Work

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

Year: 2025

Type: article

Abstract: 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 H 2 O m − 3 , 13 g H 2 O m − 3 , 19 g H 2 O m − 3 , and 25 g H 2 O 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 b max ) 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.

Source: Journal of Thermal Biology

DOI: 10.1016/j.jtherbio.2025.104324

Authors: Marc T. Freeman, Bianca Coulson, Celiwe A. Ngcamphalala, Mathome O. Makola, Andrew E. McKechnie

Institutions: South African National Biodiversity Institute, University of Pretoria, University of Cape Town

Language: English

Topic: Physiological and biochemical adaptations

Subfield: Ecology

Field: Environmental Science

Domain: Physical Sciences

Open Access status: hybrid

Estimated APC paid: $3,360

Funders: National Research Foundation, FitzPatrick Institute of African Ornithology, University of Cape Town