Sex differences in immunocompetence differ between two Peromyscus species

Males generally exhibit reduced immunocompetence and greater susceptibility to disease than females. The explanations for why males may be more susceptible to disease than females fall into two categories: 1) the proximate mechanisms mediating immunity, such as hormonal mechanisms, and 2) variation in reproductive success between the sexes. The present study examined the extent to which these factors contribute to sex differences in cell-mediated immune function in polygynous Peromyscus maniculatus and monogamous Peromyscus californicus. Prevailing hypotheses suggest that, because variation in male and female reproductive success is greater among polygynous than monogamous species, sex differences in immunocompetence should be greater among polygynous than monogamous species as well. In contrast to these predictions, sex differences in cell-mediated immunity and body mass were only observed among monogamous P. californicus, in which females exhibited higher splenocyte proliferation in response to the T cell mitogen, concanavalin A, and weighed less than male conspecifics. Male P. maniculatus had higher serum testosterone concentrations than male P. californicus, but females of the two species did not differ in circulating estradiol concentrations. Sex steroid concentrations were not correlated with either immunocompetence or body mass; however, large P. californicus males exhibited reduced immune responses. Taken together, these results do not support the hypothesis that sex differences in immunocompetence are more pronounced among polygynous compared with monogamous species. Furthermore, these data suggest that circulating testosterone does not mediate sex differences in immuno-competence or body mass in P. californicus.