Respiratory frequency in donkeys increases in response to environmental heat load

Panting is a thermoregulatory mechanism that involves a simultaneous increase in respiratory frequency (Rf) and decrease in tidal volume. Cattle and sheep pant in response to a heat load, but horses have an effective sweating response and are not thought to thermoregulate by panting except in cases of extreme heat prostration. Donkeys also sweat effectively and therefore should not pant. Donkeys in warm climates, however, have been seen to breathe rapidly even at rest. This study tests the hypothesis that donkeys do pant in response to high ambient temperatures. Four donkeys (body weight 236 ±50 kg) were placed either outside in direct sun on hoi humid afternoons (ambient shade temperature 33 ±I.3°C) or inside a cool (21 ±1.4°C) barn in the morning. Donkeys were acclimated to the thermal conditions for 3 h, then Rf was determined by visual counting of flank and nostril movements. Body temperature (BT) was measured by ivctal thermometer. Mean ±se Rf was higher (P < .05) in the heat 154 ±X breaths per minute, bpm) than in cooler conditions (24 ±4 hpm). Mean BT was significantly higher under hot conditions (37.7 vs. 37.3°C) but ihi.s small difference cannot increase VO₂ sufficiently to increase Rf. The interanimal variation in Rf was not correlated with body si/e. Rather. 1 hot black donkeys had higher Rf (72 bpm) than 2 hot tan donkeys (44 bpm). The occurrence of panting in these donkeys cannot he fully substantiated since tidal volume was not measured. Yet, the dramatic increases in Rf, prompted solely by environmental heat load, can have no function oilier than respiratory cooling. The donkey appears able to thermoregulate by panting in addition to sweating, unlike its close relative the horse.