Why do females mate multiply? A review of the genetic benefits

The aim of this review is to consider the potential benefits that females m ay gain from mating more than once in a single reproductive cycle. The rela tionship between non-genetic and genetic benefits is briefly explored. We s uggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begi n by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple matin! : can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifyin g cases where females use precopulatory cues to identify mates prior to rem ating. In the simplest case, females remate because they identify a superio r mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre -copulatory cues may be less reliable and females mate with several males t o promote post-copulatory mechanisms that bias paternity. Although a distin ction is drawn between sperm competition and cryptic female choice, we poin t out that the genetic benefits to polyandry in terms of producing more via ble or sexually attractive offspring do not depend on the exact mechanism t hat leads to biased paternity. Post-copulatory mechanisms of paternity bias ing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those tha t improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third pos sibility is that genetic diversity among offspring is directly favoured. Th is can be due to bet-hedging (due to mate assessment errors or temporal flu ctuations in the environment), beneficial interactions between less related siblings of the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevai ling environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among pr ogeny in elevating fitness. We conclude that post-copulatory mechanisms pro vide a more reliable way of selecting a genetically compatible mate than pr e-copulatory mate choice. Some of the best evidence for cryptic female choi ce by sperm selection is due to selection of more compatible sperm. Two fut ure areas of research seem likely to be profitable. First, more experimenta l evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promotin g assortative fertilization and increasing reproductive isolation between p opulations may help us to understand sympatric speciation.