TGIPF: The Weird World of Banana Slug Sex: Redux


Ed. note: this was the first in a long and distinguished line of posts about, ahem, well, you’ll see.  It was published June 22, 2012. Some things are better the second time.

Today I have the honor of kicking off a new series on LWON, a series all about  . . . (wait for it) . . . penises! Are you excited? I know I am.

There’s only so much penis talk one can tolerate, of course, so this will be an occasional column. Don’t expect to get a penis every Friday, because you won’t. And I don’t want to hear any bellyaching. Without further ado, welcome to the first installment of Thank God It’s Penis Friday (TGIPF).

Two banana slugs

In the early 1900s, Harold Heath, a zoology professor at Stanford University, noticed something odd about the banana slugs his students were dissecting. Some of them lacked a penis. Others had stumpy penises that seemed “abnormally underdeveloped.” Banana slugs, like earthworms, are hermaphrodites, so every sexually mature adult should have had one. Yet some — about five percent — didn’t. How odd.

The penis of a full-grown banana slug is hard to miss. First, it’s enormous. Banana slugs grow to be 6 to 8 inches, and the slug’s erect penis can be the same length. These slugs are so well known for their massive genitals that one species, Ariolimax dolichophallus, carries a name that literally means long penis (dolichophallus). Here’s another fun fact. A banana slug’s penis emerges from its genital pore, which is on its head. (I’m not going to make a joke about dickheads here because I am not a fan of lowbrow humor).

Perhaps, Heath speculated, the slugs’ penises had been “cast off” and the stumpy bits he and his students observed were the first stages of penis regeneration. Heath wasn’t the first one to notice that some banana slugs were penisless, but he was hellbent to figure out why. He collected about 200 slugs and brought them to the lab. On a couple of occasions, the slugs seemed frisky, but “evidently complete union never took place since no young were produced,” he wrote. So he took to the field to observe slug love firsthand. (Warning: This post contains a photo of a slug penis. Proceed at your own risk.)

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. The biting was “unusually vigorous,” he added, “and within a very few minutes the penis was entirely severed.”

You can’t unsee a slug penis.

Why, he wondered, would they engage in this bizarre behavior? Perhaps, he posited in his 1916 paper on the topic, the amputated penis acts like plug that holds the sperm in. “On the other hand,” he noted, “it may be that the presence of a human observer or some natural enemy brings with it unusual conditions,” and the penis severing is a way for the slugs to separate quickly.

Scientists now call this phenomenon – slugs chewing off of one another’s penises – apophallation. (If you’re very brave, you can watch a video of the behavior here). Studies show that when apophallation occurs, both slugs often end up penisless. And, contrary to what Heath believed, slug penises do not seem to regenerate. Researchers now know a great deal about slugs and slug penises, and they’ve observed apophallation numerous times. But why it occurs is still something of a mystery.

One theory suggests that the slugs might do the chewing because they’re stuck. We’ve all heard stories of wolves chewing off a foot to escape a trap, so it makes some sense that slugs might chew their penises off to escape each other. But Brooke L.W. Miller, who researched the behavior when she was a graduate student, doesn’t buy that. In her dissertation she writes, “After an erection, the penis becomes limp (personal observation) and it seems highly unlikely that it could get ‘accidentally stuck.'”

Miller favors another theory: sperm competition. First, let’s look at non-hermaphrodites. From an evolutionary standpoint, every organism wants to produce as many offspring as possible. Females can mate with many males, and doing so may be advantageous. More partners means a better chance her eggs will get fertilized. But from the male’s perspective, the more partners a female has, the less likely he is to father her offspring. So natural selection “favors males that can prevent females from remating,” Miller writes. And males have developed elaborate strategies to ensure that their sperm — not the sperm of their competitors — wins. Male damselflies, for example, have penises that act like scrub brushes, sweeping away competitor’s sperm as they mate.

Sperm competition can happen in hermaphrodites like the banana slug too. And apophallation may be a way of outcompeting rival slugs. Bear with me, this gets a little complicated. Imagine you have two slugs, Taylor and Bailey. They mate, and then Taylor chews Bailey’s penis off. Bailey can still receive sperm. However, because Bailey doesn’t have a penis, he can’t fertilize anyone else’s eggs. So maybe he won’t seek out more mates. Or perhaps he seeks them, but can’t find anyone willing to mate with a penisless slug. Either way, that works out well for Taylor. Chewing off Bailey’s penis helped ensure that Taylor’s sperm will fertilize Bailey’s eggs, thus Taylor gains the upper hand. Miller’s research supports this idea, but it falls short of conclusive proof.

Of course, the decision to chew another slug’s penis off isn’t as clearcut as it might seem. When one slug begins chomping, the other typically responds in kind. And then they both lose in this evolutionary penis-chewing arms race.

Image of the mating slugs courtesy of Wikimedia commons, and the slug penis image comes from Brooke L.W. Miller’s web site: Home of The Slug Love


Share Button

15 thoughts on “TGIPF: The Weird World of Banana Slug Sex: Redux

  1. Ooooo–thanks for this cool piece! I love banana slugs! Find them unique and fascinating. They live where I grew up in CA.

    In fact, I’ve written a scene that involves a pair of mating banana slugs (and a pair of potentially mating humans) into my novel in progress. However, the scene in my novel isn’t as explicit as this article, in that I don’t describe banana-slug penises. That’s because when I watched a mating pair years ago, apparently I didn’t hang around long enough to observe that part of the act! Maybe I should rework the scene …

  2. Kathleen, thanks for your comment. It’s possible the pair you saw mating didn’t exhibit this behavior. It only happens in about one out of every 25 encounters or so. The video was bad enough. I can’t imagine seeing it firsthand.

  3. Does Miller mention anything in her dissertation about auto-apophallation? I could swear I’d heard of banana slugs chewing at their *own* nether regions on rare occasions (which could complicate the sperm competition theory), but I might be making that up.

  4. Robert, you’re absolutely right. They DO sometimes chew their own penises off (and that does complicate things). But I think that’s far less common than chewing each other’s off. Miller has a video on her web site if you want to see this behavior for yourself. She notes that, in this instance, mating ended and the slug seemed unable to retract his penis.

  5. Ah, great, cheers!

    I knew I’d seen that somewhere. Of course the one place I don’t look in my brief internet investigation is Miller’s web site itself…

Comments are closed.

Categorized in: Cassandra, Evolution, Nature, Sex, TGIPF

Tags: , , , , , , , ,