Penises come in all sorts of shapes and sizes. Some are lobed and paired, some are highly maneuverable, and at least one allows an animal to inject sperm into its own head. Well there’s a diversity of female genitalia, too. Researchers studying mosquitofish have discovered that females developed genital openings of different sizes and shapes as a response to the presence of predators and also to block unwanted males from trying to mate with them. The findings were published in Evolution last week.
There are multiple, non-mutually exclusive mechanisms that could lead to female genital divergence. According to the “ecology hypothesis,” for instance, variations in the surroundings alter the context of sexual selection. The “lock-and-key hypothesis,” on the other hand, focuses on avoiding inter-population mating: Females shape their genitalia to promote copulation with desired males of their own population or species only.
When North Carolina State University’s Christopher Anderson and Brian Langerhans tested some of these hypotheses using Bahamas mosquitofish (Gambusia hubbsi) living in water-filled vertical caves (called blue holes) that vary in predation risk, they found unequivocal support for the ecology hypothesis. Females in blue holes containing fish-eating fish had a smaller genital opening; females living without the threat of predation had a larger, more oval-shaped genital opening.
Their findings also back the lock-and-key hypothesis: Females evolved differently-shaped genitalia to deter mating attempts by males from other populations. Previous work revealed that male mosquitofish have more bony, elongated genitalia when living among predators. “When predators lurk nearby, male fish must attempt to copulate more frequently – and more hurriedly – with females,” Langerhans says in a statement. “So females have evolved a way to make copulation more difficult for unwanted males.”
By having a “lock” that only works with a well-suited male’s “key,” females avoid hybridizing with maladapted populations and other species. After all, interbreeding can result in sterile offspring. “Populations with greater evidence of interbreeding had more divergently shaped genitalia in both males and females,” Anderson adds. “This suggests that genitalia can evolve, at least partly, to reduce hybridization and thus could be involved in the formation of new species.”
Female and male mosquitofish genitalia in different Bahamian locations show contrasts between living in waters with and without the threat of predation. Christopher Anderson/NC State University.