Energy metabolism, body size, and problems of scaling.

v_ywiNG to Max Kleibcr's lucid contributions, the lubjecl of energy metabolism and body size is one of the best studied and understood within the extensive field of comparative physiology. This is symbolized in Fig. I, where wc can sec our old friend Gulliver, as well as a Lilliputian who is walking down the cobblestone street. The immediate problem that the Lilliputian emperor had thrust upon him was how much food to give the Man Mountain. Swift (27) reported that il was exactly 1,7211 Lilliputian portions. Does Gulli ver's Delphic expression indicate that he is looking at the Lilliputian or at the empty space lo the left? The significance of the empty space should soon become clear. Comparative physiology is based on the premise the animals arc more or less similar and thus can bc compared. This does not mean that they arc alike, and lhe deviations from the general pattern are often as meaningful and as interesting as the similarities. Those who have dissected a racehorse or a greyhound may have noted diat these animals have larger than pro portionate hearts. In proportion to their body size mammals generally have very similar heart size, about 5 or 6 g/kg body weight, and wc arc so used lo this scale that wc immediately notice a deviation. The fact thai most mammals arc similar in lhat they have just above 0.5% of their body weight as heart, may, at first glance, bc surprising, for wc know lhat the small animal in relation to its body size has a far higher metabolic rate than the large one. To supply lhe tissues with oxygen at lhe necessary high rate obviously cannot lie achieved by merely adjusting stroke volume, which is limited by the size of the heart; thus, the heart frequency remains lhe major variable to adjust, as evident from heart rales between 500 and 1,000/min in the smallest mammals (13). Among mammals in general, perhaps the most conspicuous difference is their size. A 4-ton elephant is a million times as large as a 4-g shrew, and the largest