Hydromys chrysogaster É. Geoffroy 1804
Creators: name: Don E. Wilson
name: Russell A. Mittermeier
name: Thomas E. Lacher, Jr
Publisher: name: Lynx Edicions
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Rodentia
Muridae
Hydromys
Hydromys chrysogaster

description: 237.
Common Water Rat
Hydromys chrysogaster
French: Hydromys d’Australie / German: Goldbauch-Schwimmratte / Spanish: Rata de agua de Australasia
Other common names: Australian Water Rat, Beaver Rat, Golden-bellied Water Rat, Rakali, Sooty Beaver Rat, Water Rat, White-bellied Beaver Rat
Taxonomy. Hydromys chrysogaster E. Geoffroy Saint-Hilaire, 1804,
“I’'une des iles du canal d’Entrécasteaux” (= one of the islands in D’Entrecasteaux passage).
J. A. Mahoney and B. J. Richardson in 1988 restricted type locality to Bruny Island, Tasmania, Australia. This is type species of Hydromys. The large-bodied water rats of Australia and Melanesia have at various times been split into many species, or lumped together as one. T. Iredale and E. L. G. Troughton in 1934 listed five species in Australia alone, while Troughton in 1935 recognized ten species acrossAustralia and New Guinea. G. H. H. Tate in 1951 divided the populations into two series based on molar size, but concluded “I do not think that the two groups exhibit characters offull specific rank. Instead,it seems that there may be regional groups... that are incompletely isolated from one another.” Tate otherwise focused on variation in fur color of underparts (variably grayish white or yellowish in most populations, but plain gray in specimens from south-west Western Australia), while noting that variation in upperparts was explained by factors of age, sex, and molt condition. Troughton’s descriptions noted additional features, including impressive contrasts in hindfoot proportions and cranial morphology between geographically proximate populations, especially in northern Australia. Genetic studies to date have contributed little to our understanding ofthis group. P. R. Baverstock and coworkers in 1977 found no chromosomal variation among individuals sampled from six areas within Australia: southwest and north-west of Western Australia, north-east and south-east of Queensland, and inland and southern South Australia; and a comparable karyotype was described from Tasmania by J. A. Dartnall in 1970. C. H. S. Watts, no doubt influenced by this finding, characterized H. chrysogasteras “recently arrived” in Australia. Contrary to this interpretation, a recent and as yet unpublished study of variation in mitochondrial gene sequences by K. A. Bettink found significant divergence between some regional populations within Australia, and suggestive evidence for two distinct species in New Guinea. H. chrysogaster probably comprises more than one species, but far fewer than the number of available synonyms. Full revision is needed to untangle the taxonomic structure of the group. Currently regarded as monotypic, but in need of further assessment.
Distribution. New Guinea, mainland Australia (mainly peripheral but extending farthest inland in mesic E & SE, and along Cooper Creek to terminate in ephemeral lakes of Lake Eyre Basin), and Tasmania; also present on many islands and archipelagos, including Obi, Waigeo, Biak, Yapen, Kai, Aru, Kiriwina, Goodenough, Fergusson, Normanby, Bernier, Dorre, Barrow, Bathurst, Melville, Wessel, Inglis, Groote Eylandt, Pellew, Mornington, Dunk, Palm, Whitsunday, Dent, Fraser, North Stradbroke, South Stradbroke, Flinders, and Bruny.
Descriptive notes. Head—body 195-390 mm, tail 200-325 mm, ear 13-20 mm, hindfoot 46-80 mm; weight 210-1280 g. Data for one population in New South Wales found males to average slightly longer (head-body 310 mm vs. 290 mm) and slightly heavier (755 vs. 606 g) than females, but with no difference in average tail length. Populations in southern and eastern Australia are uniformly large-bodied and those from montane localities in New Guinea uniformly much smaller. There is more interregional variation in body size and proportions across northern Australian and in lowland Melanesia. The Common Water Ratis a heavily built, short-limbed semi-amphibious murine with dense, woolly pelt, heavily modified, flipperlike hindfeet, thick and well-furred tail, and small eyes and ears. Its degree of specialization for aquatic life makesit one of the most highly specialized Australo-Papuan murines, and one of the few species well known among members of the Australian public. Fur on upperparts is extremely dense and soft, typically grayish brown or brown overall, usually flecked with black, and often darker along middle of back than on flanks. Pelage has two distinct components—an outerlayer with body hairs measuring 12-14 mm in length, overtipped byslightly longer guard hairs, and a woolly inner layer measuring 8-9 mm; inner hairs are usually gray or brownish gray, outer hairs are yellowish at base and with brown tips, and guard hairs are uniform black. Fur on underparts is similarly dense, but both components shorter and paler, woolly inner layer to 5-6 mm and external layer to 9-10 mm; color of underparts more variable than that of upperparts, ranging from dusky gray to dull grayish white and ocherous yellow; 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 colorations is relatively abrupt. Woolly underfur tends to be more visible on limbs and sides of head, especially on vibrissal pads. Head is broad and somewhat flattened. Eyes fairly small and positioned near top of flattened head. Ears are somewhat thickened and with rounded margin, always projecting well beyond fur; skin on ears variably pigmented or pale, and usually hairy both inside and outside. Vibrissal pads and upperlips moderately inflated, contributing to swollen appearance of snout. Vibrissae are relatively numerous and suff, mostly unpigmented, and extending to ears or to rear of head. Front feet are small, with pigmented skin, and short hairs that are dark on wrist and usually paler on fingers; palmar pads broad and almost united; small nail on first digit and short but strong claws on remaining four digits. Hindfeet are elongate and very broad, upper surface with dark skin and hairs, all digits with short, strong claws; skin of plantar surface thickened and plantar pads have roughened, granular texture; interdigital pads distinct but closely adpressed; hallucal pad on inside of sole