Abstract:
Columbid birds represent a useful model taxon for examining adaptation in metabolic and
thermal traits, including the effects of insularity. To test predictions concerning the role of
insularity and low predation risk as factors selecting for the use of torpor, and the evolution of
low basal metabolic rate in island species, we examined thermoregulation under laboratory and
semi-natural conditions in a mainland species, the African Green-Pigeon (Treron calvus). Under
laboratory conditions, rest-phase body temperature (Tb) was significantly and positively
correlated with air temperature (Ta) between 0 °C and 35 °C, and the relationship between resting metabolic rate (RMR) and Ta differed from typical endothermic patterns. The minimum
RMR, which we interpret as basal metabolic rate (BMR), was 0.825 ± 0.090 W. Green-pigeons
responded to food restriction by significantly decreasing rest-phase Tb, but the reductions were
small (at most ~ 5 °C below normothermic values), with a minimum Tb of 33.1 °C recorded in a
food-deprived bird. We found no evidence of the large reductions in Tb and metabolic rate and
the lethargic state characteristic of torpor. The absence of torpor in T. calvus lends support to the
idea that species restricted to islands that are free of predators are more likely to use torpor than
mainland species that face the risk of predation during the rest-phase. We also analysed
interspecific variation in columbid BMR in a phylogenetically-informed framework, and verified
the conclusions of an earlier study that found that BMR is significantly lower in island species
compared to those that occur on mainlands.
