Brain, abdominal and arterial blood temperatures of free-ranging eland in their natural habitat

Abstract

Using implanted miniature data loggers we measured brain, arterial blood and abdominal temperatures at 5-min intervals in two free-ranging eland (Tragelaphus oryx) in their natural habitat. The animals were subjected to a nychthemeral range of globe temperature which exceeded 40 degrees C. Arterial blood exhibited a moderate amplitude (2.3 degrees C) nychthemeral rhythm, with a temperature peak at 1600-1800 hours, and a trough in the early morning at 0600-0800 hours. Mean abdominal temperature was 0.2-0.3 degrees C lower than the corresponding blood temperature, and had a peak-to-trough amplitude of 2.6 degrees C. Brain temperature closely paralleled changes in blood temperature but usually exceeded blood temperature by about 0.5 degrees C. Sporadic episodes of selective brain cooling occurred in one animal, but the duration and magnitude of such cooling was small (less than 0.4 degrees C), and took place only well above the mode of blood temperature. Our results do not support the concept that eland routinely employ adaptive heterothermy and selective brain cooling to survive in their natural environment.

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

“…Brain temperature never exceeded 40.5°C. However, the threshold for selective brain cooling in most artiodactyls is •39°C (see Fuller et al 1999a), and in horses it was reported to be •38°C (McConaghy et al 1995), near to the mode of blood temperature of zebras in our study. So if zebras have a threshold above 40.5°C, it would be the highest, by about 1.5°C, of all mammals studied so far.…”

“…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).…”

“…Such patterns of selective brain cooling did not occur in free-ranging artiodactyls in their natural environment. 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).…”

“…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).…”