Metabolism and Body Temperature during Circadian Sleep and Torpor in the Fed and Fasting Pigeon

Abstract

Shallow torpor is utilized by many small-to-medium sized birds and mammals to extend their limited energy reserves by minimizing energy expenditure when food is scarce and/or ambient temperatures are low. In some of these animals circadian torpor also occurs as an endogenous rhythm under constant conditions. In rodents, the duration of circadian torpor increases proportionally to decreases in body weight as energy reserves decline (Tucker, 1966; Brown and Bartholomew, 1969; Wolff and Bateman, 1978) while depth of torpor remains relatively constant (Walker et al., 1979; Harris et al., 1984). However, in birds the depth of circadian torpor is inversely proportional to their body weight while the duration of torpor is generally constant and restricted to the dark portion of the light-dark cycle. A characteristic pattern of progressively larger nocturnal decreases in body temperature (Tb) and metabolic rate (MR) and a daily return to euthermic levels accompanies body weight loss in the tiny willow tit (Reinertsen and Haftorn, 1984), white-crowned sparrow (Ketterson and King, 1977) and larger kestrel (Shapiro and Weathers, 1981). Declining energy reserves in these diurnally active birds are thus followed by increasing nocturnal energy conservation during the inactive portion of the circadian rhythm, which in ringed turtle doves was shown to be characterized predominantly by slow wave sleep (SWS; Walker et al., 1983).