TGIPF: Slug Sex Redux


The Slug

Slugs have sex. You probably already knew that. And if you read my last post on banana slugs’ strange sexual appetites, you had the rare opportunity to see a slug penis. So you already know what the organ looks like.

Limax maximus, the leopard slug. (And my hand for scale).

Except you don’t! That was a trick statement. You know what a banana slug’s penis looks like. What about the penis of Limax maxiumus, the leopard slug? Can you picture it?

I’m going to bet 99% of you are picturing something that looks nothing like the real thing. Because the real thing looks nothing like a penis. It looks like a canoe paddle. It looks like a tiny, translucent flower. It looks like a spaceship. In other words, it looks decidedly un-phallic.

Is that what you were picturing? I didn’t think so. In fact, the organ is so un-penis-like, when naturalist Lionel E. Adams observed a pair of these slugs mating in the summer of 1897, he had no earthly idea what the structure was. “Being anxious to determine the identity of the organs extruded, I severed the parts in question from a paired couple and submitted them and the animals to Mr. W. M. Webb, F.L.S., for dissection,” he writes in his 1898 article “Observations on the Pairings of Limax Maximus.” Webb conducted the dissection and wrote back. It’s a penis, my dear chap! (Not Webb’s exact words.)

And let me be clear, the sketch above depicts two penises. It shows how they intertwine and change shape during copulation. But I’m getting ahead of myself. Let’s back up so that I can describe the acrobatic maneuvers that occur when this impressive organ is put to use.

Leopard slug courtship begins with a single slug laying down a trail of scented slime. Another slug picks up the scent and begins to follow, nibbling and nipping. Then the slugs begin to climb, one leading the other up a tree or wall. When they reach an overhang, the slugs align their bodies tail to mouth to form a circle. I’ll let Adams take over for a bit. “The circle now grows more contracted, the slugs overlapping and showing evident excitement, the mantles flapping before and behind,” he writes. “Then, suddenly, the slugs intertwine fiercely, and launch themselves into space, heads downwards, but suspended by a thick strand of mucus.” That’s right. Leopard slugs mate while hanging on a thread made of mucus. It just doesn’t get kinkier than that, folks.

Once the slugs are dangling midair, their penises emerge from a genital pore near their heads. Slugs are hermaphrodites, so they both have one. “The organ, cylindrical at first, quickly assumes a club-shape,” Adams writes. “But presently a frilled edge appears along one side.” Banana slugs, you may remember, insert their penises into one another. Leopard slugs, on the other hand, wrap their penises tightly together so that the ends touch. The upper part of this penis knot expands to form “a mushroom or umbrella.”

The two intertwined slugs and their intertwined penises dangle motionless for five to ten minutes, swapping sperm, “and in this extraordinary manner the mutual act is consummated,” Adams writes. Then the slugs untwist and go their separate ways, in some cases climbing up the mucus string, ingesting it as they go. The whole process can take hours.

But perhaps you’d rather have Sir David Attenborough walk you through the process. [Seriously. You must watch this video.]

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Why? Why go to such great lengths to breed? Why not do it like banana slugs — swap sperm on the ground, chew each other’s penises off, and call it a day? Adams speculated that the mucus string keeps them safe from predators while they mate. But that explanation doesn’t feel very satisfying. After all, other slugs have predators too. And surely birds could pick off dangling slugs just as well as they can pick off slugs on the ground. Unfortunately an (admittedly cursory) scan of the literature didn’t turn up other hypotheses.

For scientists who study slugs, called malacologists, this elaborate mating behavior is old hat. But I’ve never seen it (or heard of it) before. So I think it’s pretty much the coolest thing ever. And I’m not the only one. Robin Rosetta, a self-proclaimed “novice malacologist” at Oregon State University and the author of an article on slug control titled “Slime and Punishment,” [PDF] had the good fortune to catch two leopard slugs in the act several years ago. “Anyone who has ever witnessed this courtship in person will likely never be the same,” she told me in an email.

Boring old slimeball slugs are engaged in elaborate sexual hijinks under our very noses. What else have we missed? I feel awestruck, an emotion I imagine Adams felt seeing this behavior for the first time. It’s proof that you don’t have to be a scientist to experience the thrill of discovery.


Image credits:

Photos for the slug animation and the slug with hand shot by Cassandra Willyard.

Slug penis sketches courtesy of J.W. Taylor via Wikimedia Commons

Slugs mating courtesy of Petter Haugen on Flickr


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4 thoughts on “TGIPF: Slug Sex Redux

  1. Such a lovely essay. Nature is truly fascinating. The mating ritual is, well, very beautiful. slugs? I had no idea. I went to UC Santa Cruz for graduate school in astronomy. It was very common to see banana slugs in the redwood forest. Their sex dance is also amazing as shown in one of your links, but it is hard not to think about the “apophallation” (great word) part if you are a guy and think “yuk…”

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