Abstract:
Physiological performance declines precipitously at high body temperature (Tb), but little attention has been paid to adaptive variation in upper Tb limits among endotherms.
We hypothesized that avian maximum tolerable Tb (Tbmax) has evolved in response to
climate, with higher Tbmax in species exposed to high environmental heat loads or
humidity-related constraints on evaporative heat dissipation. To test this hypothesis, we
compared Tbmax and related variables among 53 bird species at multiple sites in South
Africa with differing maximum air temperature (Tair) and humidity using a phylogenetically informed comparative framework. Birds in humid, lowland habitats had comparatively high Tbmax (mean ± SD = 45.60 ± 0.58 °C) and low normothermic Tb
(Tbnorm), with a significantly greater capacity for hyperthermia (Tbmax 2 Tbnorm
gradient = 5.84 ± 0.77 °C) compared with birds occupying cool montane (4.97 ±
0.99 °C) or hot arid (4.11 ± 0.84 °C) climates. Unexpectedly, Tbmax was significantly
lower among desert birds (44.65 ± 0.60 °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.
