Altitudinal and seasonal variation in amethyst sunbird physiology.
Lindsay, Claire Vicky.
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Southern Africa is characterised by an unpredictable environment with daily and seasonal temperature fluctuations. As a local or non-migratory endothermic species occurring over an altitudinal gradient from the Drakensberg to the coast of KwaZulu- Natal in southern Africa, Amethyst Sunbirds (Chalcomitra amethystina) experience challenging thermal conditions and increased energetic stress as a result of ambient temperature variation. Flexibility of metabolic rates within a species allows for the colonization of different habitats along an altitudinal and thus temperature gradient. It was predicted that over this altitudinal gradient Amethyst Sunbirds would exhibit variation in metabolic rates, particularly basal metabolic rates, pre- and postacclimation, as well as variation in hematocrit levels in winter and summer trials. It was also predicted that Amethyst Sunbirds would exhibit seasonal variation in metabolic parameters. Sunbirds were caught in a winter and summer season (2006-2007) using mist nets in three locations; Underberg (1553 m), Howick (1075 m) and Oribi Gorge (541 m). Upon capture, metabolic rate was measured indirectly by quantifying oxygen consumption (VO2) using flow through respirometry, at 5 and 25°C. Birds were then acclimated at 25°C for 6 weeks on a 12L:12D cycle. VO2 was measured postacclimation at 8 different temperatures (15, 5, 10, 20, 30, 28, 25 and 33°C). Hematocrit levels were taken pre-acclimation and pre-release. Winter and summer data were compared. In the winter trials it was found that there was little variation in VO2 between individuals from the same locality, whereas significant variation was observed at the same temperatures between localities and thus between altitudes. The subpopulation from the highest altitudinal site had the highest basal metabolic rate (BMR). Summer trials showed that metabolic rates did not differ significantly between altitudinal subpopulations of Amethyst Sunbirds, however, BMR was observed to decrease as altitude decreased. The comparison of seasonal data showed that Amethyst Sunbird subpopulations from Underberg and Howick showed higher post-acclimation VO2 values per temperature in winter than in summer trials. Post-acclimation resting metabolic rate (RMR) values for Howick subpopulations were generally higher in winter than in summer, Underberg Amethyst Sunbirds showed a significant difference between summer and winter RMR at 5 and 10°C and Howick sunbirds showed a significant difference in RMR between seasons at 5°C. The Oribi Gorge subpopulation, however, showed no significant differences in metabolic rate between any temperatures when comparing a summer and a winter season. Thermal neutral zones of all of the subpopulations of Amethyst Sunbirds shifted between the winter and summer trial period. This study thus emphasized the need to understand plasticity in metabolic rates and acknowledge altitudinal and seasonal differences within a species, in order to make accurate predictions about a species thermal physiology and responses to changes in ambient temperatures. In particular, the variation in BMR, which is usually used as a species specific value, should be acknowledged in comparative studies of avian metabolic rates or in climate change models.