The Basal Metabolism of Steers

Abstract

The character and the amount of food eaten, the time and the rapidity of eating and the muscular activity incidental to a 24-hour life are so variable with different animals and, indeed, with different humans, that physiologists have attempted to prescribe certain definite conditions of. experimenting in order to rule out the effect of these variables upon metabolism and to measure the metabolism under conditions in which all external and, so far as possible, internal factors affecting metabolism are eliminated. The main object of measuring the basal metabolism is to determine what one may call the "over-head cost" of living of the organism, that is, the irreducible minimum. The measurement of the basal metab-. olism has further importance in comparative physiology, in that it is the only logical basis for the comparison of the relative heat-production of different individuals with each other or with different species.With humans these basal measurements are made in the so-called "postabsorptive condition," which may usually be obtained 12 hours after the last meal, if the meal is not too rich in protein or calories. The subject should be lying, well covered with clothing, comfortable, psychically and muscularly in repose and obviously without fever. With animals comparable conditions are difficult to secure. In-the case of the steer, for instance, food may be withheld, the body temperature may be normal, and muscular activity may for the most part be ruled out, and yet it is impossible to make a steer lie down and remain lying for any length of time. Moreover, thus far the element of protection from the external temperature, other than the normal coating of hair, has not been considered. Obviously extremes in temperature are to be avoided, for it has been found that with the rat and the dog small differences in temperature result in considerable differences in metabolism, induced either by restlessness, shivering or possibly a stimulus to greater heat-production to combat the effect of cold. With human beings, well covered as they are, temperature variations, unless extreme, play a small r6le. Observations thus far with the steer imply that the influence of environmental temperature is likewise small, particularly if the animal is normally fed. Indeed, an extensive series of experiments has shown that the temperature factor is much less pronounced with ruminants than with either the rat or the dog, and as practically all of our experiments with steers were made at or near 20°C. (the temperature 132