Functional role of metabolic suppression in avian thermoregulation in the heat

Hypometabolism arising from active metabolic suppression occurs in several contexts among endotherms, particularly during heterothermic states such as torpor. However, observed Q₁₀ ≈ 1 for avian resting metabolic rate within the thermoneutral zone, values far below the Q₁₀ = 2-3 expected on the basis of Arrhenius effects, suggests hypometabolism also plays a role in birds' thermoregulation at environmental temperatures approaching or exceeding normothermic body temperature (T_b). We evaluated the occurrence of hypometabolism during heat exposure among birds by re-analysing literature data to quantify changes in T_b and resting metabolic rate (RMR) near the upper boundary of the thermoneutral zone, at air temperatures (T_air) between the inflection above which T_b increases above normothermic levels (T_b.inf) and the upper critical limit of thermoneutrality (T_uc). Among the ∼55 % of species in which T_uc - T_b.inf > 0, Q₁₀ < 2-3 occurred in nine of 10 orders for which suitable data exist, indicating that hypometabolism during heat exposure is widespread across the avian phylogeny. Values of Q₁₀ < 2-3 were not restricted to small body mass, as previously proposed. Our findings support the idea that metabolic suppression reduces avian metabolic heat production and hence evaporative cooling requirements during heat exposure, with reductions of 20-30 % in RMR in some species. Moreover, these findings add to evidence that hypometabolism is an important component of heat tolerance among endotherms such as birds and tropical arboreal mammals.