Abstract
Different stages during development are important when it comes to phenotypic adjustments in response to external stimuli. Critical stages in mammals are the prenatal phase, where embryos are exposed to a milieu of sex steroid hormones, and the early‐postnatal phase, where littermates interact and experience their incipient social environment. Further, the postmaternal environment will influence the development of traits that are linked to reproductive success in adulthood. Accumulated evidence of male‐driven sex allocation establishes the currently untested hypothesis that the sperm sex ratio is a plastic trait that can be mediated to align with prevailing social conditions. Here, we used natural variation in the maternal environment and experimentally manipulated the postmaternal environment to identify the importance of these developmental phases on sperm sex ratio adjustments in wild house mice (Mus musculus domesticus). We found that male density in both environments was predictive of sperm sex ratios at sexual maturity: males from more male‐biased litters and males maturing under high male density produced elevated levels of Y‐chromosome‐bearing sperm. Our findings indicate that the sperm sex ratio is a variable phenotypic trait that responds to the external environment, and highlight the potential that these adjustments function as a mechanism of male‐driven sex allocation.
Highlights
Males have traditionally been dismissed as active players underlying sex ratio biases
Males did not differ in body mass prior to treatment, males exposed to the male environment during development reached a significantly larger size than males exposed to the female environment at sexual maturity (Table 1; Fig. 1A)
We explored the relationship between testes mass and the proportion of Y-chromosome-bearing sperm (CBS) as a possible mechanism accounting for variation in sperm sex ratio at sexual maturity by including testes mass as a covariate in our model
Summary
Males have traditionally been dismissed as active players underlying sex ratio biases. Our study tests whether the sperm sex ratio is a plastic trait that can respond to prevailing social conditions. We used natural variation in the maternal environment and manipulated the postmaternal environment in mice, and provide evidence that exposure to high male density during development leads to higher levels of Y-chromosome-bearing sperm at sexual maturity. Evolution Letters published by Wiley Periodicals, Inc. on behalf of Society for the Study of Evolution (SSE) and European Society for Evolutionary Biology (ESEB). Genetic sex determination is no longer viewed as the all-powerful constraint on sex allocation that it once was considered to be
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