Researchers Michael Jennions and Vega Trejo. Image: Stuart Hay/ANU

The size of a penis doesn’t really make a difference.

When you are a fish.

Researchers at the Australian National University (ANU) have been looking at the breeding habits of fish to test the theory that bigger genitals make males more attractive or successful in 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,” says Michael Jennions from the ANU Research School of Biology.

The results contradict two previous studies which found that larger penis size had a positive relationship with fish paternity success.

In humans, research has found that women rate men with a larger penis as more attractive.

Professor Jennions says the latest study will lead to a greater understanding of the evolution of genitals. Male genital size vary a lot among species.

The research involved studying the mosquitofish, Gambusia holbrooki, which has a penis-like structure known as a gonopodium. The normal male gonopodium is equal to about 30% of the fish’s body length.

However, the team selectively bred males for eight generations to create some with larger gonopodia and some smaller.

The males were then allowed to freely compete to mate with females. The researchers then used DNA paternity testing to see which males were more successful at fathering offspring.

The size of the gonopodia made no difference to which fish successfully became fathers.

The research used a sample size of 173 males and 165 females and paternity tested over 2,250 offspring.

Mosquitofish have live offspring rather than lay eggs. They are considered a feral pest in Australia after they were introduced in the 1920s in a failed attempt to control mosquitoes.

The research is published in the journal Nature Communications.

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