Efficient Evaporative Cooling and Pronounced Heat Tolerance in an Eagle-Owl, a Thick-Knee and a Sandgrouse

Abstract

<p>Avian evaporative cooling and the maintenance of body temperature (<i>T_b</i>) below lethal limits during heat exposure has received more attention in small species compared to larger-bodied taxa. Here, we examined thermoregulation at air temperatures (<i>T_air</i>) approaching and exceeding normothermic <i>T_b</i> in three larger birds that use gular flutter, thought to provide the basis for pronounced evaporative cooling capacity and heat tolerance. We quantified <i>T_b</i>, evaporative water loss (EWL) and resting metabolic rate (RMR) in the ∼170-g Namaqua sandgrouse (<i>Pterocles namaqua</i>), ∼430-g spotted thick-knee (<i>Burhinus capensis</i>) and ∼670-g spotted eagle-owl <i>(Bubo africanus)</i>), using flow-through respirometry and a stepped <i>T_air</i> profile with very low chamber humidities. All three species tolerated <i>T_air</i> of 56–60◦C before the onset of severe hyperthermia, with maximum <i>T_b</i> of 43.2◦C, 44.3◦C, and 44.2◦C in sandgrouse, thick-knees and eagleowls, respectively. Evaporative scope (i.e., maximum EWL/minimum thermoneutral EWL) was 7.4 in sandgrouse, 12.9 in thick-knees and 7.8 in eagle-owls. The relationship between RMR and <i>T_air</i> varied substantially among species: whereas thick-knees and eagle-owls showed clear upper critical limits of thermoneutrality above which RMR increased rapidly and linearly, sandgrouse did not. Maximum evaporative heat loss/metabolic heat production ranged from 2.8 (eagle-owls) to 5.5 (sandgrouse), the latter the highest avian value yet reported. Our data reveal some larger species with gular flutter possess pronounced evaporative cooling capacity and heat tolerance and, when taken together with published data, show thermoregulatory performance varies widely among species larger than 250 g. Our data for Namaqua sandgrouse reveal unexpectedly pronounced variation in the metabolic costs of evaporative cooling within the genus Pterocles.</p>