Big in the tropics

How big can a cold-blooded animal get? It depends how hot it is (perhaps).

Larry Li's team argues that animals have a minimum metabolic rate, below which they become too sluggish to function. This limits their size, because as animals get bigger, their relative metabolic rate declines — which is why a shrew needs to eat its own body weight each day, and you or I get by on about 2% of our weight. But body temperature also has an exponential effect on metabolic rate: a 5 °C rise leads to a 150% increase in metabolic rate. In tropical climates, then, the effects of increased temperature compensate for those of increased size, so animals can get bigger. Li and his colleagues back their idea up with evidence that in a variety of beasts — earthworms, millipedes, cockroaches, frogs and so on — the biggest tropical species is much larger than the biggest temperate species.

I suspect that this idea will be quite controversial. One can think of other reasons why tropical species might be bigger — there could be more food around, for example. It's also often argued that animals get bigger towards the poles, because it helps them keep warm (because they lose heat less quickly). This idea is known as Bergmann's rule.

Also, if I understand their earlier papers correctly (see here and here, these researchers have taken the highly unusual step of arguing that mass-specific metabolic rate does not change with size — i.e. that a big animal's cells do not burn energy more slowly than a small one's, which goes against more than 150 years of thinking about metabolic rate. This also seems to contradict their ideas on body size.

It's another entry into the strange history of metabolic rate, which everyone agrees is a fundamental biological property, but which has also generated more than its share of confusion and disagreement. My book looks at scientists' efforts to understand metabolic rate — which have been going on for about 200 years — as well as some recent powerful theories that seek to explain it (with which Li and co disagree), and the other things in nature that metabolic rate can explain. Check out the metabolic ecology link on the right to know a bit more. Check out my book next year to know more than you'll ever need.

Generality in ecology

The same issue of Oikos contains an interesting paper by Norman Owen-Smith of the University of Witwatersrand, South Africa, promoting what he calls a 'metaphysiological' approach to ecological theory. He argues (I think I have this right) that scientists studying the biology of populations — the rate at which they grow and the reasons they go extinct — have focussed too much on numbers of individuals, and not enough on flows of energy and matter, and that this approach has given their science shaky foundations, liable to fall foul of the basic laws of physics and biology. 'I believe that population ecology … has tended to seek mathematical rigour at the expense of biophysical rigour,' Owen-Smith writes. Getting it right matters, he adds, because our models will play a large part in how we try to conserve endangered populations.