is elongate;all digits are united by folded skin to level of terminal pads, with distinct webbing present between second, third, and fourth digits. Tail is relatively long, but usually slightly shorter than head-body length, rounded in cross-section and muscular for most of length; scales flattened, typically in 13-14 rows/cm, and barely visible through dense cover of dark hairs that each subtend 6-7 scale rows and form short terminal tuft; hairs of basal portion of tail are dark brown to black, often darker than those on body; all but a few individuals have cream or white tail tip that accounts for between 5% and 50% oftotal tail length. Cranium hasflattened braincase, broad interorbital region, rostrum moderately broad and with heavy incisors, auditory bullae moderately large; molars quite large but reduced to two perquadrat, crowns with “basined” cusp arrangement. Mammae two on each side, both inguinal. Karyotype with 2n = 28, including 21 pairs of telocentric autosomes in a graded series, and two small metacentric pairs; identical autosomal complements are recorded in species of Pseudomys, Mesembriomys, and Leggadina, and in some Melomys species, and this is interpreted as an ancestral arrangementfor Australian Hydromyini. X chromosome is a large acrocentric and Y chromosome is a tiny telocentric. Spermatozoa with moderately long tail (108 pm), and relatively short (7 pm) sperm head that is broad basally, with narrow apical hook and two accessory ventral hooks. Glans penis has bulbousdistal portion, large dorsal papilla, outer crater rim that is weakly papillate, low inner crater, and robust baculum with broad, weakly notched base.
Habitat. In Australia the Common Water Rat is generally restricted to vicinity of rivers, streams and freshwater wetlands, extending locally into mangroves when these are present. In several places where arid habitats directly abut coast, populations of this murine exist entirely along marine foreshore, including the Coorong of south-east South Australia, and Barrow, Bernier and Dorre Islands of Western Australia. In arid interior of central Queensland and north-east South Australia, this species inhabits scattered, permanent waterholes but, during sporadic flood events, it undergoes dramatic but short-lived population and range expansions. In New Guinea and its satellite islands it occurs in almost every habitat, including deltaic mangroves and freshwater swamps, and evergreen tropical lowland to montane rainforest. In lower-elevation rainforest it is most commonly, though not exclusively, associated with rivers and streams. Elevational range is sea level to 1900 m in New Guinea and sea level to ¢.1700 m in southeast highlands of Australia.
Food and Feeding. Common Water Rats are largely carnivorous and will consume almost any kind of aquatic or terrestrial animal. Main prey in aquatic realm are freshwater mollusks, crustaceans, fish, and adults and larvae of invertebrates. From the land they consume both invertebrates and vertebrates, including frogs, lizards, and birds, and will feed on carrion as well as living prey. Plant foods such as the roots of sedges (Cyperaceae) are sometimes consumed, but in captivity these are ignored if animal food is also offered. In southern Australia, diet varies seasonally, with more aquatic prey consumed during winter months than at other times of year. Aquatic food items are usually consumed on land, and commonly at a preferred feeding station (where discarded, inedible parts can build up into a sizeable “midden”). For most adults, feeding behavioris solitary, but a female and its incompletely weaned young will forage and consume food together, presumably providing an opportunity for social learning. G. J. Barrow described a female and young piling up a heap of freshwater bivalves and waiting for them to open in the sun. D. Fleay described a more active method of chewing the margins of the valves to open them. One consequence of the Common Water Rat’s reliance on aquatic preyis its unusually diverse suite of trematode parasites; these typically have their first intermediate hosts in mollusks and crustaceans, from which the water rats acquire their infection. In other murines, nematodes rather than trematodes typically dominate the recorded helminth communities.
Breeding. Nearly all information on breeding comes from populations in inland New South Wales, or from captive stock derived from same general area. Breeding in the wild and in captivity can occur at any time of year, but there is a strong prevalence for births in spring and summer, with 88% of captive births during September—March. Females are polyestrous, with average cycle of ten days (range 7-17 days). Most females produced one or two litters per season, but sometimes as manyas five if conditions very favorable. Gestation period averaged 34-9 days (range 33-41 days) for firstlitter of a season, and slightly longer (36-6 days) for subsequentlitters. Averagelitter size was 1-4 (average 3-3). Postpartum estrus is common, and, since weaning usually begins around four weeks of age, a proportion of females are pregnant during lactation period. Unless a second litter is produced, full weaning and independence are often delayed for a further four weeks, after which the young typically disperse. Appearance of sexual maturity is attained at c.70 days of age in males (scrotal testes) and c.95 days in females (perforated vagina),
but onset of breeding is usually delayed through ensuing winter months, so that most females are eight months or older when they producefirst litter. Growth continues after this time, with full adult size usually attained in second year oflife. Although males and females can live for up to six years in captivity, both sexes cease gamete production at c.3-5 years of age. When living at high densities, some dominant females appear to mate and breed preferentially, while others may remain in an anestrous state for long periods or even entire life. In the wild, males and females dig burrows in riverbanks, or shelter inside hollow fallen trees, or build leaf nests among tree roots. Latter options are used exclusively in mangrove or swampforest, habitats where burrows would become entirely flooded. Burrows excavated in streambanks usually have two or more entrances at water level and contain one or more expanded nesting chambers 20 cm or more in height.
