The oxygen consumption of rats is frequently used as an indication of heat production. Before we can draw conclusions regarding the effects of any procedure on the heat production of the animals it is necessary to eliminate differences arising from age, sex and weight. By using the animals as their own controls or by using control groups of the same age and sex differences due to these factors are removed. The procedure to be tested may, however, lead to changes in weight or in growth rate of the test animals. These changes may be sufficient to result in differences in weights of the test and control groups. Hence it is not often possible to follow the advice of Benedict (1938) to compare the metabolism of animals having the same weight. The usual practice has heen to refer the absolute oxygen consumption to some unit which supposedly corrects for body weight or size. While the situation has improved somewhat since Kleiber (1944) first expressed alarm, four reference units are still in general use.These are: body weight (kg), 'metabolic size' (kgi) or (kgl) and surface area (M2 calculated from body weight). Since there can be only one correct way of expressing oxygen consumption and since conclusions often depend upon the assumption that the reference unit used is the proper one, it is surprising that no comparative study of the four means of expressing heat production has been made. This paper reports the results of such a study.
METHODSMale albino rats were used for all experiments. They were kept in individual metal cages and fed on Purina Laboratory Chow and water ad libitum.Oxygen consumption was recorded for 1 hr by a closed-circuit method previously described (Hsieh & Chiu, 1959). Six units were used, making possible twelve tests each morning. During the tests the air surrounding the animals was maintained at about 250 C.In one series of experiments the ages of the rats were ignored. Ninety animals of different weights were obtained from the animal house and kept at 28°C for about 5 days. Oxygen consumption was determined after this period. This experiment was designed to imitate the usual procedure, in which rats of different ages and weights are used.To test the effects of age on oxygen consumption and to obtain rats of the same age but with different weights the following experiments were performed. Thirty-three rats about 7 weeks old were divided into three groups of eleven each. One group (WN) was kept at