Avian thermoregulation in the heat : efficient evaporative cooling allows for extreme heat tolerance in four southern hemisphere columbids

Birds show phylogenetic variation in the relative importance of respiratory versus cutaneous evaporation, but the consequences for heat tolerance and evaporative cooling capacity remain unclear. We measured evaporative water loss (EWL), resting metabolic rate (RMR) and body temperature (T-b) in four arid-zone columbids from southern Africa [Namaqua dove (Oena capensis, similar to 37 g), laughing dove (Spilopelia senegalensis, similar to 89 g) and Cape turtle dove (Streptopelia capicola, similar to 148 g)] and Australia [crested pigeon (Ocyphaps lophotes), similar to 186 g] at air temperatures (T-a) of up to 62 degrees C. There was no clear relationship between body mass and maximum T-a tolerated during acute heat exposure. Maximum T-b at very high T-a was 43.1 +/- 1.0, 43.7 +/- 0.8, 44.7 +/- 0.3 and 44.3 +/- 0.8 degrees C in Namaqua doves, laughing doves, Cape turtle doves and crested pigeons, respectively. In all four species, RMR increased significantly at T-a above thermoneutrality, but the increases were relatively modest with RMR at T-a = 56 degrees C being 32, 60, 99 and 11% higher, respectively, than at T-a = 35 degrees C. At the highest T-a values reached, evaporative heat loss was equivalent to 466, 227, 230 and 275% of metabolic heat production. The maximum ratio of evaporative heat loss to metabolic production observed in Namaqua doves, 4.66, exceeds by a substantial margin previous values reported for birds. Our results support the notion that cutaneous evaporation provides a highly efficient mechanism of heat dissipation and an enhanced ability to tolerate extremely high T-a.