Energy Conservation in a Tree-Kangaroo (Dendrolagus matschiei) and the Red Panda (Ailurus fulgens)

Tree-kangaroos and red pandas, when exposed to low temperatures, greatly reduce rate of metabolism without decreasing core body temperature, apparently by reducing peripheral circulation, skin temperature, limb temperature, and heat loss to the environment. Along with body size and insulation, peripheral circulation is a major factor influencing rate of heat loss at low ambient temperatures. The reduction in rate of heat production continues down to ambient temperatures near 0 C, below which rate of metabolism increases. Such reductions in rate of metabolism are limited in duration, from several hours at moderate ambient temperatures to a few minutes at cold temperatures, at least in the tree-kangaroo. This behavior is not shown in the laboratory when the animal is confined to small chambers. The reduction in rate of metabolism at low temperatures is found principally in tropical, arboreal mammals that have a mass between 2 and 15 kg, have sedentary habits, and feed principally on leaves and/or fruit. It reflects an existence dominated by low energy expenditures in a mild climate. This behavior demonstrates that the energetics of large endotherms is not simply the energetics of small endotherms scaled to a large mass: a large mass permits patterns that cannot be anticipated by studying small species alone.