12.8 Metabolic Heat
Porter and Gates (1969) have calculated estimates of \(M\) for several animals, including sheep, based on published data for animal oxygen consumption. Consumption of O2 implies oxidation of protein, fat, or carbohydrate. Since the heat released in each of these reactions is known, the Porter-Gates method consists essentially of identification of the animal’s diet and conversion of quantity of O2 consumed to amount of metabolic energy produced. This method is most useful in studying animal species in which metabolic data are not readily available.
It probably comes as no surprise to the reader to learn that metabolic rates of domestic animals such as sheep have been thoroughly studied by veterinary scientists. Brockway, McDonald, and Pullar (1965) describe a fascinating series of experiments in which total, sensible, and evaporative heat losses in sheep were carefully measured by direct calorimetry. For an animal maintained at 15°C with a fleece depth of 0.1 m, total heat loss by all routes was 132 kJ kg-1 day-1. Total evaporative heat loss was 52.3 kJ kg-1 day-1, of which 41.9 kJ was due to respiratory evaporation. Sweating heat loss was approximately 12.6 to 20.9 kJ kg-1day-1 regardless of temperature. The authors suggest that, since sheep have little or no physiological control over sensible heat loss, variation in respiratory evaporation heat loss is the chief means by which sheep adjust to their environment. When the weather is hot, they are obliged to pant!