eLife 8, e38187 (2019)

In the complex world of living matter, the scaling law linking the metabolic rate of an organism with its body mass almost seems peerless — both in its simplicity and its longevity. But what drives the scaling? That different species elude quantitative comparison and single species span too narrow a size range has made this a difficult question to answer. Now, Albert Thommen and colleagues have sought help from the remarkable flatworm Schmidtea mediterranea, which grows when fed and shrinks when starved — exhibiting a 40-fold range in body length.

Using microcalorimetry, Thommen and co-workers confirmed the expected scaling, then devised a theoretical framework casting growth and shrinkage as a function of the organism’s metabolic energy budget. Armed with both model and data, they identified an increase in the mass per cell due to size-dependent energy storage as the cause of the scaling in the starving worms. It remains to be seen whether the same mechanism underpins allometric scaling in all animals.