Promiscuity May Be Key Factor In Immune System Evolution, Study Suggests

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

The researchers compared 20 years of data on average white blood cell counts for 41 primate species. The 41 species 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 ten males per day while in heat. Other species have varying levels of monogamy, including some that mate with one partner for life. The researchers found a direct correlation between WBC levels and mating behavior. Data for each species come from zoos and are composed of veterinary reports of basal, or normal, WBC 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 WBC counts far more than other disease risk factors.

"We expected to see a correlation between WBC 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," says 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 WBC counts of humans to the various primate WBC counts, and found that humans are on par with the more monogamous primate species.

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

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.