title: The impact of humidity on evaporative cooling in small desert birds exposed to high air temperatures.
abstract: Environmental temperatures that exceed body temperature (Tb) force endothermic animals to rely solely on evaporative cooling to dissipate heat. However, evaporative heat dissipation can be drastically reduced by environmental humidity, imposing a thermoregulatory challenge. The goal of this study was to investigate the effects of humidity on the thermoregulation of desert birds and to compare the sensitivity of cutaneous and respiratory evaporation to reduced vapor density gradients. Rates of evaporative water loss, metabolic rate, and Tb were measured in birds exposed to humidities ranging from ∼2 to 30 g H2O m(-3) (0%-100% relative humidity at 30°C) at air temperatures between 44° and 56°C. In sociable weavers, a species that dissipates heat primarily through panting, rates of evaporative water loss were inhibited by as much as 36% by high humidity at 48°C, and these birds showed a high degree of hyperthermia. At lower temperatures (40°-44°C), evaporative water loss was largely unaffected by humidity in this species. In Namaqua doves, which primarily use cutaneous evaporation, increasing humidity reduced rates of evaporative water loss, but overall rates of water loss were lower than those observed in sociable weavers. Our data suggest that cutaneous evaporation is more efficient than panting, requiring less water to maintain Tb at a given temperature, but panting appears less sensitive to humidity over the air temperature range investigated here.
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131; 2Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa; 3DST/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: Alexander R
authors.initials: AR
authors.lastname: Gerson
authors.forename: Eric Krabbe
authors.initials: EK
authors.lastname: Smith
authors.forename: Ben
authors.initials: B
authors.lastname: Smit
authors.forename: Andrew E
authors.initials: AE
authors.lastname: McKechnie
authors.forename: Blair O
authors.initials: BO
authors.lastname: Wolf
mesh_terms: D000818:Animals; D001833:Body Temperature Regulation / Q000502:physiology*; D010856:Columbidae / Q000502:physiology*; D006358:Hot Temperature; D006813:Humidity; D046109:Passeriformes / Q000502:physiology*; D013019:South Africa; D014867:Water; D014870:Water Loss, Insensible / Q000502:physiology*
chemical_list: D014867:Water