Male Fish With Small Genitals Are in Luck

May 19, 2016 | Erica Tennenhouse

Photo credit: Stuart Hay, ANU

A new study finds females don’t care.

Mosquitofish males no longer need to fret about the size of their gonopodia — the penis-like structures that they use to inseminate females.

Researchers at the Australian National University reported in the journal Nature Communications that males of this species who have bigger genitals are no more attractive to females, and no more successful at fathering offspring.

"Our findings show the size of male genitals has no effect on their attractiveness, success in reproduction, or their ability to swim and move around in the water," study co-author Michael Jennions said in a press release.

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These results contrast previous findings by Jennions that fish with larger penis size were better at inseminating females. Human research has also shown that women rate men with a larger penis as more attractive.

A normal male mosquitofish gonopodium is equal to about 30 percent of his body length.

The researchers selectively bred mosquitofish over eight generations to produce some males with larger gonopodia and some with smaller gonopodia. No previous studies have used artificial selection to alter genital size and test the effects on reproductive success.

Males with different sized genitals were allowed to compete to mate with females, and the researchers conducted DNA paternity tests to see which males had fathered more offspring.

"To our surprise, we found the size of the gonopodia made no difference to which fish successfully became fathers," said Megan Head, co-author of the study.

Having a large gonopodium also did not hamper swimming speed for males.

Though the results were unexpected, the authors note that it’s often challenging for scientists to publish unexpected findings, making it hard to judge just how unusual their results really are.

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