Why do females mate with multiple males?

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The number of males which a female mates with shows a large variation among animal species. This interspecific variation has been studied for a long time, based on hypotheses that invoke benefits of polyandry for females. We studied the evolution of monogamy/polyandry by using a species that has the genetic variation of the degree of polyandry among populations. This feature makes the crossing among populations with different levels of polyandry possible. We used the adzuki bean beetle (Callosobruchus chinensis). First, we examined the possibility that factors in one sex is responsible for the different levels of polyandry among populations. There were three possible case : only females are responsible, only males are responsible, and both are responsible. We also showed that there was a genetic variation of levels of polyandry among populations. Artificial selection experiments showed no correlated response in the rates of mating in two sexes. This suggested that the no genetic correlation in the rates of mating of two sexes. The direct benefit of polyandry for females was not shown. Several components of offspring fitness did not show the indirect benefit of polyandry for females. Males in the population with a higher level of polyandry showed a larger ability of sperm competition over males of the population with a lower level of polyandry. The level of polyandry was also affected by conditions including the time in a day and substrate for oviposition.