Sex allocation in simultaneous hermaphrodites: Trade-offs between sex-specific costs and lifespan

Abstract

Fitness in self-incompatible simultaneous hermaphrodites incorporates gains and costs from both male and female reproductive function, and evolutionarily stable allocation of gonadal tissue to male or female function depends on these gains and costs. Paradoxically, despite the often equal expected gains but different costs associated with each sex, contributions to expected reproductive success through male and female function must be identical. Whenever allocation costs are unequal and limiting resources are energetically expensive or risky to acquire, these costs must ultimately be paid through reduced survival, resolving the paradox by equally diminishing expected reproductive success as male and as female. Maximizing fitness as lifetime reproductive success – not just reproductive rate alone, as in previous studies – maximizes the product of expected survival time and reproductive rate. The analysis shows how male-biased allocation can thereby arise and generate novel predictions on the relation between intensity of sperm competition and allocation to male function.