Big tackle a drag for male fish

It's a winner with the ladies, but having a big manhood is a drag for male fish and increases their likelihood of being eaten, new research shows.

United States scientists reporting in the Proceedings of the National Academy of Sciences say there may be an evolutionary trade-off between the ability to pull females and out-swim predators.

Brian Langerhans from the Department of Biology at Washington University and colleagues studied female preference and swimming performance in mosquitofish in Texas and the Bahamas.

Male mosquitofish transfer sperm to females through modified anal fins, which they display during courtship.

Researchers found the fish had bigger sperm-transfer fins in environments where there were no predators.

Their genitals were smaller when they were under threat of being eaten.

Researchers also showed female mosquitofish videos of males with digitally enhanced genitals.

They found the females were drawn to the enhanced males rather than those with average sized genitals.

Scientists said the slowness of the well-endowed fish was caused by greater drag in the water, which reduced their ability to dodge predators with swimming bursts.

"This swimming burst is very important in avoiding predation, and males possessing relatively large gonopodia might experience reduced burst-swimming speed owing to the enhanced drag generated by the large genitalia," they report.

Dr Rob Brooks, an expert on animal evolution and sexual selection from the University of NSW, said the research illustrated the opposing forces of evolution.

Without an evolutionary downside to having large genitalia, the male mosquitofish would simply end up with impracticably massive sex organs, Dr Brooks said.

"In this case, it seems like males doing well by having large genitalia ... are not quite as effective swimmers.

"These opposing factors are very important in evolution because it prevents things getting over-exaggerated."

- AAP