Pre- and post-copulatory traits are affected by experimental inbreeding, but they are not correlated.

backgroundIt has been suggested that the expression of males' secondary sexual traits provides reliable indicators of their sperm traits, predicting positive correlations between pre- and post-copulatory traits (Fertility Indicator Hypothesis). Yet, it has also been suggested that males face life-history tradeoffs between investing into primary versus secondary sexual traits, predicting negative correlations (Sexual Allocation Tradeoff Hypothesis). These two hypotheses are not mutually exclusive when males' sexual traits are condition-dependent and high-quality males are better able to invest into both pre- and post-copulatory traits than low-quality males. To test these hypotheses, we manipulated the genetic quality of wild-derived male house mice by experimental inbreeding and first tested whether inbreeding affects primary or secondary sexual traits (condition-dependent expression). We then tested whether pre- and post-copulatory traits are correlated. We recorded courtship behavior and vocalizations of the males during female contact and measured males' reproductive organs, sperm quality, and the expression of four genes associated with spermatogenesis.

resultsInbreeding did not reduce male courtship vocalizations, though it altered their vocal repertoire and reduced other courtship behaviors. Inbreeding negatively impacted relative testes mass and sperm quantity and quality, after two generations of inbreeding. We found no consistent correlations between pre-and post-copulatory traits, either positive or negative, regardless of inbreeding.

conclusionsOur results indicate that inbreeding impacted the expression of primary and secondary sexual traits in wild-derived house mice, which is the first such evidence to our knowledge, but we found no support for either the Fertility Indicator or the Sexual Allocation Tradeoff Hypotheses.