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.
