Thermoregulation in ratites: a review

Abstract

Laboratory and free-ranging studies on the emu, ostrich and kiwi show ratites to be competent homeotherms. While body temperature and basal metabolic rate are lower in ratites than other birds, all of the thermoregulatory adaptations present in other birds are well established in ratites. The thermoneutral zone has been established for the emu and kiwi, and extends to 10°C. Below that zone, homeothermy is achieved via the efficient use of insulation and elevated metabolic heat production. In the heat, emus and ostriches increase respiratory evaporative water loss and use some cutaneous water loss. Respiratory alkalosis is avoided by reducing tidal volume. In severe heat, tidal volume increases, but the emu becomes hypoxic and hypocapnic, probably by altering blood flow to the parabronchi, resulting in ventilation/perfusion inhomogeneities. Ostriches are capable of uncoupling brain temperature from arterial blood temperature, a phenomenon termed selective brain cooling. This mechanism may modulate evaporative effector responses by manipulating hypothalamic temperature, as in mammals. The implications of thermal physiology for ratite production systems include elevated metabolic costs for homeothermy at low ambient temperature. However, the emu and ostrich are well adapted to high environmental temperatures.