Fishy genital balancing act

Biology Tuesday, May 10, 2005. Post by David Bradley

Diversity of male genital size in animals with internal fertilization has intrigued scientists for decades. Most evolutionary explanations they have come up with centered on post-mating sexual selection, such as sperm competition advantages. On the other hand, the roles of pre-mating sexual selection and natural selection have been less well studied.

Now, biologist Brian Langerhans and colleagues at Washington University, in St. Louis, Missouri, have investigated female preference and swimming performance in western and Bahamas mosquitofish, (picture available via this link).

Mosquitofish cannot retract their gonopodia – modified anal fins through which males transfer sperm to females – and they often display or swing them during courtship. In laboratory experiments, females preferred to spend time with videos [Is this the first example of piscine porn? Ed.]of males displaying digitally enlarged gonopodia rather than those with average-sized gonopodia, which implies 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, an effect perhaps due to greater drag in the water. These results suggest that alternative mechanisms, such as mating selection favoring larger genitalia and natural selection favoring reduced size, may also drive genital diversification.

SOURCE: PNAS Piscine Press Release

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