Abstract:
Avian evaporative cooling and the maintenance of body temperature (Tb) 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 (Tair)
approaching and exceeding normothermic Tb in three larger birds that use gular flutter,
thought to provide the basis for pronounced evaporative cooling capacity and heat
tolerance. We quantified Tb, evaporative water loss (EWL) and resting metabolic rate
(RMR) in the ∼170-g Namaqua sandgrouse (Pterocles namaqua), ∼430-g spotted
thick-knee (Burhinus capensis) and ∼670-g spotted eagle-owl (Bubo africanus), using
flow-through respirometry and a stepped Tair profile with very low chamber humidities.
All three species tolerated Tair of 56–60◦C before the onset of severe hyperthermia, with
maximum Tb 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 Tair 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.
