Finding Mr Right: good genes and multiple mating by females

Understanding why males and females multiply mate is at the heart of behavioural ecology. Generally, theory suggests that while a male's reproductive success is limited by the number of mates he has, a female's success is limited by the number of viable eggs she produces. Since sperm is relatively cheap and plentiful, a female should be able to maximize her reproductive success by mating with only a single male. However, females of most animal species mate with more than one male (sometimes referred to as polyandry). This apparent contradiction with the theory has motivated extensive research over the past decade. A large number of empirical studies, particularly in insects, have now shown that males provide material resources to their mates. These resources can include such things as nutrients that the female consumes, care for her offspring, or protection from predators, and are referred to as direct benefits. Females that mate with multiple males can therefore acquire more resources and increase their reproductive success. Direct benefits provide a satisfying resolution to the apparent paradox of polyandry. However, in some mating systems, males provide only sperm (genes) and no direct benefits, yet females still multiply mate. In these systems, females might benefit indirectly by obtaining good genes or genes that better complement their own. When it is difficult to assess male quality prior to copulation, these females may mate with multiple males and rely on post-copulatory cues, such as sperm competitiveness, to select the ideal father. A recent study on guppies provides some much needed empirical evidence showing that females do in fact mate with multiple males in search of good genes.Jon Evans and Anne Magurran 1xMultiple benefits to multiple mating in guppies. Evans, J.P and Magurran, A.E. Proc. Natl. Acad. Sci. USA. 2000; 97: 10074–10076Crossref | PubMedSee all References1 examine the predator avoidance abilities of juvenile guppies that were produced by females that had either mated with four different males or a single male four times. They show that multiply mated females produce offspring that have better developed schooling and predator escape behaviours. Because male guppies provide no direct benefits, they attribute these results to good genes. Evans and Magurran1 also show that multiply mated females produce larger broods with a shorter gestation time. However, this latter result might also be explained by potential sperm limitation in the females mated to only a single male. Nevertheless, this study provides compelling evidence that females mate with multiple males in search of good genes.Although the importance of polyandry and postcopulatory paternity biasing or ‘cryptic female choice’ is becoming clear, little is still known about the proximate mechanisms involved. For example, females might play a passive role, only facilitating sperm competition, or they might play an active role by removing undesired sperm. The fact that females may be in search of not only good genes, but genes that better complement their own, might help to explain apparent disparities in mate choice, and warrants consideration in mate choice studies. Evidently, promiscuity is not only rewarding for males, but also females.