01/17/2007

Bite Size

Why Predators Are the Size They Are

New research from scientists from the Zoological Society of London explains why some predators are big while others are small -- and why polar bears are the carnivore kings.

Faced with a 3-meter, 800-kilogram polar bear -- the largest living land-based predator -- you might be tempted to think that the animal was, well, rather big. But compared to a 15-meter-long spinosaurus weighing nine tons, the polar bear begins to look somewhat puny. The question is, why are today's predators the size they are? And why are they so much smaller than the dinosaurs?

Now researchers from the Zoological Society of London have come up with an answer. It all has to do with energy.

The starting point for the study, conducted by Chris Carbone and colleagues and published in the online journal PLoS Biology, was the observation that predators tend to come in two size groups. Small predators, with a weight ranging from less than 15 kilograms up to 20 kilograms, almost exclusively hunt very small prey whose weight is far less than their own. Large predators, on the other hand, whose weight exceeds 20 kilograms, tend to hunt prey whose weight resembles their own.

To explain this, the researchers came up with a simple equation. The energy which a predator needs each day consists of the total of what the animal uses while resting and while hunting. The animal must meet these energy needs through the total calorific content of its prey, if it doesn't want to go hungry.

What the biologists discovered is that for animals with a body weight of up to 14.5 kilograms, the best energy budget is achieved when the animal concentrates on hunting small animals. For larger animals, however, the higher energy expenditure associated with hunting can only be covered if they concentrate on larger prey.

"Small-prey-feeding carnivores appear to have relatively low hunting costs -- searching and pursuit can occur at walking pace and the capture and killing phases are often very brief," the authors write. "Large carnivores, however, with their large prey, have higher hunting costs, with long high-speed chases and high costs of capture and killing."

Animals whose weight lies in the intermediate range, such as lynxes or jackals, are free to choose the strategy they prefer, the authors add.

The energy equation also places an upper limit on carnivore size. Above a weight of 1,100 kilograms, the amount of energy needed for hunting is so great that the animal can no longer achieve it even if it hunts large prey. This ties in with the fact that the largest individual ever recorded of the largest living predator, the polar bear, weighed just over 1,000 kilograms. Other now-extinct large carnivores, such as the short-faced bear and the South American sabercat, also came in under the 1,000-kilo limit, lending, ahem, weight to the authors' theory.

But how, in that case, did the dinosaurs get so big? The answer, the authors believe, lies in their metabolism. "Were we to assume lower metabolic rates (e.g., as in reptiles), we would predict a larger maximum mass," they write. "The largest terrestrial predators, such as giganotosaurus and tyrannosaurs, may have achieved their massive size by having a lower metabolic rate."

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