Evolutionarily stable strategies of age-dependent sexual advertisement

In various models of sexual selection mediated by the viability indicator (“good genes”) mechanism, a sexually selected trait will truly reflect male quality if its expression is costly for the male. However, in long-lived species, the expression of a trait often increases with age while the genotype of the male remains unchanged. This fact may obscure the indicator mechanism. Hitherto, game theory models of honesty in sexual advertisement have not taken life-history effects into account, whereas life-history models of reproductive effort have only seldom considered the dependence of mating success on the actions of other individuals. Here, the two approaches are combined, and I examine whether honesty is maintained if males can divide their advertisement effort over their lifetime. The model shows that an increase in the expression of the sexually selected trait over several years is an evolutionarily stable strategy (ESS) under a wide range of situations, so that a correlated preference for old age can emerge through a viability indicator mechanism. Honesty in the strict sense is not preserved: an optimally behaving low-quality male will in some cases advertise more than a high-quality male of equal age, to the extent that the strongest advertisement found in the population can be associated with a low-quality male. Due to life-history trade-offs, however, honesty in an average sense holds true over the lifetime of individuals: “cheater” age classes will remain small enough, that a female will obtain a higher expected mate quality if she trusts in the trait as an indicator of viability. Received: 25 June 1996 / Accepted after revision: 15 April 1997