The Impact of Humidity on Evaporative Cooling in Small Desert Birds Exposed to High Air Temperatures

• Published in: Physiological and biochemical zoology 2014-11, Vol.87 (6), p.782-795
• Main Authors: Gerson, Alexander R., Smith, Eric Krabbe, Smit, Ben, McKechnie, Andrew E., Wolf, Blair O.
• Format: Article
• Language: English
• Subjects: Animals; Birds; Body temperature; Body Temperature Regulation - physiology; Columbidae - physiology; Doves; Evaporation; Heat; Heat dissipation; High temperature; Hot Temperature; Humidity; Passeriformes - physiology; South Africa; Water; Water loss; Water Loss, Insensible - physiology; Water temperature
• ISSN: 1522-2152; 1537-5293
• DOI: 10.1086/678956

Environmental temperatures that exceed body temperature (T b) 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, andT bwere 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 maintainT bat a given temperature, but panting appears less sensitive to humidity over the air temperature range investigated here.