Avian thermoregulation in the heat: evaporative cooling capacity in an archetypal desert specialist, Burchell's sandgrouse (Pterocles burchelli)

Sandgrouse (Pterocliformes) are quintessential examples of avian adaptation to desert environments, but relatively little is known about the limits to their heat tolerance and evaporative cooling capacity. We predicted that evaporative cooling in Burchell's sandgrouse (Pterocles burchelli) is highly efficient and provides the basis for tolerance of very high air temperature (T-a). We measured body temperature (T-b), resting metabolic rate (RMR) and evaporative water loss (EWL) at T-a between 25 degrees C and similar to 58 degrees C in birds exposed to successive increments in T-a. Normothermic T-b averaged 39.0 degrees C, lower than typical avian values. At T-a > 34.5 degrees C, T-b increased linearly to a maximum of 43.6 degrees C at T-a = 56 degrees C. The upper critical limit of thermoneutrality (T-uc) was T-a = 43.8 degrees C, closely coinciding with the onset of panting and gular flutter. Above the T-uc, RMR increased 2.5-fold to 2.89 W at T-a = 56 degrees C, a fractional increase far exceeding that of many other species under comparable conditions. Rates of EWL increased rapidly at T-a > 42.9 degrees C to 7.84 +/- 0.90 g h(-1) at T-a = 56 degrees C, an 11-fold increase above minimal levels. Maximum evaporative cooling efficiency (ratio of evaporative heat loss to metabolic heat production) was 2.03, but could be as high as 2.70 if our assumption that the birds were metabolising lipids is incorrect. Thermoregulation at very high T-a in P. burchelli was characterised by large increases in RMR and EWL, and is much less efficient than in taxa such as columbids and caprimulgids.