236.

Earless New Guinea Water Rat

Crossomys moncktoni

French: Crossomys de Monckton / German: Monckton-Schwimmratte / Spanish: Rata de agua de Nueva Guinea de orejas pequenas

Other common names: Earless Water Rat, Papuan Earless Water Rat

Taxonomy. Crossomys moncktoni Thomas, 1907,

“Serigina, Brown River, N.E. British New Guinea [= Papua New Guinea]. Altitude ‘not less than 4500 feet’ [= 1372 m].”

H. Rimmler in 1938 recommended inclusion of genus Crossomys within Hydromys, but this was not adopted by other workers. Closest relatives of Crossomys clearly lie among the suite of other Australo-Papuan water rats, which include members of “typical” genus Hydromys, also highly aquatic, and other genera that show insectivorous, vermivorous, and carnivorous adaptations. Molecular-sequencing data currently favor a special affinity of Crossomys to Hydromys and Parahydromys, as originally perceived by Rummler. Because Hydromys and the less specialized Parahydromys seem to represent as single lineage, however,it is likely that the pronounced aquatic adaptations (such as strongly webbed, paddle-like hindfeet) arose independently in Crossomys and Hydromys . Treated as monotypic but in need of further assessment.

Distribution. Recorded from five discrete areas of New Guinea, including the Swart Valley of the Snow (= Surdiman) Mts, SW slopes of Mt Sisa, E Central Cordillera, Cromwell Range on Huon Peninsula, and Astrolabe Range.

Descriptive notes. Head-body 175-230 mm, tail 212-273 mm, ear 1-5 mm, hindfoot 44-53 mm; weight 165 g. The Earless Water Ratis a stocky-bodied,short-limbed aquatic murine with heavily modified, flipper-like hindfeet, a thick, well-furred tail with ventral crest (presumably assisting in propulsion and/or steering), and highly reduced external ears buried within the fur. It is the most highly specialized Australo-Papuan murine, its degree of specialization for aquatic life matched only by the Earless Water Mouse ( Anotomys leander, Cricetidae, Ichthyomyini) of high Andes. Fur on upperparts is extremely dense and woolly, distinctly bi-layered; underfur very fine, 13-14 mm long, hairs white at base and dark brown or black at tips, the latter color visible through overfur, which has somewhat shaggy texture; overfur issilvery gray to olive gray, body hairs 16-18 mm, pale gray with brown or black tips, guard hairs 18-20 mm, fine and mixed black and silvery; fur on underparts is similarly dense, with woolly all-white underfur and silvery-gray overfur, often with subtle wash created by yellowish or pale orange/ pink tipping (variable within at least some populations, e.g. Mount Sisa). Texture and color of underparts extend to inner surfaces of limbs and throat but not to sides of head, which are dark; boundary between flank and belly colorationsis abrupt. Head is broad and somewhat flattened, with no obvious neck, fur on vibrissal pads dark but with underlying white woolly layer that produces some mottling; eyes very reduced; ears extremely short and entirely buried within fur, pinnae thick and rounded, ¢.3 mm long and just large enough to close external auditory meatus; vibrissae relatively numerous and suff, mostly unpigmented, 40-45 mm long and extending to rear of head; vibrissal pads and upper lips moderately inflated, contributing to “swollen” appearance of snout. Front feet are small, with pigmented skin and short pale hairs, fingers naked, palmar pads broad and almost united, claws on four of front digits, all ivory, strongly curved and sharp; hindfeet elongate and broad, upper surface almost naked and with dark skin; all digits with strong ivory claws and joined by webbing that extends to base of terminal pads, skin of plantar surface thickened, interdigital pads broadly united, hallucal pad on inside ofsole very elongate, outer margin of sole at rear fringed with stuff hairs. Tail is relatively long (100-121% of head-body length), rounded in crosssection, but exceptionally thick and muscular for most of length; tail scales small and flattened, barely visible through cover of long gray hairs except on sides neartip, where almost naked;ventral tail crest begins at base of tail as two broad streams ofstiff white or cream hairs (6-8 mm); these converge and, 50 mm behind tail base, unite to produce a midline crest that extends to tail tip. Cranium with short and narrow snout, incisors very reduced, auditory bullae proportionally small, molars reduced to two per quadrat, crowns with “basined” cusp arrangement butless sharply crested than Hydromys . Mammae two on each side, both inguinal. Karyotype 2n = 48, four largest pairs and one small pair submetacentric, all with very small short arms; all submetacentric autosomes with prominent centromeric C-bands, X-chromosome large, acrocentric, Y unknown.

Habitat. The Earless New Guinea Water Rat is highly specialized for aquatic life and has been captured only in immediate vicinity of sizeable streams and rivers. Inspection of capture sites on Mount Sisa indicates a capacity to live in cold, fast-flowing montane streams with many rapids and small waterfalls, interspersed with turbulent pools. It has been recorded also at a still waterbody on Mount Wilhelm. Occurs at elevations of 900-2700 m.

Food and Feeding. R. S. Voss reported on stomach contents of eight individuals, all of which contained exclusively insects, mostly aquatic nymphs and larvae of mayflies (Ephemeroptera), caddisflies ( Trichoptera), true flies ( Diptera), and moths and butterflies (Lepidoptera), but also some winged adults of these. P. Dwyer found stomach of an adult male from Mount Sisa “full of tadpoles and small larvae,” the latter presumably of aquatic insects. J. I. Menzies and E. Dennis referred to consumption of tadpoles, presumably based on testimony of local hunters. Saem Majnep, a Kalam hunter from the Schrader Range, Madang Province, Papua New Guinea, reported that this species (Kalam name Kmn-kuypep or tob-adk, ‘flipper foot’) feeds on tadpoles and frogspawn and on other small aquatic creatures.

Breeding. Reduced mammary formula suggests small litter size. The Kalam hunter S. Magnep reported a local belief that the Earless New Guinea Water Rat has a single young at a time; he was unaware of local knowledge regarding either location of its dens or its nesting behavior. Menzies and Dennis reported thatit lives in holes in riverbanks, presumably based on local knowledge from elsewhere within the species’ range.

Activity patterns. The Earless New Guinea Water Rat is one of the most highly aquatic of all Murinae, and Majnep reported that it is unable to walk about freely on land on account of its flipper-like hindfeet. Menzies and Dennis suggested that “when frogs are spawning in the small mountain creeks the rats move away from the rivers to hunt for tadpoles.” H. M. Van Deusen observed animals swimming and diving in streams in the Cromwell Mountains, and the fact that he was able to obtain photographs and even movie footage points to at least some diurnal activity. Dwyer also reported some activity during daylight.

Movements, Home range and Social organization. No information.

Status and Conservation. Classified as Least Concern on The IUCN Red List. Protection of stream habitats and their amphibian and invertebrate faunasis clearly critical to the survival of this remarkable species.

Bibliography. Aplin, Singadan et al. (2016b), Donnellan (1989), Ellerman (1941, 1949), Flannery (1995b), Misonne (1969), Menzies & Dennis (1979), Musser & Carleton (2005), Rowe, Achmadi et al. (2014), Rowe, Reno et al. (2008), Rummler (1938), Tate (1951), Thomas (1907c), Van Deusen (1966), Voss (1988).