Changes in pattern of heat loss at high ambient temperature caused by water deprivation in a large flightless bird, the emu.

When exposed to high ambient temperatures, birds defend body temperature by increasing evaporative water loss, via either respiratory or cutaneous water loss. Water deprivation can lead to changes in thermal responses and lower levels of water use for thermoregulation. We have studied the effect of 2-3 wk of water deprivation on the physiological responses of emus during exposure to an ambient temperature of 45 degrees C. Water deprivation led to a delay in the onset of panting (54 vs. 24 min after start of exposure) and to higher body temperatures (38.7 degrees vs. 38.3 degrees C) at the end of exposure to 45 degrees C. After panting was initiated and body temperature stabilised, the water-deprived emus had a lower total evaporative water loss (77 vs. 101 g/h), the same respiratory water loss (70 vs. 72 g/h), and a lower cutaneous water loss (7 vs. 29 g/h) than they did when hydrated. The factor contributing most to the lower total evaporative water loss in the dehydrated emus was a 47% reduction in dry thermal conductance, which led to a decrease in the exogenous environmental heat load and therefore the level of evaporation needed to defend body temperature. We suggest that the decrease in dry thermal conductance follows from the lower level of cutaneous water loss.