Activity patterns. A study by F. E. Fish and R. V. Baudinette of locomotion found that Common Water Rats are moderately competent both on land and in water. Maximum running speed was twice that of swimming, but at same speed swimming was energetically moreefficient. During swimming, the thick pelage produces positive buoyancy, and propulsion is achieved by alternate paddling movements of hindfeet, with forefeet used only for minor directional adjustments, and tail used only as a rudder. Although more at home on ground and in water, the Common Water Rat is reported also to climb trees in search of food. In southern Australia, it isessentially crepuscular but it forages also during day, especially in cooler months. In northern Australia and New Guinea, populations of this murine appear to be morestrictly nocturnal. A study of Common Water Rat thermal physiology by F. D. Fanning and T. J. Dawson found these rats to have low tolerance of either high or low environmental temperatures: at air temperatures above 30°C they were unable to maintain their stable core temperature (36-1°C) and would rapidly enter hyperthermia, despite use of strategies such as saliva-spreading; at temperatures below 25°C, they would rapidly enter hypothermia. Several factors appear to contribute to this poor thermoregulatory capacity. One is the surprisingly poor insulating properties of the Common Water Rat’s pelage, despite its thick woolly underfur. Another is the absence of any major anatomical specializations, such as vascular retia, to achieve effective heat exchange. In absence of any specialized features, the species resorts to behavioral strategies to avoid major excursions of body temperature — resting below ground or inside logs during periods of very high temperature, minimizing time spent in very cold water by seasonal dietary shifts from aquatic to terrestrial prey, and using daytime basking to return core body temperature to acceptable levels after immersion. These strategies, however, clearly have their limits, especially at high elevations where air and water temperatures are lower overall, and prey variety and abundances are probably also lower. Under generally warmer conditions experienced in northern Australia and New Guinea, avoidance of extreme heat is probably more important than recovery from cold conditions, and it ispossible for these populations to be essentially nocturnal, thereby reducing risks of predation by large diurnal raptors and large poikilothermic reptiles.
Movements, Home range and Social organization. Some of highest recorded “natural” densities of Common Water Rats were recorded in agricultural landscapes containing numerous irrigation canals, thereby providing abundant food and den sites. In such contexts, many individuals are scarred from agonistic behaviors, with damage to tails (scars and/or missing terminal segments) and bites to rump both common among individuals of both sexes. In captive colonies, the water rats develop dominance hierarchies in both sexes, and these are defended by agonistic behaviors that include tailand rump-biting. Such high densities are rarely encountered in more natural contexts, and incidence of scarring is usually quite low. Although there are few published examples of quantitative studies, highest densities and smallest home ranges are reported from south-east Queensland, where W. F. Harris in 1978 estimated average home range size of 1-2 ha. At Badger Creek in south-central Victoria, J. L.. Gardner and M. Serena found average home range size of 11-5 ha for animals living along streams running through wet sclerophyll forest, and recorded individual nightly movements by dominant males of up to 4 km. A more limited study by P. C. Speldewinde and coworkersin coastal heath and swamp habitats in south-west Western Australia gave estimated home ranges for three males of 6-2 ha, 8-4 ha, and 42-1 ha, the small values for individuals living beside a swamp and large value for an individual living along a shallow creek. In both Queensland and Victoria studies, home ranges of males were larger than those of females, and male territories usually overlapped several female territories.
Status and Conservation. Classified as Least Concern on The IUCN Red List. The Common Water Rat is considered to be secure in all states of Australia and in Melanesia. Despite this fact, in many parts ofits range,it is not numerous and populations are probably discontinuous. In much of the wheat-growing and grazing country of southern Australia, this species has declined or disappeared regionally owing to modification of wetland habitats through clearing of surrounding vegetation or increasing of surface-water salinity with loss of invertebrate food items. On the other hand, largescale irrigation projects have clearly favored this species in some rural areas, as has management of stream and lake levels in many urban streams and ponds. The conservation status of all local populations will need to be reassessed once the taxonomy of this group has been adequately resolved.
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description: Published as part of Don E. Wilson, Russell A. Mittermeier & Thomas E. Lacher, Jr, 2017, Muridae, pp. 536-884 in Handbook of the Mammals of the World – Volume 7 Rodents II, Barcelona :Lynx Edicions on pages 684-685, DOI: 10.5281/zenodo.6887260