BioEd Online

The reason that there are no three-tonne lions chasing elephants across the Serengeti is, say researchers, because carnivorous land mammals have an upper size limit of about 1,100 kilograms. Above this, the costs of living outweigh the benefits of bringing down large prey.

The result gives a fresh perspective on why large carnivores are particularly vulnerable to extinction, and brings more bad news for polar bears. Not only is the arctic ice melting beneath them, but today's largest land carnivore also lives on a metabolic precipice, barely able to catch enough food to support its bulk, say Chris Carbone of the Institute of Zoology, London, and his colleagues.

As they get larger, carnivorous mammals show a striking shift in their eating habits. "Species below about 20 kilograms feed on really small stuff," says Carbone think of a badger snuffling for worms. "Species above 20 kilograms dramatically change tactics, and feed on prey about their own size" think of a cheetah chasing down a gazelle.

To work out the details of why this transition takes place, the team looked closely at the costs and benefits of each strategy, compiling measurements of the energy expenditure and feeding rates of different species.

Small prey is plentiful and easy to catch, but each item is barely a snack. For an animal larger than 14.5 kilograms exactly, the researchers found, the occasional big meal is much more efficient. Pound for pound, big game hunters burn more than twice as much energy per day as small carnivores. But the massive amount of calories in a single big catch makes this worth it for the larger hunters.

The costs of a large body, however, rise more quickly than the profits gained from ever-larger kills. Around the 1,000-kilogram mark, carnivores go out of business, the team reports in PLoS Biology¹. "You've got to be strong and beefy enough to catch large prey," says Carbone, "but the intake rate doesn't sustain carnivores above a certain size."

Bigger isn't always better

Polar bears weigh about 500 kilograms on average, although the largest one ever recorded was 1,002 kilograms. The largest known land-dwelling carnivorous mammals, such as the short-faced bear (Arctodus simus), which lived in the Americas until about 10,000 years ago, are estimated to have also weighed in at about 800-1,000 kilograms.

Herbivores, who don't have to expend much energy chasing down their more plentiful food, are thought to have tipped the scales at as much as 15,000 kilograms the giant group of rhinoceros-related Indricotheres, which lived around 30 to 10 million years ago and looked a bit like giant giraffes, are thought to be the largest land mammals to have ever lived.

Reptilian carnivores got much larger, of course Tyrannosaurus rex weighed about 5,000 kilograms. But some estimates of their metabolisms suggest that they may have burnt fuel at about the same rate as a 1,000-kilo mammal.

The fossil record suggests that big, fierce animals are the first to suffer in hard times, says ecologist John Gittleman of the University of Virginia, Charlottesville. "It's likely that we have lost many large carnivores, such as short-faced bears and sabre-tooth tigers, because of the energy costs of large size, coupled with declining prey."

The same principle helps to explain why so many modern large carnivores their problems compounded by habitat loss and hunting are looking down the barrel, he adds. "Conservation must look at biological reasons for why carnivores are faced with extinction. This study shows that metabolic costs are relevant to these problems."

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