Among primates, certain species are quite promiscuous. Female Barbary macaques, for example, mate with as many as 10 males each day during estrus--risky behavior as far as sexually transmitted diseases (STDs) go. Or is it? The results of a study published in last Friday's issue of the journal Science suggest that the primate immune system may have adapted to this kind of behavior. According to the new report, females with more mating partners have higher white blood cell counts than do those who stick with a single mate.

Postdoctoral researcher >Charles L. Nunn of the University of Virginia and his colleagues set out to test whether evolutionary increases in three factors of disease risk--namely group size and density, time spent on the ground (which increases exposure to soil-borne pathogens) and number of mating partners--correlated with similar increases in white blood cell counts. Their analyses reveal that of those three factors, only the risk of STDs appears to have shaped variability in primate immune systems. The team concludes that "the precise reason for this result requires further study," but they propose that "it might be that STDs are much more common in nature than previously thought, or that behavioral mechanisms to avoid infectious disease are less effective against sexually transmitted pathogens."

Not all primates have this pumped-up immune system, ready to protect against promiscuity. White blood cell counts in humans, for instance, are apparently more consistent with monogamy. At least one reason for this difference shows up in a study of bees, which appears in the same Science issue. This work demonstrates that immunity can be costly, especially in an animal that is already under stress. "When an animal is already stressed, mounting an immune response may place excessive demands on stores of an essential factor," such as a rare amino acid or energy, >Andrew F. Read and Judith E. Allen of the University of Edinburgh explain in a commentary accompanying the two reports.

Read and Allen further note that whether the correlation between STDs and higher white blood cell counts among primates will hold up remains to be seen. Although the overall white blood cell counts are higher in promiscuous primate species in this latest report, numbers of the specific cell types were sometimes counterintuitive. "Certainly the idea that immune response variations in nature can be understood in terms of fitness costs and benefits is appealing," they assert. "The principle challenge is to marry this idea with our detailed understanding of how immunity works."