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How Penis Length Affects Performance

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Caroline Reid

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207 How Penis Length Affects Performance
Seed bugs mating by Katarina Christenson via Shutterstock

Scientists chop off a portion of an insect's penis and guess what? It doesn't work as well.

The little Lygaeus simulans insect, or seed bug, is 11 millimeters long. It struts around while dragging around a seven-millimeter-long penis. The penis is about two-thirds as big as the bug itself, which isn't uncommon in the insect world. However, the female's spermathecal duct (insect equivalent of a vagina) is far too short to harbor the full length of the penis. In fact, it's only a third of the length.

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So is the male insect just making up for something? Or is there a purpose for this extra length? To find out if the seed bug's long penis was a product of selective breeding or just an evolutionary anomaly, the scientists cut off a portion to see how the insect's breeding habits changed.

It must be noted that the penis consists of a long, thin tube with no obvious muscles or blood vessels. Therefore, the insect probably didn't experience any pain, just humiliation. Or maybe courage in the face of a challenge.

“We can show that males that had the penis cut, but not reduced in length significantly, had the same reproductive success as males with normal penises,” says Liam Dougherty, a postgraduate researcher at the University of St Andrews and lead author on the paper. This was the equivalent to a tiny insect circumcision. The real damage was done when the penis was reduced by anything more than this.

If the penis was shortened by 5% of its original length, then this significantly reduced the number of offspring produced by sexual intercourse. If the penis was reduced by a drastic 30%, then the male insects seemed less interested in copulating; they spent half as much time fornicating, and they struggled to inseminate females successfully. As a result, they fathered significantly less offspring.

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“By showing that the length of the penis influences reproductive success, we can say that it has been sexually selected to some extent,” says Dougherty.

CT scans of copulating seed bugs have revealed that the male's penis does some impressive coiling inside the spermathecal duct. The penis needs to be long enough to reach the female's spermatheca—her sperm storage organ. However, it now seems that the coiling penis has a higher purpose than just dramatic flair.

[Via Proceedings of The Royal Society B]


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