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For Fish, Size Doesn’t Matter After All

While studying the mating behaviors of several species of fish, researchers at the Australian National University have found that when it comes to fish, it’s not the size of the boat that matters, but the motion of the ocean. Biologists at the ANU Research School of Biology were collecting data on whether or not female fish prefer male mating partners with larger genitals. To the dismay of male enhancement pill manufacturers everywhere, the team found that female fish do not, after all, seek out mates who are more well-endowed than others.

The team’s findings, published in the journal Nature Communications, detail the methodology used to reach such a conclusion. The researchers selectively bred eight generations of the minuscule Gambusia holbrooki, otherwise known as the mosquitofish, to create nine separate lines of 540 females.

Gambusia holbrooki, the mosquitofish

Gambusia holbrooki, the mosquitofish

After dividing the females into separate tanks, males derived from wild-caught specimens were introduced – after having their sex organs, or gonopodium, carefully photographed (no fish were shamed in the making of these photographs; the team had an animal ethics permit from the Australian National University Animal Ethics board). The researchers then meticulously observed and photographed the mating behaviors of the fish to determine if gonopodium size had an effect on the mating success of the different male specimens.

The female fish were observed to select males across a broad range of gonopodium sizes, implying that female mosquitofish do not have a preference for genital size in their mating partners. According to the team behind this study, such a finding goes against some research about animal mating behaviors and presents a problem of contradictory results:

The lack of evidence for selection against deviations from the natural line of allometry in our study is therefore a genuine conundrum. Unfortunately, difficulties in reporting unexpected findings lead to well-known publication bias that systematically distorts science. As such, it is difficult to assess whether our results are genuinely anomalous or reflect a larger file drawer problem in evolutionary biology.

Thus, while these female fish do not seem to select male mates based on their level of endowment, the implications of the results are somewhat difficult to assess. Similar studies in humans have found that, unlike fish, smaller genital size in male humans is tied with reduced sperm health. Oh, to be a fish.