Growth clocked

Size and temperature predict pace of life.

If you know how big and how warm an animal is, you can work out how long it will take to grow, say researchers. These two factors account for nearly all of the variation in developmental time from plankton to birds¹.

"Despite the incredible complexity of development, and the diversity in form and function, once you account for body size and temperature, hatching time is the same for a salmon, a grasshopper or plankton," says biologist James Gillooly, of the University of New Mexico in Albuquerque.

The model could be extended to measure many other rates, such as lifespan and the speed of population growth, Gillooly believes. "We've devised a formula for a biological clock," he says, "which ticks not in units of time, but in units of energy."

Time for change

Size and temperature are fundamental to the pace of life. Their effects on development have been studied separately for more than a century; this is one of the first attempts to unite them.

Gillooly's group considered how temperature affects the speed of the chemical reactions that power living cells. They combined this with a mathematical model of how size influences an animal's metabolic rate.

Broadly speaking, increasing temperature speeds biological processes, and increasing size slows them down. Bigger animals take longer to develop, and have relatively slower metabolisms.

From size and temperature, the model accurately predicts the time from birth to hatching for many species, including birds, fish, amphibians, aquatic insects and plankton.

Despite its good fit to the data, the model has "simplifications that not everyone would go along with", comments ecologist David Atkinson of the University of Liverpool, UK. It works best as a description of broad trends, he says, rather than variation over narrow size ranges, such as within a species.

But, he concludes, "it's a powerful theory - you can explain an awful lot of nature very quickly with only a little information".

Pattern emerging

Egg-layers are particularly suited to this analysis, because all the embryo's food is provided in one go. So growth is not affected by a mother's ability to get food while pregnant, or by the young's luck in fending for itself.

When the animal gets out into the environment, other factors kick in. Gillooly's model is less good at predicting how long an animal will take to grow from birth to its final size, although measuring an organism's chemical composition, and so its diet, improves their predictions.

In recent years, many ecologists have come to believe that, by studying how size, temperature and chemistry influence the way that organisms use and transport resources, they can explain the large-scale patterns seen across the animal and plant kingdoms.