Rakali

## Description

The rakali (Hydromys chrysogaster), also known as the rabe or water-rat, is an Australian native rodent first described in 1804. The change to the aboriginal name Rakali was intended to foster a positive public attitude by Environment Australia.

One of four described species in the genus Hydromys, it is the only one with a range extending beyond Papua New Guinea and Indonesian West Papua. Having adapted to and colonised a unique niche of a semiaquatic and nocturnal lifestyle, this species lives in burrows on the banks of rivers, lakes and estuaries and feeds on aquatic insects, fish, crustaceans, mussels, snails, frogs, birds' eggs and water birds. Rakali have a body 231–370 millimetres (9.1–14.6 in) in length, weigh, 340–1,275 grams (0.750–2.811 lb) and have a thick tail measuring around 242–345 millimetres (9.5–13.6 in). Females are generally smaller than males but tail lengths are normally the same. They have partially webbed hind legs, waterproof fur, a flattened head, a long blunt nose, many whiskers and small ears and eyes. The body is streamlined with a skull that is large, flat and elongated, with two molars on the upper and lower jaw, similar to the water mouse. They are black to brown in colour with an orange to white belly, and dark tail with a white tip.

Hunted for their soft fur and considered a nuisance animal, numbers were under threat until a protection order in 1938. They were still under destruction permits from 1938 to 1957 due to alleged destruction of irrigation banks and destruction of fishing nets. Additionally from 1957 to 1967 a number of licensed seasons were also held for this reason.

## Appearance

The rakali is a large Australian rodent that lives in burrows on the banks of rivers and lakes and feeds on aquatic insects. Females are generally smaller than males but tail lengths are normally the same. They have partially webbed hind legs, waterproof fur, a flattened head, a long blunt nose, many whiskers, and small ears and eyes. Their body is streamlined with a skull that is large, flat, and elongated, with two molars on the upper and lower jaw. Rakali are black to brown in color with orange to white belly and dark tail with a white tip.

## Distribution

Rakali are found in Australia, New Guinea, and offshore islands. They live near permanent water in a diverse range of habitats that varies from fresh slow-moving streams, brackish inland lakes, and creeks to wetlands, rivers, estuaries, sheltered marine bays, and beaches on coastlines.

## Behavior

Rakali are active during the night. Their home range typically comprises 1-4 km of waterways and they can travel a distance of between 200 m to a kilometer in a night when foraging. They search for their food at dusk, partly by sight when in the water and hunting prey on the water's edge returning to a favorite feeding place where evidence of previous catches can be seen. They can be found swimming and feeding in the early morning and evening. Rakali do not tolerate low temperatures well, so in winter they prefer to hunt terrestrial prey. They also spend long periods in the warmth of the burrow. These well-adapted semi-aquatic and territorial creatures may become very aggressive in high-density populations as they are mostly solitary. Fighting is common, leading to scarred tails from being bitten during fighting amongst both adult males and females.

## Diet

Rakali are carnivores. They feed on a variety of aquatic animals including fish, crustaceans, shellfish, small birds, eggs, mammals, frogs, reptiles. In winter, when resources are limited, they will also take plants.

## Reproduction

Rakali breed from late winter to early spring. Under good seasonal conditions, these solitary animals come together with their mate and may produce several litters (if young are removed will reproduce again within two months), but separate on the birth of the young. Rakali usually nest in burrows which they construct in river banks but they may also build their nests within sunken logs and reeds, in areas surrounded by roots and dense riparian vegetation for cover from predators. Females typically produce litters of 4 to 5 young after the gestation period that lasts about 34 days. The young are born blind but they grow quickly and are usually independent after about 4 weeks. Reproductive maturity develops at around 12 months of age and females are able to breed until they are 3.5 years old.

## Population

### Population threats

Hunted for their soft fur and considered a nuisance animal, rakali were under threat until a protection order in 1938. They were still under destruction permits from 1938 to 1957 due to the alleged destruction of irrigation banks and the destruction of fishing nets. Although nationwide today the rakali populations appear stable there are individual populations facing a significant threat. Near threatened in Western Australia's wheat belt, urbanization, salinization, waterway degradation, and pollution have all attributed to this status change. In the Victorian Lake Wellington, Lake Victoria, and Lake Tyers region numbers have declined in the past fifty years attributed to commercial eel-fishing, predation, loss of habitat due to commercial development, and poisoning from baiting programs placing this species at risk.

### Population number

According to IUCN, the rakali is locally common throughout its range but no overall population estimate is available. Currently, this species is classified as Least Concern (LC) on the IUCN Red List.

## Fun Facts for Kids

The rakali was first described in 1804.

The Gunwinggu (Kunwinjku) people of western Arnhem Land (Australia) call this animal Yirrku.

Rakali are not well adapted to hot and cold extremes as they have poor insulation qualities. They control body temperature by remaining in the burrow in the heat of the day in summer and by foraging during the day in winter.

Rakali have the unusual ability to kill cane toads without being poisoned.

Although having adapted to a unique niche of a semiaquatic lifestyle rakali have the capacity to run at twice their maximum swimming velocity. Their metabolic rate increases during running at a rate of 13-40% greater than when swimming.