown at bases but, in different populations, variably tipped with pale gray-brown, cinnamon or brighter reddish hues, guard hairs pale and inconspicuous, projecting 2-3 mm; fur on flanks and outer surfaces of limbs paler; fur of underparts distinctly lighter and contrasting with flanks, in different populations pale buff, creamy white, or white to hair bases, sometimes with reddish wash; coloration of underparts extends onto throat, chin, and cheek. Head strikingly patterned in most populations, usually with dark patch on each vibrissal pad and dark mask around each eye, sometimes a pale spot between eye and ear, or behind ear; vibrissae dark and very long, extending back to shoulders; ears relatively short and rounded, pale but with dark outer rim, thinly furred; foreand hindfeet unpigmented, hairs white or pale buff, claws short, pale, and with strongly curved tips, plantar pads broad; forefeet with claws on four digits, flat nail on inner digit; hindfeet relatively elongate in insular populations, shorter in peninsular populations, claws present on all digits. Tail very elongate (140-160% of head-body length), uniformly brown,scales relatively large (6-9 rows per cm), hexagonal to diamond-shaped, strongly overlapping, some populations with distinct longitudinal keels; each scale with three hairs equal in length to two scales, overall appearance slightly hairy; tail tip with prehensile pad 20-30 mm long on upper surface. Cranium has very short rostrum,strongly flaring zygomatic arches, molarsrelatively unreduced (compared with other Chiruromys). Mammae three on each side, one axillary and two inguinal. Karyotype is 2n = 44, with two pairs of large submetacentric autosomes and the remainder telocentric, and a large submetacentric X chromosome; Y chromosome unknown.

Habitat. Most records are from primary evergreen tropical rainforest habitat; T. Flannery reported one exception—a group “taken from a hole in a coconut palm in a plantation near disturbed forest.” On the Southeast Papuan Peninsula, Forbes’s Tree Mouse is restricted to hill forest mainly to ¢.700 m, and appears to be replaced above c.1000 m by its smaller congener, the Broad-headed Tree Mouse ( C. lamia). On D’Entrecasteaux Islands, where it is the only recorded member of its genus, it extends to higher elevations, at least to 1300 m in lower montane rainforest on Goodenough Island (which peaks at 2531 m but remainsunsampled at high elevations). On Southeast Papuan Peninsula,it may be sympatric with the Lesser Tree Mouse ( C. vates) in the Astrolabe Range.

Food and Feeding. Dental morphology suggests a general dietary focus on fruits and leaves.

Breeding. Forbes’s Tree Mouse occupies nests in tree hollows, and information on breeding comesfrom field observations on groups and dissection of females taken from these contexts. In lower montane forest on Fergusson Island, Flannery observed the nest to be “high up the trunk ofstraight, tall trees, around 30-60 cm in diameter... the entrance consisted of a small natural hollow that had been enlarged (on the inside only) by chewing. A tunnel led downwardsinside the trunk to a nesting chamber 30-40 cm from the entrance. The small nesting chamber was filled with rotting leaves, which was very wet. According to local informants who regularly encounter this animal, the nesting chamberis always wet.” Dennis and Menzies reported two observations on breeding activity in December—a pregnant female with three embryos, and a group of four adults and a single newborn young. The young are said to be quite small and totally hairless and thus less precocial than in many other groups of Murinae. Flannery’s observations on the sex and age composition of six groups suggested the possibility that they often comprise an adult female and male together with young from one or two consecutive litters, each litter with one or two surviving young. If this inference is correct, the species displays an unusually high level of parental care (and tolerance) of the young, with young remaining in the parentalterritory and nest almost until sexual maturity.

Activity patterns. Although this species can be locally common,it is rarely trapped on the ground, and is more commonly taken from nests situated in natural or modified tree hollows. These can be 10 m above ground, and it is possible that activity is more or less confined to the canopy.

Movements, Home range and Social organization. Like other species of Chiruromys, this species nests communally. Menzies and Dennis reported two groups, one with two males and two females (one pregnant), and the other with four adults and one newborn young. Flannery reported on six groups observed on Fergusson Island in August 1992: group size was 2-5, always including asingle adult female, usually a single adult male, and up to four immature individuals, often of contrastingsizes and probably derived from consecutive litters. Flannery also commented on the docile nature of newly captured individuals.

Status and Conservation. Classified as Least Concern on The IUCN Red List. This species was being heavily exploited on Fergusson Island as a wild food resource at the time of Flannery’s visit in August 1992, but he remarked that “As yet, this does not seem to have affected the species adversely.” Perhaps a greater concern is that its elevational range on the Southeast Papuan Peninsula corresponds with peak forestry activity and the growth of plantations, especially of oil palm. Similar developments are also affecting the lower-elevation habitats of Forbes’s Tree Mouse on each of the larger islands of the D’Entrecasteaux group.

Bibliography. Dennis & Menzies (1979), Ellerman (1941), Flannery (1995a, 1995b),Laurie (1952), Laurie & Hill (1954), Menzies & Dennis (1979), Musser & Carleton (1993, 2005), Rowe et al. (2008), Rimmler (1938), Tate (1951), Tate & Archbold (1935b), Thomas (1888b, 1895d), Watts & Baverstock (1994a).