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Study finds some female fish evolve bigger brains when males have bigger genitals

The ConversationDespite what you might think, evolution rarely happens because something is good for a species. Instead, natural selection favours genetic variants that are good for the individuals that possess them. This leads to a much more complicated and messy world, with different selective forces pushing in many directions, even within a single species.

One prominent consequence of this is “sexual conflict”. This is the term evolutionary biologists use when one sex evolves a feature that gives benefits to the sex carrying it, but disadvantages the other sex, which in turn develops its own adaptations to counter this. Sexual conflict seems to explain some of the most bizarre manifestations of reproductive biology that we know of. The enormous, curly penises of some duck species or the tendency of male bed bugs to punch holes in their partners’ abdominal walls to inseminate them are good examples of this.

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.

Mosquitofish: surprisingly violent. Shutterstock

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 fertilising 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.

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