Absence of selective brain cooling in free-ranging zebras in their naturalhabitat

We used implanted miniature data loggers to measure brain and arterial bloo d temperatures in three free-ranging zebras (Equus burchelli) in their natu ral habitat, every 5 min for 9 days. The animals experienced globe temperat ures exceeding 40 degrees C, and radiant heat load of about 1000 W m(-2). A rterial blood exhibited a moderate amplitude (1.7 degrees C) nychthemeral r hythm, with an acrophase at similar to 19.00 h and a nadir late in the morn ing, at similar to 10.00 h. Brain temperature consistently exceeded blood t emperature, on average by 0.2-0.4 degrees C, and changes in brain temperatu re closely tracked changes in blood temperature. There was no evidence of s elective 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 u nlike that reported for horses in the laboratory. Our results do not suppor t the view that mammals lacking a carotid rete can achieve selective brain cooling.