Hormones and mating system affect sex and species differences in immune function among vertebrates.

Males generally exhibit reduced immune responses as well as increased intensity and prevalence of infections compared to female conspecifics. Physiologically, these sex differences may reflect the immunosuppressive effects of androgens. In addition to suppressing immune function, androgens maintain several characteristics important for reproductive success. Thus, a dynamic relationship is assumed to exist among hormones, secondary sex traits, and the immune system. Ultimately, the extent to which this relationship exists may be related to the mating system. Because polygynous males generally have higher circulating testosterone concentrations and rely more heavily on testosterone-dependent traits for reproductive success than monogamous males, sex differences in immune function are hypothesised to be more pronounced among polygynous as compared to monogamous species. Additionally, if secondary sex traits are used to advertise infection status, then females should be able to use the condition of male secondary sex traits to discern the immune/infection status of males during mate selection. The purpose of this review is to survey current studies that examine both the proximate mechanisms and ultimate function of variation in immune function and susceptibility to infection and determine whether immunological variation influences mate preference and possibly reproductive success.