Burning Energy as Equals

Universal. Fish, trees, and humans share roughly the same metabolic rate.

Ever wondered what you have in common with a banana? Scientists have found that the resting metabolic rate is roughly the same in every living species, from bacteria to plants to people, when body size and temperature are taken into account. The discovery suggests that widely diverse species burn energy in predictable patterns. It may help researchers predict how quickly a given creature will develop or reproduce.

Metabolic rate--how fast an organism breaks down and burns food--sets the pace for much of development. Researchers have long known that bigger and hotter species have higher metabolic rates than cooler, smaller ones (think Dalmatian versus Yorkie). But putting numbers on this trend has been tricky. Historically, metabolic rate studies have focused on either body size or temperature. But, a team of researchers began wondering, why not put the principles together?

The group, including ecologist James Brown at the University of New Mexico in Albuquerque and Geoffrey West at Los Alamos National Lab, fine-tuned a model they advanced 4 years ago (Science, 4 April 1997, p. 122). That used body mass and fractal geometry to explain why metabolic rate varies in proportion to the 3/4-power of an organism's mass. The new study, reported in the 20 September issue of Science, also takes temperature into account for 250 species, showing that metabolic rate scales according to these two factors. "Once you have a fundamental sense of the combined effects of size and temperature," Brown says, "it looks like you can account for an awful lot of biology." That means that other variables like ecological adaptation may do less to set metabolic rate than some scientists believe. Already, the group is using its formula to predict developmental rates for aquatic organisms.

"This work gives us a constraint envelope, a parameter space, in which life can evolve that's more limited than expected," says Carlos Martinez del Rio, an ecological physiologist at the University of Wyoming in Laramie. "All organisms must satisfy these basic biophysical laws." Del Rio cautions, however, that even though the metabolic rate varies much less than thought, it's not strictly universal. Organisms still vary 20-fold, a range that may clarify all sorts of differences between living species.

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