Variation in the size of male genitalia in animals that have internal fertilization has long been of interest. While competitive advantages of sperm robustness and other post-mating sexual selection criteria have been widely studied and posed as evolutionary explanations, the roles of pre-mating sexual selection and natural selection have been less well studied.
Post-doctoral fellow Craig A. Layman in Yale's Department of Ecology and Evolutionary Biology, working with lead investigator Brian Langerhans at Washington University and Thomas J. DeWitt at Texas A&M University, analyzed the gonopodia, or modified fin that transfers sperm to females, in western and Bahamas mosquitofish. Mosquitofish cannot retract their gonopodia and they often display or swing them during courtship.
"Our results suggest that both mating selection, favoring larger genitalia, and natural selection, favoring reduced size, may direct evolution and diversification of genitals," said Layman.
The researchers studied female preference and male swimming performance in these two species of mosquitofish. In laboratory experiments, females preferred to spend time with videos of males displaying digitally enlarged gonopodia rather than those with average-sized gonopodia, implying sexual selection for increased gonopodium size. Yet the researchers also found that males with larger gonopodia were slower to escape potential predators through evasive swimming bursts.