203.

Least Philippine Forest Mouse

Apomys musculus

French: Petit Apomys / German: Zwerg-Philippinenwaldmaus / Spanish: Raton de bosque minimo de Filipinas

Other common names: Least Forest Mouse, Least Philippine Apomys

Taxonomy. Apomys musculus G. S. Miller, 1910,

“Camp John Hay, Baguio, Benguet, Luzon (altitude 5000 ft [= 1524 m]),” Philippines.

Apomys musculus is a member ofsubgenus Apomys and is most closely related to A. microdon . It is locally sympatric with latter on Luzon, but often replaces it at higher elevations;it is presumably sympatric with A. gracilirostris on Mindoro and with A. insignis on Dinagat. G. G. Musser and M. D. Carleton in 2005 gave range of A. musculus as Luzon, Dinagat, and Mindoro, but commented (citing unpublished work by LL. R. Heaney as a source) that what is now referred to as A. musculus may in fact represent three species: typical musculus in northern Luzon, a separate species in southern Luzon, and a furtherspecies on Mindoro. Heaney and others in 2016 gave distribution of A. musculus as “only on Luzon, where they are widespread.” Currently regarded as monotypic but in need offurther assessment.

Distribution. Luzon, Mindoro, and DinagatIs, Philippines.

Descriptive notes. Head-body 76-124 mm, tail 97-121 mm, ear 14-17 mm, hindfoot 20-25 mm; weight 16-24 g. Species of Apomys are soft-furred, small to medium-sized murines with relatively unspecialized body form; distinguished from other Philippine murines by a distinctive suite of features, including long, narrow hindfeet with sharp, recurved claws on all digits, thinly furred tail with weakly overlapping scales and three hairs per tail scale, unreduced eyes, relatively large and thinly furred ears,vibrissae on snout very elongate and,folded back, reach to shoulder or beyond, reduction to two mammae on each side, both inguinal, and a simplified molar pattern with cusps united into transverse laminae. Members of subgenus Apomys are small, long-tailed species active primarily in canopy. The Least Philippine Forest Mouse is smallest member of both subgenus and genus. Fur on its upperparts, flanks, and outer surfaces of limbs is overall bright tawny brown,lightly flecked with black on back and with brighterorange tints on flanks and cheeks, while that on underparts and inside oflimbs is rich ocherous buff or buffy gray, often with white patches on midline of chest and abdomen; abrupt border between upperside and underside. Dark hues of foreand hindlimbs extend to ankle and wrist, giving way to buffy white on foreand hindfeet, and white hairs on digits; hindfeet elongate and narrow, with six plantar pads. Tail is equal to or slightly longer than head-body length, brown above to tip and white or mottled with pale brown below, with small but conspicuous scales and short hairs. Cranium has globular braincase and relatively short, narrow rostrum. Chromosomal complement has 2n = 42, FN = 52; autosomes include one pair of medium-sized and two pairs of small metacentric, one pair of large submetacentric, one pair oflarge subtelocentric, and 16 pairs oftelocentric chromosomes graded from large to small. Sex chromosomes not obvious, hence X and Y chromosomes presumably both telocentric.

Habitat. Typically found in evergreen tropical forests, including both montane and mossy communities. Least Philippine Forest Mice occur also in secondary forest, and sometimes in shrubby and grassy habitats. Elevational range on Luzon is 300-2370 m, but most records are from above 1460 m; on Mindoro recorded at ¢.1370 m. Record from Dinagat is from 750 m.

Food and Feeding. Reported as “omnivorous, feeding on seeds, insects and other invertebrates, and earthworms.” Breeding. In sample of females from Mount Isarog (S Luzon), estimated litter size from counts of embryos and uterine scars was 1-3 (average 2). Males in this population had small testes, measuring up to 5 mm x 11 mm.

Activity patterns. Least Philippine Forest Mice are generally nocturnal, though occasionally active during day in rainy season. They spend most time in forest canopy, but sometimes descend to ground or close toit.

Movements, Home range and Social organization. On Mount Isarog in 1993-1994, Least Philippine Forest Mice occurred at density of 8-8 ind/ha, the highest density for any small terrestrial mammal at the site. Average distance moved between captures was 42 m, maximum distances varying from 150 m to 226 m. Home-range size was estimated at 0-22 ha (range 0-01-0-56 ha).

Status and Conservation. Classified as Least Concern on The IUCN RedList.

Bibliography. Balete & Heaney (1997), Danielsen et al. (1994), Ellerman (1941), Heaney (2016d), Heaney, Balete, Dolar et al. (1998), Heaney, Balete, Gee et al. (2005), Heaney, Balete & Rickart (2016), Heaney, Tabaranza, Rickart et al. (2006), Hollister (1913), Justiniano et al. (2015), Miller (1910), Musser (1982a), Musser & Carleton (1993, 2005), Rickart et al. (1991).