717.

Australian Dusky Rat

Rattus colletti

French: Rat de Collett / German: Australische Dunkelratte / Spanish: Rata oscura de Australia

Other common names: Collett’s Rat, Dusky Rat, Mulbu, Territory Dusky Rat

Taxonomy. Mus colletti Thomas, 1904, “S[outh]. Alligator River, Northern Territory, Australia.”

Rattus colletti is in the sordidus speceis group and has been placed as a subspecies of R. sor- didus. It 1s sister to R. villosissimus, with R. sor- didus being sisterto this clade. Monotypic.

Distribution. N Northern Territory and Melville I, Australia.

Descriptive notes. Head—body 65-211 mm, tail 78-150 mm, ear 17-21 mm, hindfoot 28-33 mm; weight 22-213 g. The Australian Dusky Ratis large, with long muzzle. Pelage is long and harsh. Dorsum is dark grizzled grayish brown to grizzled blackish brown, fading to yellowish brown on sides and throat. Venteris grayish to yellowish gray and sharply demarcated from dorsum. Feet are grayish brown dorsally, with black sole. Ears are short, rounded, and dark brownish gray; vibrissae are comparatively short. Tail is only c.70% of head-body length, sparsely covered in black hairs, and dark gray to black. Skull similar to that of the Canefield Rat ( R. sordidus) but smaller. Cestodes, trematodes, nematodes,ticks, mites, lice, and fleas have been recorded from the Australian Dusky Rat, including Echinoparyphium, Nippostrongylus, Hymenolepis, Raillietina, and Mastophorus. There are six pairs of mammae: three axillary and three inguinal. Diploid numberis 2n = 42.

Habitat. Flat coastal and subcoastal monsoonal flood plains in dense grassland on swampy treeless plains with long periods of floodingand rainfall. During wet season (November—March), heavy rainfall and run-off floods plains for 3-6 months or longer at lower elevations. During dry season (April-May), water retreats, and plains dry out.

Food and Feeding. The Australian Dusky Rat is primarily herbivorous. During wet season,it primarily eats stem bases of grasses and corms of sedges. When sheltered on higher ground during this time, it forages on the forest floor of woodland areas. During dry season, it mainly eats sedge corms in cracks and depressions where they seek shelter.

Breeding. Reproduction occurs during transition from wet to dry seasonin April, although it can be earlier or later depending on amount of flooding that occurred during wet season. Nests are made in humid cracks, and litters are born in them. Estrous cycle lasts c.4=6 days. Births occur mainly in June, although they can extend into July depending on amount of flooding. Females can reproduce year-round, but they are usually restricted to transition between wet and dry season. Litters usually have nine or more young that generally emerge from nests at ¢.3 weeks of age, usually in June-July.

Activity patterns. The Australian Dusky Rat is primarily nocturnal. Foraging occurs at night, and sheltering occurs during the day in burrows, trees, roots, or logs in wet season and in cracks in soil during dry season. In winter, the species is more active at night, and foraging occurs often, but during dry season, foraging at night diminishes, and individuals stay incracks and get all their nutrients from sedge corms. After wet season begins, individuals become more active at night and forage aboveground more often. The Australian Dusky Rat is a specialized terrestrial species, adapted to living in a highly variable habitat, although it does regularly climb to find shelter during wet season. During wet season flooding, it retreats to higher ground on levee banks or in clumps of grasses and fallen sedges in shallower parts of plains.

Movements, Home range and Social organization. The Australian Dusky Rat is found at high densities in these areas during wetseasons because they are restricted to smaller areas. They dig burrows in areas with soft soil that is not waterlogged, or they take refuge on nearby higher ground in trees, roots, and logs. During dry season, they leave highlands and spread throughout grass and sedge areas. During transition between wet and dry season, breeding occurs, and nests are made in humid cracks. When dry season commences and cracks widen as aridity increases, breeding stops, and individuals find refuge in dry, shallow depressions on plains. During dry season, growth slows, densities of ectoparasites increase, nocturnal activity aboveground is reduced, and populations decrease due to deaths of older individuals. Population decreases during dry seasons can be fairly small or rather extensive depending on how long aridity lasts.If it lasts long, many young individuals die. During transition from dry to wet seasons, Australian Dusky Rats become more active at night aboveground and move back to higher ground and shallower parts of plains as ground swells filling cracks and as vegetation regrows. The Australian Dusky Rat goes through extreme fluctuations in density depending on length and severity of wet and dry seasons. These fluctuations are non-cyclic and depend on conditions during each season. If there is mild flooding during wetseason, more breeding occurs, increasing densities to extremely high levels. This can lead to much more activity aboveground during the day, which causes high mortality from predation in late dry season or the following season. If flooding in wet season is severe or extreme, there will be few individuals to recolonize areas, and breeding will be less common. If long or extreme wet season is followed by long dry season or drought, Australian Dusky Rats can become extremely rare.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Australian Dusky Rat is very common throughoutits fairly large distribution, although populationsfluctuate greatly over time. It is found in several protected areas, including Kakadu National Park. It is a minor threat to agriculture, specifically rice. Although it seems currently unthreatened, saltwater intrusion caused by introduced Water Buffalo (Bubalus bubalis) might alter vegetation. Habitat alteration caused by various exotic and introduced plant species and various invasive species, including the cane toad (Rhinella marina), might have substantial effects on the Australian Dusky Rat. A major natural predator, the water python (Liasisfuscus), tends to increase and decrease parallel with population changes of the Australian Dusky Rat.

Bibliography. Aplin, Chesser & ten Have (2003), Baverstock, Gelder & Jahnke (1983), Breed (1978), Friend et al. (1988), Oakwood & Woinarski (2016), Madsen, Ujvari, Shine, Buttemer & Olsson (2006), Madsen, Ujvari, Shine & Olsson (2006), Menkhorst & Knight (2010), Mulder & Smales (2010), Musser & Carleton (2005), Redhead (1979), Rowe et al. (2011), Van Dyck& Strahan (2008), Williams (1987).