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 m23 (0%–100% relative humidity at 307C) at
air temperatures between 447 and 567C. 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 487C, and these birds showed a high degree
of hyperthermia. At lower temperatures (407–447C), 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.
