How hornbills handle heat: sex-specific thermoregulation in the southern yellow-billed hornbill [RESEARCH ARTICLE]
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.