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Sex differences in immunocompetence differ between two Peromyscus species
1Department of Psychology, Johns Hopkins University, Baltimore, Maryland 21218-2686, USA.
Abstract:
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.
Insights
Sex differences in immune function were not greater in polygynous mice than monogamous mice. Instead, females of the monogamous species showed higher immunocompetence and lower body mass than males.
Area of Science:
- Immunology
- Evolutionary Biology
- Animal Behavior
Background:
- Males often display reduced immunocompetence and increased disease susceptibility compared to females.
- Explanations involve proximate mechanisms (e.g., hormones) and variation in reproductive success.
- Hypotheses predict greater sex differences in immunocompetence in polygynous species due to higher reproductive variance.
Purpose of the Study:
- To investigate the contribution of hormonal mechanisms and reproductive success variation to sex differences in cell-mediated immunity.
- To compare these factors between a polygynous species (Peromyscus maniculatus) and a monogamous species (Peromyscus californicus).
Main Methods:
- Assessed cell-mediated immune function via splenocyte proliferation (concanavalin A response).
- Measured body mass and circulating sex steroid concentrations (testosterone, estradiol).
- Compared immune function, body mass, and hormone levels between sexes and species.
Main Results:
- Sex differences in cell-mediated immunity and body mass were observed only in monogamous Peromyscus californicus, with females showing higher splenocyte proliferation and lower body mass.
- Males of both species had different testosterone concentrations, but females did not differ in estradiol.
- Sex steroid concentrations did not correlate with immunocompetence or body mass; larger males of P. californicus had reduced immune responses.
Conclusions:
- Results do not support the hypothesis that sex differences in immunocompetence are more pronounced in polygynous species.
- Circulating testosterone does not appear to mediate sex differences in immunocompetence or body mass in Peromyscus californicus.
- Immune function and body mass sex differences may be influenced by factors beyond mating system and sex steroids.