The regular decrease in specific metabolic rate with increasing body size must somehow be reflected in the metabolic rates of the various organs that make up the whole animal. To extend this reasoning, the observed differences should also be reflected in the metabolic rates of the cells that make up these organs. We could therefore ask if it is fruitful to study the scaling problem from the viewpoint of cell metabolism.

Tissue metabolism and cell size

The peculiar situation is that large and small animals have cells that are roughly of the same size, within an order of magnitude of 10 µm (Teissier, 1939). For example, a microscopist would be hard put to recognize differences between microscopic sections of a horse muscle and a mouse muscle, except that the mitochondrial density is higher in the muscle from the smaller animal.

Because cell size in various animals is much the same, independent of body size, a large organism is not made up of larger cells, but of a larger number of cells of roughly the same size. It could therefore be expected that, as more cells of the same size are added to make up a larger organism, the metabolic rate should increase in proportion to the increased number of cells. As we have seen, this is not so, and we must therefore explore alternatives.