Physiological correlates of daily torpor in hummingbirds

Summary

Introduction

Hypometabolism is a widespread physiological feature among animals (Hochachka & Guppy, 1987). It can be regarded as an important strategy to overcome fluctuations of environmental parameters, such as cold ambient temperatures, lack of food or water, hypoxia, etc. Temperature, of course, is one of the most important environmental variables directly affecting the energy metabolism of animals, and some of the mechanisms that allow animals to adapt to different ambient temperatures have been extensively investigated over the years (Precht et al., 1973). Endothermy combined with a high body temperature has to a great extent enabled some animals to be fairly independent of environmental temperature fluctuations, either diurnal or seasonal, and has evolved in different Phyla. Insects among the invertebrates, and some fish, some reptiles, birds and mammals among the vertebrates are well known examples of endothermic animals (McNab, 1983). This chapter will focus on some questions involving the temperature relationships as well as the energy metabolism of some birds and mammals that are capable of undergoing a deep metabolic depression within a 24-h cycle, commonly referred to as daily torpor (as opposed to hibernation, referred to as prolonged torpor).

Daily torpor in birds and mammals may be viewed as a well-regulated hypometabolic state which may last up to several hours within a 24-h cycle. It is expressed by a significant reduction of metabolism and body temperature (T_B). There are two important requirements for endothermic animals to meet the above definition.