The role of hyperthermia in avian tolerance of hot and humid conditions

Pronounced hyperthermia is thought to be important for thermoregulation during hot weather in animals inhabiting humid environments, where high atmospheric humidity constrains evaporative cooling. To assess the role of facultative, reversible hyperthermia in avian thermoregulation under hot, humid conditions, we examined how humidity influences thermoregulatory performance at air temperatures (T_air) exceeding normothermic body temperature (T_b) in red-billed queleas (Quelea quelea), a species capable of extreme hyperthermia (T_b > 48 °C). Using flow-through respirometry, we exposed queleas to T_air = 34 °C-50 °C at four experimental absolute humidities (6 g H₂O m^{- 3}, 13 g H₂O m^{- 3}, 19 g H₂O m^{- 3}, and 25 g H₂O m^-3). Across these humidity treatments, resting metabolic rate, evaporative water loss and maximum evaporative cooling efficiency responded similarly to increasing T_air, and maximum tolerable body temperature (T_bmax) remained consistent (48.60 ± 0.40 °C). Heat tolerance limits (HTL; environmental temperatures associated with thermoregulatory failure) averaged T_air = 48.41 ± 1.03 °C among humidity treatments. At T_air > ∼44.5 °C, evaporative cooling efficiency plateaued and queleas became poikilothermic, with T_b tracking T_air ≤ ∼48.4 °C. These findings demonstrate extreme hyperthermia tolerance can offset the constraints on evaporative cooling imposed by high humidity, allowing some birds to tolerate extreme combinations of heat and humidity. We propose a modification to an existing conceptual framework of the thermoregulatory roles of facultative hyperthermia to accommodate the effects of humidity on evaporative cooling and heat tolerance limits.