If energy is the currency of life, biologists are closing in on the cost of living.

No matter whether you're talking elephants or bacteria, a new study proposes that, pound for pound, all living things' at-rest metabolisms use similar amounts of energy. Though living things vary greatly in complexity and size, their energy usage falls between 3 and 90 watts per kilogram of biomass. For comparison, a MacBook Pro is supposed to draw about 12 watts when operating from its battery.

"Our interpretation is that there aren't very many accidents in nature, so it's not just a coincidence that all these different organisms fall within this narrow window," said Peter Reich, an ecologist at the University of Minnesota, and a co-author of the work. "That suggests that natural selection selects for this range."

Using energy to stave off entropy is one of the basic functions of life. Different forms of life use different energy sources. Plants use photosynthesis to turn sunlight into chemical energy. Other organisms feed on plants or other organisms to obtain the energy needed to sustain themselves. The new paper suggests that no matter how an organism gets its energy, the basic biochemistry of life requires that all organisms' cells use energy at fairly similar rates.

The new work means that if you had an elephant-sized mound of bacteria, it would use, within about an order of magnitude of variation, the same amount of energy as an actual elephant.

That contradicts earlier, highly-influential studies led by James Brown, Brian Enquist and Geoffrey West, of the University of New Mexico, University of Arizona and Los Alamos National Laboratory respectively. They found a strong correlation between the size of an animal and its metabolism. Under their rubric, small creatures used energy efficiently while large creatures did not. As organisms grow larger, they produce less energy relative to their bulk.

The UNM ecologists claimed that this relationship between size and metabolic rate, known as allometric or power scaling, was a general law of life that resulted from the difficulty of transporting nutrients around larger and larger bodies.

"It's been a very exciting theory that people tend to love or hate," Reich said. "It sounds like a general theory of relativity for biology.... It's a major advance because they are proposing something novel to unify us all and how we understand the world."

But when scientists in individual areas, like Reich who specializes in forest ecology, began to look at the data, they found that the law didn't seem to hold for all types of living things.

The old model predicts that organisms' metabolisms would vary by thousands of times, but Reich and his colleagues found that the metabolisms of living creatures were much more similar than that. For example, an elephant is one trillion million times larger than a single-celled bacterium — that's 20 orders of magnitude — but their metabolisms fall roughly within an order of magnitude.

"If there was power scaling, you'd have a 4000-fold metabolic variation. Whereas we only see a 30-fold variation," Reich said. "They're not even anywhere in the ballpark of power scaling."

Instead, the new Proceedings of National Academy of Sciences paper provides strong evidence that the basic energy usage of living creatures is much more universal than scientists' anticipated.

"There are fundamental sweet spots for life," Reich said.

And it appears his team has found one, at least here on Earth.

Image: flickr/John & Mel Kots

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