We used implanted miniature data loggers to measure
brain and arterial blood temperatures in three
free-ranging zebras (Equus burchelli) in their natural
habitat, every 5 min for 9 days. The animals
experienced globe temperatures exceeding 40°C, and radiant heat load
of about 1000 W m-2. Arterial blood exhibited a moderate amplitude
(1.7°C) nychthemeral rhythm, with an acrophase at 19.00 h and
a nadir late in the morning, at 10.00 h. Brain temperature
consistently exceeded blood temperature, on average by 0.2-0.4°C, and
changes in brain temperature closely tracked changes in blood
temperature. There was no evidence of selective brain cooling, even
during the hyperthermia which followed surgery or that associated with
intense, short-duration exercise. The relationship between brain and
arterial blood temperatures in free-ranging zebras was unlike that
reported for horses in the laboratory. Our results do not support the
view that mammals lacking a carotid rete can achieve selective brain
cooling.