The branching veins in a hand are eerily akin to the spreading limbs of a treeand that basic pattern leads to a surprising variety of other similarities among species. Indeed, a wide range of traits, including life span and metabolic rate, vary according to one simple proportion: an organism's body mass raised to some multiple of 1/4.

By extending this model to include a relevant measure of body temperature, James Gillooly of the University of New Mexico and his colleagues have more accurately predicted the metabolic rate of nearly all living things, according to a report in Friday's issue of the journal Science. They successfully tested their model against data for hundreds of species that depend on oxygen, including aerobic microbes, plants, multicellular invertebrates, fishes, amphibians, reptiles, birds and mammals. In general, the rate at which metabolic chemical reactions proceed depends on the rate at which a body can cirulate and exchange fresh materials with the environment, and on the kinetic energy, or temperature, of the system, which drives the reactions. Gillooly and his colleagues knew that "the fractal-like design of exchange surfaces and distribution networks in plants and animals"the branching veins and spreading limbsmeant that metabolic rates vary with body mass raised to the three-quarter power, they write. But prior research had used a measure of temperature that poorly predicted metabolic rate.

Gillooly's team considered temperature across species in terms of the activation energy of the metabolic chemistry in each creature, which averages about 0.6 electron volts. This term, when coupled with the Boltzmann constantan expression of the relationship between energy and absolute temperatureyielded a simple equation. Though it contains only three terms, the equation predicts metabolic rate for creatures as diverse as hydrated seeds and hibernating bears.

The model is not perfect, the scientists note: the measured metabolic rates of birds and mammals exceed those of plants and unicells by a factor of twenty, even when normalized for body mass and temperature. And an active warm-blooded creature can exceed it's predicted metabolic rate by as much as a factor of ten. But it's much better than what they had before.