If the genitals of male fish are large, females of some species can develop bigger brains to enable them to avoid sexual harassment.
This certainly seems to be the case in the Eastern mosquitofish, Gambusia holbrooki — a livebearing species in which sex is far from consensual, with males forcing themselves on females using their modified anal fin (gonopodium). This can make life difficult for females as they are constantly harassed and are unable to select the best males. Even feeding can be a challenge when you are constantly being sneaked up on from behind. Those males with larger genitalia are more successful in breeding with unwilling partners.
Now scientists from the University of Stockholm and the Australian National University have discovered that females of this species develop bigger brains if they are subjected to continuous harassment by more well-endowed males.
In their experiments the scientists used three groups of fish — one line bred for longer gonopodia, a second group line bred for shorter gonopodia, and a third control group where the research did not select for size.
After nine generations, the brains of the female fish kept with the more well-endowed males were found to be up to 6% heavier than those of females kept alongside males with smaller genitalia.
What was particularly interesting was that the females with bigger brains were better at reproducing. It’s unclear whether this was because they were more able to avoid unwanted attention, leading to them being less harassed and so able to carry on with normal day-to-day things such as eating — or whether it’s because they were better placed to select the best males to reproduce with and avoid those of lesser quality.