Mate choice and reproductive conflict in simultaneous hermaphrodites

Abstract

Simultaneous hermaphroditism defines sexual systems in which animals display male and female sex at the same time. Given that each individual now inherently expresses the ‘interests’ of both sexes, this form of gender expression can have profound consequences for the evolution of behavioural reproductive strategies. The outcome of reproductive interactions between two hermaphrodites is likely to vary with (i) intra-individual trade-offs between male and female reproduction, (ii) inter-individual courtship and mate choice between the sexes, (iii) inter-individual competition within the sexes, and (iv) competition with rival individuals in the social group. While earlier work has argued that precopulatory mate choice may only be weakly expressed in simultaneous hermaphrodites, the first section of the chapter documents that, instead, mate choice is prevalent and may include sophisticated mate discrimination based on traits such as body size, mating history, or relatedness. The second section illustrates putative conflicts that individuals may face during copulation, primarily focussing on the decision over mating roles that is central to understand hermaphrodite reproductive behaviour. Available evidence indicates that conditional reciprocity, where individuals accept matings in their disfavoured mating role in order to achieve access to their favoured role, are less widespread than initially though. The final section explores the idea that simultaneous hermaphroditism should enhance selection on postcopulatory mate choice and sperm discrimination mechanisms, including harmful male mating strategies. Support for this prediction remains limited to date, calling for much broader empirical quantifications of the fitness costs and benefits associated with hermaphrodite mating strategies.