A team of researchers has found that female mosquitofish evolve larger brains when they’re exposed to males that are evolving larger genitalia. This phenomenon is caused by sexual conflict; males evolve larger genitals to be more successful at forced mating attempts while females become smarter to avoid these quarrels. The findings are in a paper that was just published in the journal Proceedings of the Royal Society B: Biological Sciences.
Sexual conflict occurs when both sexes in an animal species are using separate mating strategies that become antagonistic. For example, males might become persistent and forceful in their mating behaviors due to a lack of courtship rituals or pair-bonding. This persistence could harm the females, which may become hurt, tired, or unable to choose a healthy father for her offspring. So in turn, the females may adopt their own strategies such as the ability to flush out sperm or easily escape a male’s advances. Although this type of evolution is well-known, there has been little focus on how females evolve traits specifically to avoid male harassment and forced copulation.
Researchers from Stockholm University studied groups of eastern mosquitofish (Gambusia holbrooki). Male mosquitofish have a modified fin called a gonopodium that they use to inseminate females. They are very aggressive and often harass females to the point of exhaustion. The research team used artificial selection to breed males with very large gonopodia, giving them an edge in these coerced matings. The team also maintained normal control groups with no selected traits.
The researchers found that as the males evolved larger gonopodia, the females evolved larger brains. The females in the experimental enclosures had 6% more brain mass than the control groups. The team believes that larger brains could help the females avoid male advances. A larger brain could also help females strategize and find better males to fertilize their eggs.
The team’s findings show that sexual conflict can affect the evolution of both sexes in an animal species. In the experiment with eastern mosquitofish, females evolved larger brains in response to the males gaining larger genitalia. The extra brain mass might help females avoid male harassment to spend more time looking for food or healthy mates. Although often not as extreme, sexual conflict is common in the animal kingdom and understanding these examples will help scientists fully understand evolution.
Buechel et al. Artificial selection on male genitalia length alters female brain size. Proceedings of the Royal Society B: Biological Sciences (2016).