Female Fish Grow Bigger Brains to Outsmart Horny Males, Researchers Say

Published in "The Royal Society," the study looked at mosquitofish, which are tiny (males measure at just over an inch) freshwater fish. To examine the different ways male harassment can create behavioral and cognitive changes in different animals, the researchers chose mosquitofish because of their unique mating habits.

Séverine Buechel, an evolutionary biologist at Stockholm University and author of the study, tells Broadly, "Males want to mate with as many females as possible to have the most offspring." Female mosquitofish, on the other hand, "want fewer matings, as they also have to ensure the survival of the offspring." In other words, the male fish do not court the females and most inseminations happen a result of forced copulations by the male fish.

To learn more about the sexual conflict and cognitive tools developed by females to avoid harassment, the researchers bred two different types of male mosquitofish with females. Previous studies concluded that male mosquitofish with larger genitals were better at coercing females. Because of the previous research, different male fish were artificially selected for their larger gonopodiums (a fin that also acts as a penis) while the other group had smaller ones. After breeding the fish for eight generations, scientists then looked at how both the male and female fish changed physically.

The results were surprising. The brain size of female fish paired with the male fish with larger genitals increased, but male brain size did not. Initially, Beuchel and her team expected both male and female brain sizes to increase because a male fish pursuing a female fish will be cognitively demanding for both. However, because only female fish saw an increase in brain size, the researchers suggest that avoiding sexual coercion requires more cognitive ability.

Most importantly, the study concluded that this research could lay the groundwork for future studies on evolution and sexual habits. According to the study, "Sexual conflict driven by male coercion is an important factor in brain anatomy evolution."

However, Beuchel insists that while this may be the case with mosquitofish, it's still too early to draw conclusions about any other species — humans included. "We will have to look into other mating systems where female mate choice and male coercion coexists to understand the implications of male coercion on brain evolution."

Mosquitofish aren't the only species that can evolve to facilitate sexual coercion. In a report published by the American Psychological Association that looked at the issue of rape through the lens of evolutionary psychology, researchers found that male scorpionflies "have a notal organ that is designed exclusively to facilitate sexual access to a female in a coercive fashion."