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Sex ratio manipulation in wild house mice: the effect of fetal resorption in relation to the mode of reproduction.

First generation laboratory-born descendants of wild-caught house mice (Mus musculus domesticus Rutty) were bred to produce litters of primipares and of dams that had conceived a second litter either after lactational anestrus or within the postpartum estrus. At the day of birth, pups were sexed and the number of implanted and resorbed embryos was determined to evaluate the influence of mode of reproduction on litter gender composition and its relation to fetal resorption. No significant deviations from an even sex ratio occurred in the sample. The results indicate that primipares produced litters with subnormal dispersion of the gender distribution, but this could not unequivocally be attributed to fetal resorption. No significant bias in the litter gender composition was detectable within litters conceived after lactational anestrus. In contrast, the dispersion of the gender distribution was significantly supernormal in the litters of dams inseminated at postpartum estrus. Within this group, fetal resorption had a significant effect upon the sex ratio, and this relationship was significantly affected by the number of implanted embryos: resorbing dams produced male-biased litters at small and intermediate numbers of implantation sites and female-biased litters when the number of implanted embryos was large. It is argued that this is most likely attributable to sex-selective fetal resorption.