Abstract:
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 yellowbilled
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,RMRand 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-scalemicroclimatic factors. In addition,Q10 for
RMR varied substantially, with Q10 ≤2 in some individuals, supporting
recent arguments that active metabolic suppression may be an
underappreciated aspect of endotherm thermoregulation in the heat.
