Post‐copulatory Reproductive Strategies

Abstract

Abstract In most sexually reproducing organisms females obtain sperm from multiple males. Such widespread female promiscuity prolongs the operation of sexual selection after copulation, through the competition of the ejaculates of different males over fertilisation (sperm competition) and mechanisms by which females systematically bias the outcome of sperm competition (cryptic female choice – CFC). Post‐copulatory sexual selection can either consistently favour certain male or ejaculate phenotypes or certain combinations of male and female genotypes. Sperm competition and CFC drive the evolution of a diverse range of post‐copulatory strategies resulting in differential ejaculate allocation by males and differential sperm utilisation by females. The post‐copulatory reproductive strategies of each sex have considerable impact on the fitness of the opposite sex, giving rise to an intimate process of intersexual coevolution. The concept of post‐copulatory sexual selection is therefore a powerful heuristic tool to explain variation in individual fitness and the staggering variation in sexual strategies both across and within species. Key Concepts: Sexual selection, or intrasexual competition for mates and fertilisation, can continue after copulation when females mate with multiple males (polyandry). The post‐copulatory processes of sperm competition and cryptic female choice are equivalent to pre‐copulatory mate competition and female choice. Molecular and behavioural studies have demonstrated the female multiple mating is nearly ubiquitous and therefore post‐copulatory sexual selection is a major evolutionary force. Sperm competition occurs when the ejaculates of different males compete to fertilise the same set of eggs, and selects for male adaptations both to prevent and win such competition A male's relative share of paternity is increased through adaptations of sperm quantity and quality, in addition to behavioural, morphological and physiological adaptations in somatic traits. Cryptic female choice (CFC) refers to female‐driven biases in the outcome of sperm competition, which are repeatable within females. CFC and sperm competition are often difficult to distinguish because they can interact to bias sperm use. Females have a wide variety of adaptations that mediate CFC; examples include differential sperm ejection, spermicide, and differential maternal allocation. Patterns of CFC can either differ between different combinations of male and female genotypes, or be consistent among females sharing similar preferences. Post‐copulatory sexual selection can foster the maintenance of polymorphism and alternative reproductive strategies within a species. The post‐copulatory reproductive strategies of one sex can reduce the fitness of the other sex, reflecting sexual conflict over mating and fertilisation. Such conflict can fuel sexually antagonistic coevolution of post‐copulatory traits.