Torpor in songbirds and the evolution of avian heterothermy

One of the most dramatic ways in which small mammals and birds offset the high energetic cost of endothermy is through facultative, regulated reductions in metabolic rate and body temperature below normothermic levels. Daily torpor involves pronounced reductions in body temperature - sometimes by as much as 35 °C below normothermic levels – within a single circadian cycle and occurs widely in small mammals. Among birds, however, the phylogenetic distribution of daily torpor is far more restricted and appears to largely be confined to a handful of predominantly basal clades, most notably the hummingbirds, swifts, nightjars and mousebirds. Although torpor has been reported in several passerines, the most recently derived avian order, closer inspection of these data raises doubts that this is in fact true torpor, as relationships between air temperature, body temperature and metabolic rate resemble more closely shallow rest-phase hypothermia, a near-ubiquitous avian response to mismatches between energy supply and demand.

This study will investigate patterns of body temperature and metabolic rate in two passerine taxa in which “torpor” has been reported, the sunbirds and swallows, and use these data to test the hypothesis that the responses documented in these taxa do not in fact represent true torpor, but rather a pronounced form of shallow rest-phase hypothermia. Target species include the White-breasted, Amethyst and Dusky Sunbird, as well as the Southern African Cliff and Barn Swallow. Collectively, these data will provide a powerful test of the hypothesized plesiomorphic origins of torpor in birds.

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