Promiscuity and the primate immune system

The behavioral and ecological factors involved in immune system evolution r emain poorly explored. We present a phylogenetic analysis of white blood ce ll counts in primates to test three hypotheses related to disease risk: inc reases in risk are expected with group size or population density, exposure to soilborne pathogens, and mating promiscuity. White blood cell counts we re significantly greater in species where females have more mating partners , indicating that the risk of sexually transmitted disease is likely to be a major factor Leading to systematic differences in the primate immune syst em.