Cloacal evaporation: an important and previously undescribed mechanism for avian thermoregulation.

We present the first experimental evidence that a bird is capable of evaporating enough water from the cloaca to be important for thermoregulation. We measured rates of evaporation occurring from the mouth, the skin, and the cloaca of Inca doves Columbina inca Lesson and Eurasian quail Coturnix coturnix Linnaeus. Inca doves showed no significant increase in cutaneous evaporation in response to curtailment of buccopharyngeal evaporation. Cloacal evaporation in doves was negligible at ambient temperatures of 30 degrees , 35 degrees and 40 degrees C. However, at 42 degrees C, the apportionment of total evaporation in doves was 53.4% cutaneous, 25.4% buccopharyngeal and 21.2% cloacal, with cloacal evaporation shedding, on average, 150 mW of heat. In contrast, the evaporative apportionment in quail at 32 degrees C (the highest ambient temperature tolerated by this species) was 58.2% cutaneous, 35.4% buccopharyngeal and 6.4% cloacal. These results suggest that, for some birds, cloacal evaporation can be controlled and could serve as an important emergency tactic for thermoregulation at high ambient temperatures.