The Romantic Lives of Banana Slugs

Fair warning: The mating rituals of banana slugs are somewhat (read “unbelievably”) unusual and there will be things in this post you can’t unlearn. I’m telling you this because I’ve seen it — ALL of it — and I’ll seriously never look at the redwood forest the same way again.

For those of you who’ve never seen one up close, Pacific banana slugs (Ariolimax) are the second-largest slug in the world at up to 9.8 inches long, and one of the slowest species on the planet with a whopping maximum speed of 6.5 inches per minute. They get their name from their coloring, similar to that of a ripe banana although they’re frequently “overripe” with large, speckled brown spots. Complete with one lung and no spine, you can see why this brightly colored creature seems to take its sweet time moving around our redwood forests.

With the return of our winter rains, these moisture-dependent slugs are once again moving safely across the forest floor, looking for things to eat and that special slug with whom they can make adorable slug babies. That’s right, it’s the season for these redwood-dwelling gastropods and the love, my friends, is in the slime.

Banana slugs are the world’s second largest slug and, although not the most exalted, are a critically important member of the redwood forest community.

The Love Network

Like all aficionados in the language of love, banana slugs are smooth operators. Oozing from their skin are dry granules of mucus (think of it like rice), which absorb the surrounding water found under the damp canopy of massive redwoods. One granule can absorb several hundred times its volume in water, coating the slug’s skin in a handsome layer of slime that helps keep it from drying out. But this slime is more than just fashionable for slugs, from their locomotion, to predator defense (it’ll numb your tongue!), it’s their do-it-all secret sauce.

If you look closely, you can see the trails of slime residue they leave behind as they scoot around the forest floor. During the mating season, they spice things up by adding pheromones to these slime trails, like little love notes to other slugs that may cross their path. Think of it as a giant, dating network for slugs to conveniently search for and find mates with “just the right chemistry.”

Love at First Bite

After finding each other through the slime network, banana slugs begin their somewhat complicated and painful courtships. Sounds familiar right? Like many great love affairs, they start on the dance floor. But as slugs encircle each other, they lunge and bite and smack each other with their tails as a way of ensuring that they’re the same species and a good match for what comes next. Toward the end of this “dance” both slugs may also display their sexual organs before they eventually intertwine.

Second warning: If you’re prone to awkwardly laughing like a sixth grader at the mention of male genitalia you may want to skip this next part.

The Other “Distinguishing Feature”

You may have already heard about this slug’s other “distinguishing feature” as it has a bit of a reputation in the animal kingdom. A banana slugs’ male genitalia can be as long as its entire body, making its penis to body ratio one of the largest on the planet. I’ll stop there while you try to unpack this fun fact. Ok, ready? These slugs are so well known for the size of their male genitals that one species, Ariolimax dolichophallus, even carries a name that literally means “long penis” (dolichophallus).

A Romantic Meal

Though its male genital gets all of the press, banana slugs are, in fact, hermaphroditic, meaning they have both male and female genitalia. In extreme cases, a slug can make itself pregnant — though this is very uncommon and doesn’t make for as fun a story. Remember all of that lunging, biting and smacking that makes up the mating dance? Well after that somewhat vicious courtship, the slugs finally let nature take its course and take turns swapping sperm from one to the other over the course of several hours. Yes, I said hours! After all of this appetite-inducing activity, they find themselves in a giant puddle of their own slime which, in slug-land, makes the perfect post-copulation snack.

A is for Apophallation 

In most cases, after intercourse both of the slugs become pregnant and cordially withdraw to go their separate ways maybe promising to call each other on the slime network in few days. But occasionally, to my horrific surprise, things don’t go so smoothly.

Sometimes, both penises can become stuck and in a seemingly intense act of desperation, the slugs begin gnawing at the other member’s penis. It’s an act known scientifically as “apophallation” — when one slug starts chewing, the other follows suit in arguably the most devastating quid pro quo in the animal world. I wish I were kidding, but that’s quite literally the painful truth and, if you have the stomach for it, you can watch the whole thing here.

Science is still looking for an answer to this unusual behavior. There are several theories based on nutrition, or survival of the offspring, but whatever the case, the decision to chew off another slugs’ penis isn’t yet clearcut.

Still with me? I hope so because the closer we look at nature, the more it promises to surprise and inspire a deeper curiosity. The wild world of banana slug romance and procreation is no exception.

Next Generation

After what now may sound more like warfare than a romantic encounter, the expectant slugs search for an appropriate place to lay their fertilized eggs, typically under a chunk of bark or a small hole in the ground. They can lay 20 to 30 eggs in one clutch, which are abandoned once laid. About a month later, the baby slugs emerge — usually pale and white — maturing within a few months to often look identical to their parents (who must be so proud). And, just like that, the cycle begins again with an enthusiastic class of young slugs hitting the slime-based dating network.

Fertilizing the Future

Although they’re not the most exalted, banana slugs are critically important members of the redwood forest community. As decomposers, they eat dead plant material, animal droppings and just about anything they can get their slime on. And what goes in comes back out as high-octane plant food with the nutrients needed to, in turn, feed insects, birds and deer, right on up the food chain to apex predators like mountain lions.

When it comes to banana slugs, love truly does make the forest world go ’round. However small, these slow, juicy creatures have a big story to tell and, like you, I’ll never look at them quite the same way again.

Learn more about our role protecting the redwood forest habitats in the Santa Cruz Mountains.