You might think it's grand to be a well endowed fish. After all, some female fish prefer mates with larger sex organs, a new study finds.
But the guys' prowess has a price.
The studs with larger gonopodia, which is what scientists call male fish sex organs, can't swim as fast as their less impressive counterparts, so they're more likely to get eaten by predators.
Bigger is better
The study was done on mosquitofish, which are like guppies. They're only about an inch long. That's body length. For the appendage, we're talking millimeters. Nonetheless, biologist Brian Langerhans of Washington University in St. Louis managed to put a tape on 350 male mosquitofish. Langerhans took pictures of the gonopodia to measure their outlines.
"The organ is quite obvious, even on such small fish," he told LiveScience.
Data in hand, Langerhans exposed about 50 females, one at a time, to video images of a male of average proportions at one end of an aquarium and an outsized male at the other end.
"They chose the larger one over and over," Langerhans said. "All females had the same preference."
Mosquitofish bear their young live, bypassing the whole egg-in-the-gravel hassle. Among such livebearing fish species, gonopodia range from less than 20 percent of a fish's body length to more than 70 percent. Don't ponder that too long, but trust that it fits into an evolutionary puzzle that spawned this study.
But first, what exactly is a gonopodium?
"In the sense that gonopodia are sperm-transfer organs, they are analogous to a mammal penis," Langerhans explained. "They evolved independently, but they serve the same copulatory function. The gonopodium must be inserted into the female gonopore, and then eject the sperm into the female body, in order to achieve insemination."
The study involved mosquitofish from two places, one where there were predators and one without.
"A male with a larger gonopodium has a higher chance of mating, but in a predator environment he has a higher probability of dying," Langerhans said.
So, for the sake of argument, let's say there are no predators around. What does evolution do?
"We found that in predator-free environments gonopodia size was larger, as there is minimal cost for large genitalia in that environment," Langerhans said. For the record, the sex organs of the predator-free guppies were 15 percent longer, on average.
The results are detailed this week in the online issue of the Proceedings of the National Academy of Sciences.
Why size matters
There's a larger point to this research. Male genitalia, scientists tell us, come in many shapes and sizes, with more variety than most body parts. These differences are sometimes the best way to distinguish one species from another.
For years, experts have figured that this remarkable diversity in genitalia had to do with sperm competition or some other after-the-act effect. The new study shows that female fish, like women, make some decisions beforehand -- conscious or not -- about the physical dimensions of the fathers of their children.
"Overwhelmingly that choice is made with size being the prize," Langerhans and his colleague report on the little guppies.
So perhaps, the logic goes, differences in male genital shape between populations lead to "reproductive incompatibility," which means two groups would split and become separate species. Langerhans is now looking into this possibility in other creatures.
He also plans to investigate whether a preference for large gonopodia caused male swordtail fish to develop their long, showy tails, which serve about as much everyday purpose as large biceps on an insurance salesman.
"Male ornamentation of the tail fin may have evolved largely due to the pre-existing preference for an elongate structure of a similar shape -- the gonopodium," Langerhans said.
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Robert is an independent health and science journalist and writer based in Phoenix, Arizona. He is a former editor-in-chief of Live Science with over 20 years of experience as a reporter and editor. He has worked on websites such as Space.com and Tom's Guide, and is a contributor on Medium, covering how we age and how to optimize the mind and body through time. He has a journalism degree from Humboldt State University in California.