Abstract
Male senescence has detrimental effects on reproductive success and offspring fitness. When females mate with multiple males during the same reproductive bout, post-copulatory sexual selection that operates either through sperm competition or cryptic female choice might allow females to skew fertilization success towards young males and as such limit the fitness costs incurred when eggs are fertilized by senescing males. Here, we experimentally tested this hypothesis. We artificially inseminated female North African houbara bustards with sperm from dyads of males of different (young and old) or similar ages (either young or old). Then, we assessed whether siring success was biased towards young males and we measured several life-history traits of the progeny to evaluate the fitness costs due to advanced paternal age. In agreement with the prediction, we found that siring success was biased towards young males, and offspring sired by old males had impaired hatching success, growth and post-release survival (in females). Overall, our results support the hypothesis that post-copulatory sexual selection might represent an effective mechanism allowing females to avoid the fitness costs of fertilization by senescing partners.