The Body. H. chrysogaster is a large animal in Rat terms. Males grow larger than females, with continued growth throughout life.

The average physique is:

Head and body length 290 mm.

The Rakali's streamlined body has a thick water-repellent coat ranging in colour, from almost black to slate grey or golden brown on the back, and white to orange on the belly. Although the Rakali have retained the dexterity in their front feet, their hind feet, perfectly adapted for aquatic hunting, render them relatively clumsy on land. They build their burrows in hollow logs, in stream and river banks, and other available places such as man made structures, where they sleep, mate, and rear their young.

Aquatic and semi-aquatic mammals with a large body surface area to volume ratio have difficulty maintaining constant body temperatures in cold habitats. Rakali, unlike the Platypus, lack highly insulative fur and efficient heat conserving vascular networks. The Rakali is able to maintain a constant body temperature in cold air temperatures, but cannot withstand long periods of time in cold water, especially during periods of activity, as they have adopted limiting physiological mechanisms necessary for true thermoregulation under such circumstances.

Rakali have adopted vasoconstriction, a narrowing of the peripheral blood vessels to lower the skin temperature when in the water. This extends the insulative shell of the animal, acting a bit like blubber does on a Whale, to minimise heat loss. A protective layer of air, trapped in the coat next to the skin, also aids in extending the time the Rakali can stay in the water. However it still has difficulty maintaining core temperature over extended periods of exposure.

In addition to vasoconstriction, the Rakali has adopted behavioural thermoregulation. Thermal stress can be minimised by escaping the heat of the day in a burrow or retreating to the water. Evading the cold is done by entering the burrow, by postural responses in the terrestrial environment, and by adaptive foraging. Initial measurements on rakali fur indicate poor insulative qualities, adding to the threat of hypothermia.

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