Tuesday, January 16, 2007

How big can a meat-eater get?

Today's news@nature has a story by me (free for a week) about an extremely ITBOAH-esque paper on carnivore energetics and ecology by Chris Carbone and colleagues. This looks at the costs and benefits of different hunting strategies — basically, whether you eat stuff much smaller than you (as tends to be the case with small carnivores, such as hedgehogs), or whether you try and bring down things about your own size (which is what big carnivores such as cheetahs and wolves tend to do).

Eating small stuff is a cheap, low-return strategy, unable to support big carnivores — this limits the maximum size of insectivores. Eating big stuff is high return, but costly, because hunting takes a lot of energy. And it becomes more costly the bigger you get — until at about 1,100 kg, carnivores go out of business.

The largest fossil carnivores are about this size. It also suggests how evolution might paint carnivores into a corner — being big and fierce has obvious advantages, in that you're a top hunter, and you can boss your own species about. But in hard times, you starve. The fossil record seems to show high turnover for fossil carnivores, as the follow this bigger, bigger, bust pattern.

The new paper is the latest in a now quite impressive series of papers by Carbone and his colleagues building links between body size, metabolism, behaviour and population biology. Here, for example, they describe the shift in prey size described above, and here they relate body size, metabolism, prey density and predator population density.


RPM said...

An important distinction that you make in the article is that this result refers to land mammals only. Aquatic mammalian carnivores get much larger than 1000kg, as do reptilian carnivores.

Has anyone looked at this scaling issue in aquatic organisms?

John Whitfield said...

Good point - this does only apply to terrestrial mammalian carnivores. I asked Carbone about marine mammals, and he pointed out that costs of movement were not related to size in the same way for marine mammals - big ones do not have the same trouble supporting themselves, because water does it for them, and moving through the water is probably more efficient for big mammals, because drag is not such an influence on them.

Another difference is that the biggest marine carnivores - baleen whales - prey on tiny stuff, such as krill. I guess their being filter feeders might make their ecology more similar to herbivores. It'd be interesting to look at the hunting behaviour of toothed whales, such as dolphins and orcas. Not sure how good the data are.