Brain temperatures in running quail.

Hypothalamic (Tb) and colonic (Tc) temperatures of adult bobwhite (colinus virginianus) were measured simultaneously while the birds were resting and running on a motor-driven treadmill at fixed belt speeds ranging from 0.42 to 0.75 m.s-1 at two air temperatures, room and 30 degrees C. For comparison, Tb and Tc were also recorded simultaneously in resting bobwhite exposed to air temperatures of 30 to 45 degrees C. Hypothalamic temperatures of both environmentally heat-stressed and running bobwhite were consistently below colonic temperatures. The mean magnitude of this body-to-brain temperature difference in birds running at the highest speeds (i.e., with the greatest exercise heat loads) and at a high air temperature (30 degrees C) was similar to that of extremely heat-stressed resting birds [ambient temperature (Ta) = 45 degrees C]. Furthermore, the body-to-brain temperature difference in quail running at 30 degrees C increases with running speed. That these birds running at maximum speeds can maintain their Tb nearly constant while Tc increases suggests enhanced heat transfer in the rete mirabile ophthalmicum, an extracranial site of arteriovenous heat exchange.