The metabolic rate of an isolated mitochondrion, sitting in a lab dish, also falls on Kleiber’s line. Even organelles have their own distribution networks. At the molecular level, their respiration is run by large, complex molecules. And even in these isolated molecules, the reactions of respiration run at the speed predicted by Kleiber’s rule, suggesting that within these molecules’ structure there are nanoscale distribution networks that move individual oxygen and ATP molecules around. From monsters to molecules, this line of reasoning extends the network theory’s reach over a size range of 27 orders of magnitude, or a thousand trillion trillion times. The same logic extends the theory to the bacteria, the vast group of single-celled organisms lacking mitochondria, chloroplasts, or nuclei, but that do have these large and complex molecules. West, Brown, and Enquist’s model describes these as “virtual fractals”: Resources are distributed in a branching pattern through the cell, even if they are not in tubes. For the model devised by Banavar and his colleagues, this isn’t such an issue—resources spread out from a central point, like food being shared from the head of a table to the guests seated around it.
It seems that natural selection has such a strong preference for efficient distribution that the same fractal network solution has evolved many times, at scales from molecules, to cells, to plants and animals, taking on many different forms and for many different substances but always converging on the same fundamental properties. These networks are such a versatile solution to the problem of supplying a body with resources that they have allowed life to evolve into a remarkable range of sizes. It’s as if a human engineer had invented a single mechanism that could power everything from silicon chips to supertankers.
So model networks can explain why the slope of the line linking mass and metabolism has a gradient of 3/4. But that leaves lots of variation. Animals of the same size in different groups of organisms have very different metabolic rates. Reptiles are slow, birds fast, and mammals somewhere in between. Plants are slowest of all. Even accounting for size, the speediest metabolic rates seen in nature run 200 times more quickly than the slowest. Body size controls the rate at which cells can be supplied with resources, but this is not the only