Male fecundity stimulation: conflict and cooperation within and between the sexes: model analyses and coevolutionary dynamics

Abstract

Female promiscuity forces the ejaculates of different males to compete for fertilization through sperm competition. In turn, competing ejaculates often influence female promiscuity and fitness, for example, when ejaculate products increase female fecundity. Here we develop theory examining situations where males stimulate female fecundity and compete for fertilization and female remating is influenced by male fecundity stimulation. We consider the fitness consequences that fecundity stimulation has simultaneously for males inseminating the same female and for the female herself, and we show that the way fecundity increases with male stimulation shifts the coevolutionary dynamics of female remating and male ejaculate expenditure from conflict to cooperation among all mating partners. When fecundity stimulation is weak and males "know" their mating roles, the second male to inseminate a female can exploit the fecundity stimulation of the first male, fostering intra- and intersexual conflict over female remating. However, in a diversity of species where fecundity more than doubles with remating, we show that the female and both males can gain from female remating, leading to intra- and intersexual cooperation over female remating. This coevolutionary perspective yields new insights into the connection between promiscuity, fecundity, and sperm competition and the complex interplay between sexual conflict and cooperation.