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

Abstract

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 (Ta). We measured body temperature (Tb), resting metabolic rate (RMR) and evaporative water loss (EWL) at Ta between 25 °C and ~58 °C in birds exposed to successive increments in Ta. Normothermic Tb averaged 39.0 °C, lower than typical avian values. At Ta > 34.5 °C, Tb increased linearly to a maximum of 43.6 °C at Ta = 56 °C. The upper critical limit of thermoneutrality (Tuc) was Ta = 43.8 °C, closely coinciding with the onset of panting and gular flutter. Above the Tuc, RMR increased 2.5-fold to 2.89 W at Ta = 56 °C, a fractional increase far exceeding that of many other species under comparable conditions. Rates of EWL increased rapidly at Ta > 42.9 °C to 7.84 ± 0.90 g h-1 at Ta = 56 °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 Ta 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.