Regulation of Body Temperature in the Rock Hyrax, Heterohyrax brucei

Abstract

The effects of ambient temperature on the body temperature, oxygen consumption, heart rate, breathing rate, evaporative water loss, and behavior of Heterohyrax brucei from the vicinity of Nairobi, Kenya, were studied. The mean rectal temperature of animals at rest in the absence of heat stress was 36.4° C, and telemetered abdominal temperatures of a normally active individual in an outdoor inclosure ranged from slightly below 35 to almost 37°. No periods of profound nocturnal hypothermia were observed. However, body temperature was labile and markedly affected by ambient temperatures, especially above 30°. Following exposures of 60 to 90 minutes to temperatures from 0 to 42.5°, the upper and lower limits of observed body temperatures were separated by 7 to 8°. Exposure to direct sunlight produced marked hyperthermia, increased rate and amplitude of breathing (mean resting rate, 37 per minute), sweating on the soles of the feet, and a marked decrease in tonus of skeletal muscles. The lower critical temperature was near 25°, but because of an anomalous decline in oxygen consumption at temperatures above 35°, no upper critical temperature was apparent. The standard metabolic rate was 0.52 milliliter O2 (gram hour)−1, which is about 20 per cent below the level predicted on the basis of body weight. Mean minimal total thermal conductance was 0.184 calories (gram hour ¼)−1. Above 25°, evaporative water loss increased directly with Ta. At an ambient temperature of 42.5° and a water vapor pressure of 12 millimeters Hg, the hyraxes evaporated enough water to dissipate all their metabolic heat production. About 22 per cent of the water was evaporated from the feet, which sweated profusely during heat stress. The mean minimum heart rate of 118 per minute occurred between ambient temperatures of 24 and 35°. This rate is about 52 per cent of that predicted on the basis of size. Heart rate increased at both higher and lower temperatures. Despite their labile body temperatures, low metabolic rate, and high thermal conductance rock hyraxes maintain body temperatures in the usual mammalian range with a minimum expenditure of energy. Thermoregulation is facilitated by the equable climate in which they live and by their use of patterns of behavior which contribute to control of body temperature. These include diurnality, basking, restricted periods of surface activity, gregarious fossorial habits, and huddling.