William F. McComas, USC

Barbary macaques (Macaca sylvanus), a primate species found in Northern Africa, were found to have a naturally high white blood cell count. U.Va. researchers think it is an evolutionary response to the macaque's promiscuous mating behavior, especially the female's. In their social system, males also take an extensive part in caring for the young.

Promiscuity could be the key factor in immune system evolution, study suggests

By Fariss Samarrai

A new study indicates that evolution of the immune system may be directly linked to the sexual activity of a species. Reported in last week's issue of Science, the comparative analysis of 41 primate species demonstrates that the most promiscuous species have naturally higher white blood cell counts -- the first line of defense against infectious disease -- than more monogamous species.

"Our findings strongly suggest that the most sexually active species of primates may have evolved elevated immune systems as a defense mechanism against disease," said principal investigator Charles L. Nunn, a research associate in the biology department. "We looked at animal species with a range of mating behaviors and found a strong relationship between high white blood cell counts and high promiscuity in healthy animals. The more monogamous species have lower counts."

The researchers compared 20 years of data on average white blood cell counts for 41 primate species. The species studied represent the major primate evolutionary groups and the full range of mating behaviors. Some of the species are highly promiscuous, such as the Barbary macaque, whose females may mate with up to 10 males per day while in heat. Other species have varying levels of monogamy, including some that mate with one partner for life. Data for each species come from zoos and are composed of veterinary reports of basal, or normal, white blood cell counts for healthy females.

"The implication of our finding is that the risk of sexually transmitted disease is likely to be a major factor leading to systematic differences in the primate immune system," Nunn says. "This puts the evolution of sexual behavior in close relation to the evolution of the immune system."

The researchers also compared other behavioral and social factors that might affect the animals' immune systems, including high population density, which increases the risk of exposure to disease, as well as exposure levels to soil-borne pathogens, namely fecal contamination. They found that mating promiscuity affected white blood cell counts far more than other disease risk factors.

"We expected to see a correlation between white blood cell counts and various behavioral and ecological factors, but were surprised to find that sexual activity appears to be the key factor in how the immune system develops," said co-author John L. Gittleman, U.Va. associate professor of biology. "This opens up many new questions about behavior and the immune system."

The researchers also compared mean white blood cell counts of humans to the various primate white blood cell counts.

"Based on this comparison, humans are more similar to the more monogamous primate species," Nunn said.

The research is funded by the National Science Foundation and the National Institute of General Medical Sciences, and was conducted by three U.Va. scientists in the department of biology: Nunn, who specializes in primates; Gittleman, who uses computational methods to study evolution; and Janis Antonovics, who studies sexually transmitted diseases.