It doesn’t matter if it’s a tiny bacterium, a growing tree or a gigantic mammal – it seems most groups of organisms favour the same optimum metabolic rate.

Previous studies had shown that, within many groups of organisms, smaller species generally produce more energy within each cell than larger species. But according to Anastassia Makarieva from Petersburg Nuclear Physics Institute in St Petersburg, Russia, no studies had compared resting metabolic rates across the whole range of life on Earth.

Makarieva’s team trawled through a database of 3,006 different species, ranging from bacteria to elephants. They found that the average resting metabolic rate per unit mass varied by a factor of 10,000 – despite the fact that body mass varied by a gigantic factor, 1020.

For most species, the metabolic range was even narrower, with the majority lying between 1 and 10 Watts per kilogram – a factor of 10 difference. There was no consistent relationship between metabolic rate and body mass.


Elephantine metabolism

“The largest organism we studied is the elephant, which has a metabolic rate of 1 Watt per kilogram, and the smallest is a bacterium with a metabolic rate of 4 Watts/kg,” says Makarieva.

Using the formulae that had previously been used to calculate the metabolic rate within separate classes of animals, you would have expected a multimillion-fold difference, she says.

Since such a large number of species falls within this narrow range, she hypothesises there may be an optimum metabolic rate for all organisms. “Organisms that lie close to this value may be the fittest to survive,” she says.

Although the team don’t yet know what evolutionary advantage it may offer, they believe the need to stay close to this value may help explain certain aspects of evolution, such as the size at which invertebrates needed to evolve a breathing mechanism, or the shape and size of tree leaves.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0802148105

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