Oscillatory pattern in oxygen consumption of Hummingbirds

Abstract

Mammals and birds offer the most conspicuous example of homeothermic endothermy, a metabolic feature that implies maintenance of a constant body temperature along broad ranges of ambient temperature. The concept of homeothermic endothermy has been developed in close association with the terms thermoneutral zone and basal metabolic rate. These two metabolic parameters, however, are not easily estimated in micro-endotherms, a difficulty that might emerge from intrinsic aspects of endothermy in minute animals. To address this issue, we used empirical work derived from theoretical considerations. Our theoretical analysis is based on a model of body temperature control by shifts in metabolic rate, and assumes that micro-endotherms lose heat very quickly due to body size, and exhibit a remarkable capacity to rapidly increase metabolic output. We found that these two metabolic traits can lead to non-equilibrium metabolic rate and body temperature. We then measured metabolic rate and body temperature during euthermia in two species of hummingbirds, and analyzed data using the χ 2 periodogram statistic and a power spectral analysis. We found long-range correlation in both oxygen consumption and body temperature during euthermia, a finding that suggests non-random 1/ f oscillations. A similar pattern was not found in the rat, a much larger endotherm. Hummingbirds, then, do not appear to maintain steady-state metabolic conditions during euthermia. If, as we suggest, this pattern applies to micro-endotherms in general, the traditional concepts of thermoneutral zone and basal rate of metabolism might not apply to these animals.