Does Size Matter? Not To Mosquitofish, It Doesn’t

Contrary to previous studies—and popular belief—genital size does not affect a male mosquitofish’s ability to attract a mate.

AsianScientist (May 24, 2016) – When it comes to the size of male genitals, big isn’t always better, according to researchers in Australia.

The research team from the Australian National University (ANU) looked at the breeding habits of fish to test the theory that bigger genitals make males more attractive or successful in fathering offspring. They found that female fish don’t find males with big genitals any more attractive than those with normal-sized or smaller genitals. They published these findings in Nature Communications.

“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,” said researcher Professor Michael Jennions from the ANU Research School of Biology.

The findings contradict two previous ANU studies which found that larger penis size had a positive relationship with fish paternity success, and research in humans which found women rate men with a larger penis as more attractive.

Jennions said the present study would lead to a greater understanding of the evolution of genitals, particularly since male genital size varies hugely among species.

The research involved studying male genital size of Gambusia holbrooki, or the mosquitofish, which has a penis-like structure known as a gonopodium. The normal male gonopodium is equivalent to about 30 percent of the mosquitofish’s body length. However, the team selectively bred male mosquitofish for eight generations to create some fish with larger and some with smaller gonopodia.

The males with different size genitals were then allowed to freely compete to mate with females. The researchers used a sample size of 173 males and 165 females, and then paternity tested over 2,250 offspring to see which males were more successful at fathering offspring. Unusually, instead of laying eggs, mosquitofish have live offspring.

“To our surprise, we found the size of the gonopodia made no difference to which fish successfully became fathers,” said researcher Dr. Megan Head, also from the ANU Research School of Biology.



The article can be found at: Brooksmythe et al. (2016) Fitness Consequences of Artificial Selection on Relative Male Genital Size.

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Source: Australian National University.
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