BLOOD AND BRAIN TEMPERATURES OF FREE-RANGING BLACK WILDEBEEST IN THEIR NATURAL-ENVIRONMENT

Using miniature data loggers, we measured the temperatures of carotid blood and brain in four wildebeest (Connochaetes gnou) every 2 min for 3 wk and every 5 min, in two of the animals, for a further 6 wk. The animals ranged freely in their natural habitat, in which there was no shelter. They were subject to intense radiant heat (maximum similar to 1,000 W/m(2)) during the day. Arterial blood temperature showed a cir cadian rhythm with low amplitude (< 1 degrees C) and peaked in early e vening. Brain temperature was usually within 0.2 degrees C of arterial blood temperature. Above a threshold between 38.8 and 39.2 degrees C, brain temperature tended to plateau so that the animals exhibited sel ective brain cooling. However, selective brain cooling sometimes was a bsent even when blood temperature was high and present when it was low . During helicopter chases, selective brain cooling was absent, even t hough brain temperature was near 42 degrees C. We believe that selecti ve brain cooling is controlled by brain temperature but is modulated b y sympathetic nervous system status. In particular, selective brain co oling may be abolished by high sympathetic activity even at high brain temperatures.