How hornbills handle heat: sex-specific thermoregulation in the southern yellow-billed hornbill

At a global scale, thermal physiology is correlated with climatic
variables such as temperature and aridity. There is also evidence that
thermoregulatory traits vary with fine-scale microclimate, but this has
received less attention in endotherms. Here we test the hypothesis that
avian thermoregulation varies with microclimate and behavioural
constraints in a non-passerine bird. Male and female southern
yellow-billed hornbills (Tockus leucomelas) experience markedly different
microclimates while breeding, with the female sealing herself into a tree
cavity and moulting all her flight feathers during the breeding attempt,
becoming entirely reliant on the male for provisioning. We examined
interactions between resting metabolic rate (RMR), evaporative water loss
(EWL) and core body temperature (Tb) at air temperatures (Ta) between 30°C
and 52°C in male and female hornbills, and quantified evaporative cooling
efficiencies and heat tolerance limits. At thermoneutral Ta, neither RMR,
EWL nor Tb differed between sexes. At Ta > 40°C, however, RMR and
EWL of females were significantly lower than those of males, by ~13% and
~17%, respectively, despite similar relationships between Tb and Ta,
maximum ratio of evaporative heat loss to metabolic heat production and
heat tolerance limits (~50°C). These sex-specific differences in hornbill
thermoregulation support the hypothesis that avian thermal physiology can
vary within species in response to fine-scale microclimatic factors. In
addition, Q10 for RMR varied substantially, with Q10 ≤ 2 in some
individuals supporting recent arguments that active metabolic suppression
may be an under-appreciated aspect of endotherm thermoregulation in the
heat.