Scaling of avian heat tolerance limits and evaporative cooling capacity

Andrew McKechnie

University of Pretoria, South Africa

Interactions between avian body temperature, evaporative water loss (EWL) and resting metabolic rate (RMR) during heat exposure remain poorly studied compared to those below thermoneutrality. Using data collected under standardised conditions of very low humidity from 56 species across three continents, we examined how heat tolerance and evaporative cooling scale with body mass and vary among taxa. Maximum air temperature at which body temperature can be defended at sublethal levels varies by >10ºC among orders, whereas maximum body temperature varies between 42.5ºC and 44.5ºC. Minimum evaporative water loss at thermoneutrality and maximum values during acute heat exposure scale with exponents of 0.79 and 0.81, respectively, and evaporative scope (maximum EWL / minimum EWL) averages 9.2 with a range of 3.1-18.4. The scaling of RMR at high air temperature is considerably more variable than that of EWL, reflecting the differing metabolic costs of the three major avian avenues of evaporative heat dissipation. Above thermoneutrality, RMR increases far more rapidly in passerines (panting) compared to columbids (cutaneous evaporation), whereas caprimulgids (gular flutter) often show no discernable increase. Maximum ratios of evaporative heat loss to metabolic heat production vary from 1.3-2.3 in passerines to >4.5 in some doves, nightjars and sandgrouse. Models of the risks of lethal dehydration and hyperthermia parameterized using our EWL and heat tolerance data reveal that, under a business-as-usual emissions scenario, several of the world’s arid regions will likely lose substantial fractions of their avian biodiversity by the end of the century.