Battle of the sexes Stuart Hay/ANU

Size matters. Bigger genitals mean more mating success for male mosquito fish, a relative of the guppy. But the development of longer male organs prompts females to evolve bigger brains to help them escape overeager mates.

Mating among mosquito fish is far from romantic. The male makes no effort to court partners, instead sneaking up and attempting to copulate by force up to a thousand times a day. It uses a modified anal fin, the gonopodium, to deliver sperm into the female.

In this sort of mating system, the relationship between males and females can resemble that between predators and prey, which commonly involve an evolutionary arms race where adaptations on one side are closely matched by changes on the other. For example, big-brained predators tend to prey on big-brained prey, as the two try to outsmart each other.


Séverine Buechel and colleagues at Stockholm University in Sweden wondered if a similar arms race was going on between male and female mosquito fish. Do females evolve bigger brains to defend against sneaky males, and do males evolve bigger brains in response?

To test this, the team looked at what happened to brain size when males were bred to have longer gonopodia. Male mosquito fish have long gonopodia compared with related species in which coercion is not the dominant mating strategy, and males with longer gonopodia tend to be more successful at mating.

The researchers found that breeding more well-endowed males led to bigger-brained females. But there was no arms race: male brains didn’t get bigger at the same time.

Sexual harassment

Harassment by males poses many risks for females: injury, disease transmission and lower survival rates for offspring, for example. Having a larger brain might help females see better or pay more attention to their surroundings, making it easier for them to spot males and get away.

Mating systems are an overlooked factor in brain evolution, says Buechel. This has been studied more in species in which females exert some choice over whom they mate with and when.

For example, bigger brains help male bowerbirds win mates by building more complex nests, and unpublished work by Buechel’s group suggests that choosing a mate can be cognitively demanding for females too.

“Depending on what mating strategy has been adopted, brain evolution might have gone in different directions,” says Buechel.

It is an interesting idea, but we can’t say for sure if selection for bigger-brained females happens in the wild, says Jonathan Evans at the University of Western Australia.

The next step might be to show that females with larger brains really are better at avoiding male mating attempts. “That would be a simple but quite convincing bit of evidence that this sex-specific selection for brain size is indeed because of sexual conflict,” he says.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2016.1796