Aspects of nasal heat exchange in resting reindeer.

Total respiratory heat loss (t.r.h.l.) in resting reindeer has been calculated from exhaled air temperature (Te) and respiratory minute volume (V) obtained during heat and cold stress in both summer and winter. Exhaled air temperature decreased with decreasing ambient temperature (Ta) in both summer and winter. The lowest Te recorded was +6 degrees C at a Ta of -45 degrees C in winter and the highest +38 degrees C at a Ta of + 29 degrees C in summer. At any Ta, Te was higher in winter than in summer. V was constant with increasing Ta up to +25 degrees C and -10 degrees C in summer and winter, respectively. Further increase in Ta resulted in thermal tachypnea. During summer t.r.h.l. did not change significantly in the ambient temperature range from -25 to +25 degrees C. In winter, however, t.r.h.l. increased linearly with increasing ambient temperature from -30 to +15 degrees C. Minimum t.r.h.l. was found to coincide with the animals' lower critical temperature. We suggest that the heat exchange in the upper respiratory tract of resting reindeer is actively regulated to attain fine tuning of thermal balance in this high arctic mammal undergoing great seasonal changes in fur insulation.