For monkey and man alike, promiscuous behavior carries a risk of sexually transmitted diseases. A new study suggests that some nonhuman primates simply accept the danger and deal with it by keeping the immune system on heightened alert.
Charles Nunn, a behavioral ecologist at the University of Virginia in Charlottesville, and his colleagues analyzed blood samples from healthy females of 41 primate species in zoos. On average, the white blood-cell counts of promiscuous species, such as yellow baboons, were 50 percent higher than the counts of monogamous species, such as gibbons.
The researchers think they see a battle of evolutionary selection that pits the advantages of having many mates against the hazards of sexually transmitted diseases. Some species end up on one side, some on the other. Although Nunn and his colleagues didn't study people, average human white blood-cell counts lump us with the monogamous primates. "That fits with the pattern we see, because humans really aren't highly promiscuous," he says. But is it possible that more sex would be good for human health because it would exercise the immune system?
"I wish I could tell you that, but our study doesn't deal with it," he says.