Data for: Adaptive variation in the upper limits of avian body temperature

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 body
temperature (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 temperatures (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 -Tbnorm gradient = 5.84 ± 0.77 °C)
compared to 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 suggests thermal generalization.