Sexual promiscuity comes at a price. Species with a wandering eye have to invest much more energy in their immune systems to ward off sexually transmitted diseases, according to findings reported in the 10 November issue of Science. The work suggests that sexual behavior may have a profound effect on the evolution of the immune system.
Scientists know little about how the immune system evolves. They assume that factors such as crowding, the number of soil microbes, and multiple sexual partners all increase the risk of disease; but how these forces have shaped immune systems over many generations is a mystery. One clue is the observation that numbers of white blood cells--a first line of defense against disease--vary widely among species. That led Charles Nunn of the University of Virginia in Charlottesville to wonder whether a high risk of infectious diseases might result in an especially vigilant immune system.
Nunn and his co-authors John Gittleman and Janis Antonovics looked at veterinary records of animals from 41 primate species kept at zoos around the world, all of which had been deemed healthy by veterinarians. They compared white blood cell counts to published data about promiscuity and other disease risk factors for each species. A species' natural group size didn't seem to have an impact; neither did how much time a species normally spends on the ground (a measure of their exposure to soil bugs). But the white blood cell count was significantly higher in species where females mate with multiple partners, such as most macaques, than in those with monogamous females, such as gibbons. Apparently, the more promiscuous species have to spend more energy on their immune systems, the authors say. Humans' immune cell counts, the authors note, are more consistent with those of monogamous primates than promiscuous ones.
The study is "provocative," says evolutionary parasitologist Andrew Read of the University of Edinburgh. He's surprised by the fact that white blood cell counts correlate with behavior. But Read notes that the differences in cell counts were relatively small, and he wonders whether they would actually make a difference in resisting disease. Read would like to see the study repeated with other groups of mammals.