Banana slugs can grow up to 10 inches long, and their penises—which emerge from their heads—can be equally impressive. After mating, one slug sometimes gnaws off its partner's penis in a bizarre process called apophallation.
Banana Slugs Have Head Penises and Chew Them Off
If you thought your love life was complicated, spare a thought for the banana slug. These bright yellow gastropods—native to the Pacific Northwest's misty forests—have one of the most spectacularly weird reproductive systems in the animal kingdom.
Let's start with the basics: banana slugs are hermaphrodites, meaning each individual has both male and female reproductive organs. You'd think this would make finding a mate easier. You'd be wrong.
The Head Penis Situation
A banana slug's penis doesn't emerge from where you might expect. It unfurls from a pore located on the side of its head, just behind the right tentacle. When erect, this organ can stretch to the full length of the slug's body—up to 10 inches.
For perspective, that's proportionally like a human having a six-foot-long member. Scientists call banana slugs one of the most well-endowed creatures relative to body size in the entire animal kingdom.
Then Things Get Really Strange
Banana slug mating is a marathon, not a sprint. The process can take several hours as the slugs exchange sperm with each other. They coil together, covered in slime, in what researchers describe as an elaborate and slow-motion dance.
But here's where it gets dark.
After mating, something called apophallation sometimes occurs. One slug will begin gnawing on its partner's penis—and won't stop until it's completely severed. Scientists aren't entirely sure why this happens, but theories include:
- The penis gets stuck due to size mismatch or dried mucus
- It prevents the other slug from mating again as a male
- It may simply be easier than untangling
The slug that loses its penis isn't necessarily doomed. Since banana slugs are hermaphrodites, the de-penised slug can still reproduce—it just has to play the female role for the rest of its life.
Why So Big?
The evolutionary advantage of such an enormous organ likely comes down to sperm competition. A longer penis can deposit sperm deeper, potentially giving that slug's genetic material a better chance of fertilizing eggs. In the slow-motion world of slug reproduction, every advantage counts.
Banana slugs are also surprisingly important to their ecosystem. They're decomposers extraordinaire, munching through dead leaves, animal droppings, and decaying plant matter. Their slime even contains an anesthetic, which is why some indigenous peoples reportedly licked them to numb toothaches.
A Mascot With Baggage
Perhaps most famously, the banana slug is the official mascot of UC Santa Cruz. Students chose it in 1986, rejecting the more conventional sea lion. It's become a point of pride—a celebration of the weird, the slow, and the gloriously unconventional.
Just maybe don't think too hard about the apophallation thing during graduation ceremonies.