Blood and brain temperatures of free-ranging black wildebeest in their natural environment

Abstract

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 approximately 1,000 W/m2) during the day. Arterial blood temperature showed a circadian rhythm with low amplitude (< 1 degree C) and peaked in early evening. 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 selective brain cooling. However, selective brain cooling sometimes was absent even when blood temperature was high and present when it was low. During helicopter chases, selective brain cooling was absent, even though brain temperature was near 42 degrees C. We believe that selective brain cooling is controlled by brain temperature but is modulated by sympathetic nervous system status. In particular, selective brain cooling may be abolished by high sympathetic activity even at high brain temperatures.

Absence of Selective Brain Cooling in Free‐Ranging Zebras in Their Natural Habitat

“…The zebras exhibited a biphasic nychthemeral rhythm of blood temperature, with a mean amplitude of •1.7°C, around a mode of •38°C. This oscillation in blood temperature was greater than that measured in free-ranging black wildebeest (Jessen et al 1994) and springbok (Mitchell et al 1997), but similar to the variation in rectal temperature previously reported in tame donkeys exposed to natural environmental variables (SchmidtNielsen et al 1957;Yousef & Dill, 1969). In donkeys, rectal temperature closely tracked the prevailing globe temperature, reaching a minimum between 24.00 and 04.00 h, and a maximum at 12.00 h (Yousef & Dill, 1969).…”

“…Our measurements on zebra add a fourth species, and the first perissodactyl, to our studies of ungulates, which previously included black wildebeest (Jessen et al 1994), springbok (Mitchell et al 1997) and eland (Fuller et al 1999a); these four studies remain the only continuous measurements of brain and arterial blood temperatures in free-ranging animals in their natural habitat. Our new study was the first to measure brain and blood temperatures in a free-ranging mammal that does not have a carotid rete, the anatomical structure usually associated with selective brain cooling.…”

“…Similarly, the minimum blood temperature was not correlated with air temperature in the early hours of the morning. In their anomalous late-morning nadir of body temperature, zebras differ from free-ranging artiodactyls, in which blood temperature was lowest in the early morning (near 06.00 h), at a time when solar radiation and air temperature were low (Jessen et al 1994;Mitchell et al 1997;Fuller et al 1999a). Sudden peripheral vasodilation initiated by direct exposure to solar radiation is thought to cause a fall in the body temperature of kangaroos after dawn (Brown & Dawson, 1977).…”

“…In black wildebeest (Jessen et al 1994), springbok (Mitchell et al 1997) and eland (Fuller et al 1999a), selective brain cooling occurred only sporadically, and its magnitude rarely exceeded 0.5°C. Moreover, selective brain cooling in wildebeest and springbok, rather than being enhanced during exercise, was abolished during intense physical activity, even though brain temperature was then at its highest, and exceeded 40°C (Jessen et al 1994;Mitchell et al 1997). To date, brain and blood temperatures in free-ranging animals have been measured only in species that possess a carotid rete, the anatomical substrate for efficient cooling of arterial blood destined for the brain.…”

“…Our recent studies on the brain and blood temperature of freeranging ungulates (Jessen et al 1994;Mitchell et al 1997;Fuller et al 1999a) have shown that the thermoregulatory responses of animals in their natural environment cannot be predicted from measurements made on tame, captive or restrained animals. In a laboratory setting, heat-stressed artiodactyls routinely maintain brain temperature below arterial blood temperature by •1°C at rest, with the cooling increasing to as much as 2.7°C during exercise (for review see Mitchell et al 1987).…”