Some female fish evolve bigger brains as males develop larger genitals

For eastern mosquitofish, sex is far from consensual. To avoid harassment, research suggests female fish evolve larger brains. 

Brooks Hays

Sexual conflict among fish can drive the development of bigger females brains and larger male genitals. Photo by Alex Hofford/European Pressphoto Agency

New research suggests sexual conflict among fish can encourage both bigger brains and larger genitals.

In lab tests involving eastern mosquitofish, scientists found males with larger genitals triggered the development of bigger female brains.

Previous fish studies have shown the antagonistic relationship among predators and their prey inspires the evolution of larger brains. More recently, scientists from Sweden and Australia wanted to find out if the hostile nature of female-male relationship might yield similar results.

Scientists chose to study the eastern mosquitofish because of the unseemly nature of their copulation. For eastern mosquitofish, sex is far from consensual. Males force themselves upon females. They use a modified anal fin called a gonopodium to impregnate their victims. Fish with larger gonopodia are better able to terrorize and penetrate female fish.

In the lab, scientists bred one lineage of male fish for the trait of enlarged genitals. Another lineage was bred for small genitals. A third lineage was used as a control. Researchers released the different groups into tanks of female fish and allowed the populations to procreate.

After nine generations of evolution, researchers measured the brain sizes of female fish. The brains of females subjected to males with longer gonopodia were 6 percent heavier than the brains of the other two groups.

Researchers described their work in a new paper published this week in the journal Proceedings of the Royal Society B.