Physiological variation in amethyst sunbirds (Chalcomitra amethystina) over an altitudinal gradient in winter

Flexibility of metabolic rates within a species allows for the colonization of different habitats along an altitudinal and thus temperature gradient. The distribution range of amethyst sunbirds (Chalcomitra amethystina) within southern Africa includes an altitudinal gradient from the Drakensberg to the coast of KwaZulu-Natal. We expected that over this altitudinal gradient amethyst sunbirds would exhibit variation in winter metabolic rates, particularly basal metabolic rates (BMR) and resting metabolic rate (RMR) pre- and post-acclimation. Sunbirds from three locations, Underberg (1553 m above sea level; asl), Howick (1075 m asl) and Oribi Gorge (541 m asl) were used for this study. Upon capture, metabolic rate was measured indirectly by quantifying oxygen consumption (V(O(2))) using flow-through respirometry, at 5 and 25 degrees C. Birds were then acclimated at 25 degrees C for 6 weeks on a 12 h:12 h L:D cycle. V(O(2)) was measured post-acclimation at eight different temperatures (15, 5, 10, 20, 30, 28, 25 and 33 degrees C). We found little variation in winter V(O(2)) between individuals from the same locality, whereas significant variation was observed in V(O(2)) at the same temperatures between individuals from the different localities and thus between altitudes. In particular, winter BMR decreased significantly with decreasing altitude post-acclimation. This study emphasizes the need to understand plasticity/flexibility in metabolic rates and to acknowledge altitudinal differences within a species, to make accurate predictions about the thermal physiology of a species and its responses to changes in ambient temperatures.