Thermal and Energetic Problems of Semiaquatic Mammals: A Study of the Australian Water Rat, including Comparisons with the Platypus

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Abstract
The Australian water rat, Hydromys chrysogaster, faces thermoregulatory problems due to the variation in its thermal environment. The influence of temperature on the rodent's heat budget in air and in water at three levels of activity was examined. The water rat maintained at low air temperatures, but hyperthermia occurred when approached . During thermoneutral conditions was 36.6 ± 0.6 C (mean ± SD) and standard metabolism was 3.28 ± 0.19 W kg⁻¹, similar to predicted values. In water, was not maintained at water temperatures below 25 C at any level of activity, suggesting thermoregulatory abilities inferior to the other Australian semiaquatic mammal, the platypus. The water rat possesses fur insulation comparable with that of the platypus, but the platypus has extensive vascular retia which the water rat lacks. In cold water the water rat may accept a degree of hypothermia, with the rate of heat loss being retarded by the maintenance of an air layer in the fur and by controlled regional heterothermia. An estimate of the cost of swimming gave values for water rats similar to those for ducks.