What do a hummingbird and an elephant have in common? And a scorpion, a warthog, an amoeba, and a palm tree? According to a study of 3,006 different species, the answer may be one of biology’s most fundamental traits: the amount of energy needed to sustain life.

The acquisition and processing of food sources is, self-evidently, a critical aspect of life that drives behavior and is molded by natural selection. But just how much energy does an organism need? And are we fated by our body size to spend more or less time foraging for energy?

At first glance, a huddled mouse (whose heart beats about 500 times per minute) seems to be burning through bioenergetic fuel a lot faster than a great whale (7 beats per minute). This intuition has seeped into scientific studies for years, as multiple reports suggest that metabolic rate (normalized by mass) decreases with increasing body mass.

But Anastassia Makarieva and her international group of colleagues were skeptical of the reports of power laws – those all-too-convenient fudge factors for quantitative relationships – and they decided to compile as many measurements as they could get their hands on. It soon became clear that the task of aggregating data wasn’t as straight-forward as it sounded: energy measurements were often reported in different units, for different numbers of individuals, and at different phases of an organism’s life.

For example, microbial researchers often quantify maximal energy use during exponential growth while studies of mammals typically examine resting rates. Microbes’ metabolic rates are generally reported per unit dry mass; larger animals’ are given per unit wet mass. And plant studies tend to focus on certain parts – like leaves or roots – rather than the whole organism.

In order to compare apples with proverbial apples (and literal oranges), Makarieva had to set some ground rules. She limited prospective data sets to those reporting metabolic rates in nongrowing, resting, post-feeding organisms to make sure things weren’t thrown off by cell division, movement, or food comas.

By the time they had finished, the team had collected data from 3,006 species and discovered something remarkable: rates of energy use were surprisingly consistent, clustered around 0.3 to 9 Watts per kilogram of (wet) biomass. This finding is particularly stunning given the enormous range of body sizes considered in the study: the smallest organism – Francisella tularensis – weighs in at 10-14 grams, while the largest – the elephant Elephas maximus – tips the scale at 4 million grams. That’s a full 20 orders of magnitude, and if the mass-dependent power law school of thought were correct, metabolic rates would have shown a >65,000-fold range rather than the 30-fold span Makarieva’s team reports.

The convergence of metabolic rates around such a relatively narrow range suggests, with the benefit of evolutionary hindsight, that it was meant to be. Perhaps, Makarieva proposes, we’re looking at the optimal rate of energy use – the narrow range that has been hemmed in through natural selection as our high-octane ancestors flamed out and our more laid back ones fizzled. The discovery of such a narrow range of energy use casts an even brighter light on the variation that does exist. If all forms of biology seem to converge on particular metabolic rate, what is to be gained from being above or below that rate?

Viewed from the other direction – by looking at an environment’s ability to supply nutrients – the finding of an apparent Goldilocks zone may point astrobiologists to energetically ideal planets or habitats. If 0.3 – 9 Watts per kg proves to be a universal feature of biology, perhaps the search for life beyond Earth would be best served not only by “following the water” as NASA proposes, but also by “following the energy.”