Adaptive variation in the upper limits of avian body temperature

Physiological performance declines precipitously at high body temperature (T-b), but little attention has been paid to adaptive variation in upper T-b limits among endotherms. We hypothesized that avian maximum tolerable T-b (T(b)max) has evolved in response to climate, with higher T-b max in species exposed to high environmental heat loads or humidity-related constraints on evaporative heat dissipation. To test this hypothesis, we compared T-b max and related variables among 53 bird species at multiple sites in South Africa with differing maximum air temperature (T-air) and humidity using a phylogenetically informed comparative framework. Birds in humid, lowland habitats had comparatively high T(b)max (mean +/- SD = 45.60 +/- 0.58 degrees C) and low normothermic T-b (T-b norm), with a significantly greater capacity for hyperthermia (T(b)max - T-b norm gradient = 5.84 +/- 0.77 degrees C) compared with birds occupying cool montane (4.97 +/- 0.99 degrees C) or hot arid (4.11 +/- 0.84 degrees C) climates. Unexpectedly, T(b)max was significantly lower among desert birds (44.65 +/- 0.60 degrees C), a surprising result in light of the functional importance of hyperthermia for water conservation. Our data reveal a macrophysiological pattern and support recent arguments that endotherms have evolved thermal generalization versus specialization analogous to the continuum among ectothermic animals. Specifically, a combination of modest hyperthermia tolerance and efficient evaporative cooling in desert birds is indicative of thermal specialization, whereas greater hyperthermia tolerance and less efficient evaporative cooling among species in humid lowland habitats suggest thermal generalization.