Banana slugs are hermaphrodites. Every slug has both a penis (which pops out of a pore on its head, like you do) and a vagina. Or, rather, every slug should have a penis. The truth is that quite a few of them don't and the story behind that discrepancy is rather strange and horrifying. Since there's little I love more than strange and horrifying stories from nature, you get to hear all about it.
At The Last Word On Nothing, Cassandra Willyard tells the story of a nearly 100-year-old effort by scientists to understand why some banana slugs appear to be missing their penises, or have penises that are stunted. We have known since 1916 how those penises came to be missing. Willyard describes the situation, which you can also watch in action in the video above:
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Banana slugs begin their mating with a few vicious love nips. Then the animals curl around each other, forming a bright yellow yin-yang symbol. Next, they insert their penises. (Remember, they both have one.) In some cases, one slug provides sperm and the other slug receives it. More often, the slugs swap sperm. Copulation can last many hours. Then, in most cases, the slugs withdraw and part ways.
Heath caught a couple of slugs in the act. He noted the biting and the insertion. And then Heath observed something puzzling. As the slugs were withdrawing their penises, “one of the animals turned its head and commenced to gnaw upon the walls of the organ,” Heath wrote.