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Now a new study has suggested that, in some species, this conflict between the sexes can have some surprising results. Specifically, avoiding conflict with males can cause females to evolve bigger brains.

To understand the effects of sexual conflict, sometimes it can help to think about evolution in other antagonistic systems. In 2010, Japanese researcher Michio Kondoh showed that brain size evolution can depend on predator-prey conflicts.

Both avoiding predators and catching prey demand brain power. By studying several hundred species of fish, Kondoh showed that prey eaten by large-brained predators tend to have larger brains themselves. It seems that both predator and prey tend to evolve towards higher cognitive functioning to give themselves an edge in their competition.

Recently, a team of Swedish and Australian researchers led by Séverine Buechel from Stockholm University, noticed that predator-prey conflict is, in some ways, like sexual conflict. This is because it features two antagonistic partners constantly evolving to better outwit the other. The researchers wondered if, like predator-prey conflict, sexual conflict might also affect the evolution of brain size.

To test this idea, they carried out a laboratory evolution experiment using a fish called the eastern mosquitofish, a relative of the guppy originally found in the Southern US. Male mosquitofish are particularly unpleasant. Unlike many fish, these animals reproduce by fertilizing eggs inside the female’s body. But instead of wooing a female and trying to impress her with his prowess, the male mosquitofish simply sneaks up on her and tries to force her to mate.