Size relationship of metabolic rate: Oxygen availability as the “missing link” between structure and function?

The fact that the specific (mass-related) metabolic rate of living beings decreases with increasing body size, has intrigued biologists for a long time. Several attempts have been made to explain the “allometric” (non-proportional) size relationship of metabolic rate, ranging from the thermoregulatory “surface law” and the fractal branching of supply systems up to the mutual interplay of biochemical reactions in varying degrees of physical exercise. Only a few conditions are known where the metabolic size allometry is temporarily suppressed so as to help the smallest animals with the highest size-related metabolic rates (hibernators, neonates) to withstand periods of reduced oxygen and food supply. Remarkably, a similar metabolic size relationship is known to calorimetrists in that the specific heat production of cell suspensions or tissue samples decreases with increasing cell density or tissue diameter. This is known as the “crowding effect” and is usually explained by impaired diffusion conditions with increasing sample size. Thus, what results as a passive consequence of supply conditions in calorimetry, seems to have been established as an active metabolic adaptation in biology. In fact, recent paleobiological and perinatological evidence suggests that an increasing oxygen availability is not only a prerequisite for improved metabolic performance, but has to be followed by an elevated oxygen consumption to avoid toxic side-effects. Hence, the overall validity of metabolic size allometry (with the few exceptions being confined to conditions of reduced supply) might result from an “escape from oxygen” which urges cells to consume the more oxygen, the better they are supplied, even though, in the smallest animals with the best supply conditions, the resulting substrate demand might be difficult to meet.