Torpor as an emergency solution in 'Galago moholi': heterothermy is triggered by different constraints

Abstract

The expression of heterothermy in the African lesser bushbaby, 'Galago moholi', seems to be strikingly different to most other heterotherms: 'G. moholi' uses its ability to enter torpor only rarely and torpor is only used by a small fraction of the population. The aim of this study was, therefore, to summarize the parameters of torpor use in 'G. moholi' to conclude the general patterns and discuss them in comparison to other heterotherms to elucidate possible causes and constraints that underlie these differences in deployment of heterothermy. Our study was carried out on wild animals using temperature loggers and open-flow respirometry for measurements of body temperature and metabolic rate, respectively. 'G. moholi' uses torpor only as a last resort and not as a routine, seasonal behavior. Nevertheless, we found that the general physiological patterns of torpor, e.g., torpor bout duration or entry and arousal times from torpor, were mainly consistent with those described for other nocturnal daily heterotherms. The greatest difference found was the unusual low rewarming rates during arousal from torpor, probably due to already depleted internal energy stores and thus inability to mobilize sufficient energy for endogenous heating. We therefore conclude that while general physiological parameters of heterothermy seem to have remained conserved in heterotherms, the underlying causes which elicit this physiological response, and thus the extent of expression and timing of heterothermy, have evolved very differently in different groups, depending on body mass and the specific habitat and lifestyle of the species.