Temperature Regulation in the Platypus, Ornithorhynchus anatinus: Production and Loss of Metabolic Heat in Air and Water

Abstract

The platypus, Ornithorhynchus anatinus had a minimum resting metabolism in air of 2.2 W·kg^−0.75 at a of 25 C. In absolute terms, this metabolism was 8% less than that of marsupials in general, and 35% lower than that of eutherian mammals. The increase in metabolism in response to cold in air was 1.8 times the resting level at a of 5 C. However, the immersion in cold water raised this to 3.2 times the resting metabolism (7.0 W·kg^−0.75 at a water temperature of 5 C). The resting metabolism of the platypus was approximately 32% higher than that of its terrestrial monotreme relative, the echidna (Tachyglossus aculeatus). Evaporative heat loss (EHL) by the platypus was low, being 0.62 to 1.36 W·kg⁻¹ over a temperature range from 5 to 30 C. The maximum measured water loss at 30 C (probably due to sweating), although lower than the level considered to constitute thermal sweating in large mammals, possibly plays a significant part in cooling the platypus during moderate heat stress and exercise. At 30 C, EHL represented 60% of the total heat production at that temperature. The conductance of the tissues and fur of the platypus was relatively constant at temperatures below 15 C in air. The mean values of 1.349-1.198 W·m⁻²· C⁻¹ were lower than those found for most terrestrial mammals. The unfurred extremities of O. anatinus, including the tail, constituted approximately 31.9% of the total surface area of the body. The largest proportion was made up of the webbed forefeet, which are the main organs of propulsion in water. The fur of the platypus was found to have an overall insulation of 0.33 W⁻¹·m²·C in air. However, in water the insulative qualities of platypus fur fell to 30%-40% of the value in air. Below an air temperature of 15 C less than 50% of the total insulation of the body of the platypus was made up by the integument, and tissue insulation became more important. The high tissue insulation in O. anatinus may be facilitated by the presence of anatomical specializations in the cardiovascular system of the extremities which allow countercurrent exchange of heat between arteries and veins. The occupancy of a burrow and the curled sleeping posture adopted by the platypus are probably very efficient in helping to conserve body heat.