Thermoregulatory Adaptations to Starvation in Birds

Abstract

Birds have body temperatures that are typically higher than those of mammals, and thus spend a large proportion of their total energy budget maintaining these temperatures—particularly in cold environments. Birds also have high surface to volume ratios and comparatively small energy reserves causing additional energetic challenges during periods of food limitation or complete starvation. During starvation, energy can be saved if the need for active thermogenesis can be reduced. Such a hypometabolic state can be achieved by reducing body temperature in a regulated manner or by increasing thermal insulation, or by employing both of these mechanisms. Adaptive changes in heat loss (thermal conductance) is well known among birds, but a growing number of studies are documenting how birds are able to conserve limited energy by reducing body temperature in a regulated manner. Rest-phase hypothermia and shallow torpor involve decreases in body temperature ranging from 1 to 10°C, with the birds retaining responsiveness to the environment, whereas deep torpor is characterized by a larger decrease, with body temperatures often approaching ambient temperature and resulting in true torpidity. Starvation is well known to induce deep torpor in some avian groups, notably hummingbirds and swifts; however, recent studies show that basically all avian groups can save energy during starvation by entering shallow torpor during the rest-phase of their daily cycle. So far, such responses have been found in at least 29 avian families. This chapter reviews our current understanding of how birds alter their thermoregulatory patterns in the face of starvation and underscores the need to: (1) investigate the neurohumoral responses underlying hypothermia and (2) better quantify the energy savings ensuing from small decreases in body temperature.