Yann Arthus-Bertrand/CORBIS

Bring it on!
Predators like this lion tend to attack other predators (here, a hyena) when the victim is smaller.

When Predators Attack

John is a Science contributing correspondent.

Even predators have enemies, and having a set of claws and sharp teeth isn't a guarantee against attack. But just which killers do predators have to watch out for? A rare look into carnivore-on-carnivore violence indicates that size matters: Predators tend to strike only those species somewhat smaller than themselves, while avoiding true pipsqueaks and similarly sized or bigger animals.

Snacking on cuddly herbivores is one thing, but getting into a tussle with another predator is a serious risk. As a result, ecologists have traditionally believed such clashes play little role in population dynamics. But instances of predators killing other predators are widely reported. The carcasses of the losers are usually left uneaten, suggesting that competition might be the motive.

To find out what might go into the decision to attack a fellow killer, Emiliano Donadio and Steven Buskirk, wildlife ecologists at the University of Wyoming in Laramie, mined data from published papers about predator-predator interactions. The survey of 59 different predatory mammal species--including tigers, leopards, foxes, and wolves--revealed thousands of instances of one species slaughtering another. The researchers then combined these data with several biological factors for each species, including body mass, diet, and geographic range.

Not surprisingly, the team found that if one predator was bigger than the other, the fight was on. The vast majority of lethal brawls happened when the attacker's body mass was between 2 and 5.4 times heavier than that of the victim. Yet, if the size difference was too great, no fight ensued. Similarly sized predators also ignored each other. Attacking a like-sized predator is just too risky, the researchers conclude in the April issue of American Naturalist, and those that are far smaller aren't worth the trouble because they are unlikely to hunt the same prey. Going after a slightly smaller predator makes sense, though: the chances of victory are high, and it's a good way to eliminate someone who competes for your resources.

The study is the first to quantify predator-predator interactions for mammals, says Craig Benkman, a zoologist also at the University of Wyoming. The results are surprising, he says, because one would think that predators of similar size might attack each other, as they're more likely to compete for the same prey.

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