Tropical birds live longer, have smaller clutches and invest more resources into self-maintenance than temperate species. These "slow" life-histories in tropical birds are accompanied by low basal metabolic rate (BMR). It has recently been suggested that the low BMR of tropical species may be related not to their slow "pace of life" or high ambient temperatures (Ta ) in tropical latitudes, but to the stability of environmental conditions in tropics. Since the repeatability of metabolic traits is higher in stable environments, such as laboratory conditions, we predicted that long-term repeatability of BMR in a tropical climate should be higher than in a temperate one. Contrary to our predictions, the repeatability of mass-independent BMR in 64 individuals of free-living tropical birds from Vietnam was low and insignificant after the species affiliation was taken into account. It indicates that BMR cannot be used as an individual long-term characteristic of tropical birds. On the other hand, tropical birds showed consistent differences in their mass-independent BMR at the interspecific level. Using BMR measurements from 1543 individuals of 134 species, we also found that different characteristics of Ta within the week preceding BMR measurements had a significant impact on the mass-independent BMR of tropical birds. The most significant effect was the difference between the absolute maximum and minimum Ta within a single week. Our results indicate that the physiology of tropical birds is more subject to changes than would be expected based on the notion of the stability of climatic conditions in the tropics.
Keywords: BMR; ambient temperature; basal metabolic rate; energy metabolism; individual variation; repeatability; tropical birds.
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