A new study has said that metabolic rate and body mass do not have any consistent relationship.
Anastassia Makarieva, a researcher from Petersburg Nuclear Physics Institute in St. Petersburg, Russia, says that most groups of organisms-be it a tiny bacterium, a growing tree, or a gigantic mammal-favour the same optimum metabolic rate.
While it has been found in past studies that smaller species within many groups of organisms generally produce more energy within each cell than their larger counterparts, Makarieva highlights the fact that none of those studies ever compared resting metabolic rates across the whole range of life on Earth.
Leading a team of researchers, Makarieva studied a database of 3,006 different species, ranging from bacteria to elephants.
The researcher 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.
According to them, the metabolic range was even narrower for most species, with the majority lying between 1 and 10 Watts per kilogram, a factor of 10 difference.
The team said that there did not appear any consistent relationship between metabolic rate and body mass.
"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," New Scientist magazine quoted Makarieva as saying.
She said that the formulae used to calculate the metabolic rate within separate classes of animals in previous studies could have shown a multimillion-fold difference.
As a large number of species fall within this narrow range, according to her, there may be an optimum metabolic rate for all organisms.
"Organisms that lie close to this value may be the fittest to survive," she said.
While the researchers are still not clear as to what evolutionary advantage it may offer, the team insists that the need to stay close to this value may be helpful in discerning 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.
An article on the research has been published in the journal Proceedings of the National Academy of Sciences.