Power corrupts

For chimpanzees, it's not lonely at the top. A new study by Yale anthropology professor David Watts has found that alpha-male chimps may have more kinds of friends—tiny, disease-causing friends—than they ever wanted. "What we have found," he says, "is that parasite loads are higher in high-ranking males than in low-ranking males."

High-ranking males' intestinal diversity is related to their testosterone, according to Watts and his coauthor, Indiana University professor Michael Muehlenbein '04PhD. Alphas have about 25 percent more testosterone than low-ranking males, enabling the aggressive behavior that catapults alphas to the top of the social and sexual hierarchy.

But the alphas pay a price. "There's a tradeoff between maintaining high testosterone levels—being in good shape to compete—and keeping your immune system working well," Watts says. The weaker a chimp's immune system, the more diverse his parasite infestation. (The study, in BioPsycho-Social Medicine, doesn't provide exact numbers.)

Watts and Muehlenbein conducted their study in Uganda's Kibale National Park, home to the world's largest known chimpanzee community. The researchers gathered 67 fecal samples from 22 adult male chimpanzees for testosterone and parasite analysis.

The researchers still don't know the long-term effects of the higher parasite loads—or even if there are any. Parasites, Watts says, could make alpha males' lives shorter and less healthy. But, he added, it could be that those males capable of making it to the top "can bear the cost of having parasites better than other males."