Male reproductive senescence causes potential for sexual conflict over mating

Abstract

The realization that senescence, age-dependent declines in survival and reproductive performance, pervades natural populations has brought its evolutionary significance into sharper focus. 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 gallus domesticus, where socially dominant males monopolize access to females and the ejaculates of multiple males compete for fertilization. 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, and the intensity of this conflict is modulated socially, by the probability of old males dominating reproductive opportunities.