Male Reproductive Senescence Causes Potential for Sexual Conflict over Mating

The realization that senescence, age-dependent declines in survival and reproductive performance [1-4], pervades natural populations has brought its evolutionary significance into sharper focus [5, 6]. However, reproductive senescence remains poorly understood because it is difficult to separate male and female mechanisms underpinning reproductive success. We experimentally investigated male reproductive senescence in feral fowl, Gallus gal/us domesticus, where socially dominant males monopolize access to females and the ejaculates of multiple males compete for fertilization [7]. We detected the signal of senescence on multiple determinants of male reproductive success. The effect of age on status was dependent upon the intensity of intrasexual competition: old males were less likely to dominate male-biased groups where competition is intense but were as likely as young males to dominate female-biased groups. Mating and fertilization success declined sharply with male age largely as a result of population-level patterns. These age-dependent declines translated into sexually antagonistic payoffs: old males fertilized more eggs when they were dominant, but this resulted in females suffering a drastic reduction in fertility. Thus, male senescence causes potential for sexual conflict over mating [8], and the intensity of this conflict is modulated socially, by the probability of old males dominating reproductive opportunities.