The tiny eastern mosquitofish, indigenous to the southern and eastern US, is unlike much of the rest of the animal kingdom when it comes to reproduction, starting with the differing objectives of the females and males of the species. Because they have to bear the burden of actually carrying the offspring, the females are typically picky about potential partners. But since the males don't have that problem, they're out to get busy as often as possible—except instead of trying to seduce females, the males attack, sometimes up to 1,000 times a day, reports Live Science. Each male uses its gonopodium, an anal fin that's the fish version of a penis and which is inserted into the female to fertilize the eggs inside her body (most other fish use an external fertilization process called spawning, releasing eggs and sperm into the water). Now researchers report in the Proceedings of the Royal Society B that when faced with males whose gonopodia are large, the brains of females get bigger.
It's all part of what's known in evolutionary biology circles as "sexual conflict," where one sex has evolved a feature that's not mutually beneficial, so the other sex evolves some countermeasure, reports Phys.org. In the case of mosquitofish, when scientists artificially selected for males with larger genitalia, females responded by getting smarter as they worked to avoid the attacking males and maintain some semblance of selection—so much so that in just nine generations their brains were 6.5% heavier than females thrown in with males sporting smaller genitals. Male brains, meanwhile, didn't change in size, which surprised the researchers, who thought chasing the females would be similarly "cognitively demanding." The findings hint at the possibility that sexual conflict can in fact lead to the evolution of bigger brains, though whether this applies to other species is for future research. (Our brains appear to be part male and part female.)