title: Avian thermoregulation in the heat: scaling of heat tolerance and evaporative cooling capacity in three southern African arid-zone passerines.
abstract: Many birds can defend body temperature (Tb) far below air temperature (Ta) during acute heat exposure, but relatively little is known about how avian heat tolerance and evaporative cooling capacity varies with body mass (Mb), phylogeny or ecological factors. We determined maximum rates of evaporative heat dissipation and thermal end points (Tb and Ta associated with thermoregulatory failure) in three southern African ploceid passerines, the scaly-feathered weaver (Sporopipes squamifrons, Mb≈10 g), sociable weaver (Philetairus socius, Mb≈25 g) and white-browed sparrow-weaver (Plocepasser mahali, Mb≈40 g). Birds were exposed to a ramped profile of progressively increasing Ta, with continuous monitoring of behaviour and Tb used to identify the onset of severe hyperthermia. The maximum Ta birds tolerated ranged from 48°C to 54°C, and was positively related to Mb. Values of Tb associated with severe heat stress were in the range of 44 to 45°C. Rates of evaporative water loss (EWL) increased rapidly when Ta exceeded Tb, and maximum evaporative heat dissipation was equivalent to 141-222% of metabolic heat production. Fractional increases in EWL between Ta<40°C and the highest Ta reached by each species were 10.8 (S. squamifrons), 18.4 (P. socius) and 16.0 (P. mahali). Resting metabolic rates increased more gradually with Ta than expected, probably reflecting the very low chamber humidity values we maintained. Our data suggest that, within a taxon, larger species can tolerate higher Ta during acute heat stress.
authors.affiliation: DST-NRF Centre of Excellence at the Percy FitzPatrick Institute, Department of Zoology and Entomology, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa.
authors.forename: Maxine C
authors.initials: MC
authors.lastname: Whitfield
authors.affiliation: Department of Zoology, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa.
authors.forename: Ben
authors.initials: B
authors.lastname: Smit
authors.affiliation: DST-NRF Centre of Excellence at the Percy FitzPatrick Institute, Department of Zoology and Entomology, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa aemckechnie@zoology.up.ac.za.
authors.forename: Andrew E
authors.initials: AE
authors.lastname: McKechnie
authors.affiliation: UNM Biology Department, University of New Mexico, MSC03-2020, Albuquerque, NM 87131-0001, USA.
authors.forename: Blair O
authors.initials: BO
authors.lastname: Wolf
mesh_terms: D000818:Animals; D001481:Basal Metabolism / Q000502:physiology; D049628:Body Size; D001831:Body Temperature / Q000502:physiology; D001833:Body Temperature Regulation / Q000502:physiology*; D006358:Hot Temperature; D006813:Humidity; D046109:Passeriformes / Q000502:physiology*; D013045:Species Specificity; D014870:Water Loss, Insensible / Q000502:physiology*
keywords: Body temperature; Critical thermal maximum; Evaporative water loss; Hyperthermia; Metabolic rate; Upper critical limit of thermoneutrality