Thermoregulation in African Green Pigeons (Treron calvus) and a re-analysis of insular effects on basal metabolic rate and heterothermy in columbid birds
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 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 which found that BMR is significantly lower in island species compared to those that occur on mainlands.