Social interactions unmask sex differences in humoral immunity in voles

Sex differences in immune function are well established among laboratory ro dents, with males typically having lower immunity than females. This sex di fference may reflect the suppressive effects of testosterone on immune func tion. Because polygynous males generally have higher circulating testostero ne concentrations than monogamous males, sex differences in immune function are hypothesized to be more pronounced among polygynous as compared to mon ogamous species. Sex differences in immune function have not been consisten tly observed among individually housed Microtus in the laboratory; thus, so cial interactions are hypothesized to be necessary for the expression of se x differences in immune function. We assessed the effect of differential ho using conditions on humoral immunity and steroid hormone concentrations in polygynous meadow voles, Microtus pennsylvanicus, and monogamous prairie vo les, M. ochrogaster. We examined humoral immunity by immunizing voles with keyhole limpet haemocyanin (KLH) and measuring antibody production 5, 10, 1 5 and 30 days postimmunization. Overall, meadow voles mounted higher anti-K LH immunoglobulin (Ig)M and IgG responses than prairie voles, regardless of the housing condition. Sex differences in antibody production were only ob served among meadow voles housed in pairs, in which females had higher anti -KLH IgM and IgG responses than males. Sex differences in antibody producti on were not observed among prairie voles or meadow voles housed individuall y. Sex and species differences in circulating oestradiol, testosterone, and corticosterone concentrations were not related to differences in humoral i mmunity. These data suggest that sex differences in immune function are mor e pronounced among polygynous species than monogamous species, but may be c ontext dependent. (C) 1999 The Association for the Study of Animal Behaviou r.