ABSTRACT
272Organisms evolve by natural selection toward traits that increase survival, while sexual selection creates an often opposing selective force toward traits that enhance reproductive success. Sexual selection can act on behavioral and physiological processes that take place before and after copulation. Precopulatory processes include competition for mates and mate choice; postcopulatory processes include sperm competition and cryptic female choice. While much-investigated in separate-sexed species, simultaneous hermaphrodites have remained underexposed in this field of research. This is surprising, because hermaphroditism is a common reproductive strategy for many organisms and allows for addressing outstanding questions that cannot be tackled in separate-sexed animals. Therefore, using hermaphroditic freshwater snails, this chapter aims to provide an overview of the importance of pre- and postcopulatory sexual selection and link this to the underlying physiological mechanisms. Several freshwater snail species have served as model organisms for decades, since they are amenable to a variety of different approaches, including cellular, biochemical, neurobiological, endocrinological, behavioral, and ecological. At the precopulatory level, the main focus of this chapter will lie on the neurobiological and physiological detection mechanisms that could underlie mate choice and strategic sperm allocation, with a special focus on the involvement of pheromonal cues. At the postcopulatory level, the physiological mechanism involved in effects caused by seminal fluid proteins, such as on behavior, paternity, and sex allocation, will be reviewed. Attempting to connect the ultimate and proximate levels of sexual selection leads me to the identification of a number of gaps in our knowledge and important research opportunities that should be exploited in future. Such integration will ultimately lead to a synthesis that is necessary for a full understanding of the evolution of hermaphroditism as a reproductive strategy in freshwater snails and will provide new insights into the process and validity of sexual selection in general.
