Vital Energetics: A Study in Comparative Basal Metabolism

By Francis G. Benedict | Go to book overview

COMPARISON OF WARM-BLOODED AND COLD-
BLOODED ANIMALS

As a result of the exact surveys of the metabolism of small cold-blooded animals by Krogh1 and his associates and of larger cold-blooded animals at the Nutrition Laboratory, and as a result of this present comprehensive survey of the metabolism of warm-blooded animals, we are in an excellent position to note whether there is any connecting bridge between these two great groups in so far as their heat production is concerned. As most of the cold-blooded animals available for study are relatively small in weight, for any general comparisons between the two groups there will always exist the problem of trying to find some method of calculation that will equalize great differences in body weight.. To be sure, there are small warm-blooded animals of about 20 gm., such as the canary and the sparrow, that are nearly comparable in size to the common laboratory frog and goldfish, but for comparison with large warm-blooded animals cold-blooded animals weighing over 2 kg., such as the larger snakes, are rarely available. The heaviest cold-blooded animal whose metabolism has thus far been studied is the Galapagos giant tortoise of 130 kg., but here there is the puzzling factor as to what proportion of the weight of this great animal must be deducted from the total weight in order to make allowance for what is obviously a great percentage of inactive material in the shape of the shell. Furthermore, as the cell temperature is usually much lower in the cold-blooded animals, the temperature factor adds to the difficulty of the comparison. The fact that weight for weight the cold-blooded animal has an infinitely lower heat production than the warm-blooded animal is not debatable. Since, however, the cell temperature of the cold-blooded animal when measured is usually much below the average cell temperature of most warm-blooded animals, any metabolic difference between these two groups may at first sight be explainable exclusively on the basis of a great difference in cell temperature. To compare cold-blooded animals at a cell temperature of 16* with warm- blooded animals at a cell temperature of 37°C is not justifiable, for, as was clearly shown by the investigation on large reptiles,2 the cold-blooded animals have a metabolism several-fold higher at 37° than at 16°. It is more logical, when possible, to compare the two groups of animals at the same cell temperature as that of the warm-blooded animals. Too often in comparing animals in these two groups, writers have been disinclined to give due weight to these great temperature differences. In spite of these basic group differences calculations have been made in the attempt to equalize the animals on the two well-known bases of the heat production per kilogram of body weight and the heat production per square meter of surface area. With the available data, therefore, comparisons may be, made of the metabolism

____________________
1
Krogh, A., Internat. Zeitschr. f. physik.-chem. Biol., 1914, 1, p. 491; Ege, R., and A. Krogh , Internat. Revue f. Hydrobiol., 1914, 6, p. 48.
2
Benedict, F. G., Carnegie Inst. Wash. Pub. No. 425, 1932, fig. 104, p. 478.

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