Banana Slugs

For such an innocuous forest dweller, the banana slug (a mollusk in the genus Ariolimax ) harbors some wildly bizarre secrets. Of course, a few normal factoids apply—they grow to almost 10 inches long, can travel 6 ½ inches per minute and maintain a diet of leaves, dead plants, mushrooms and animal droppings. Banana slugs are the second largest slug in the world and are found in the western coastal coniferous forest floors between Santa Cruz and Alaska, where they crawl beneath coastal redwoods, Douglas firs and Sitka spruces. They typically sport a slimy overripe banana look complete with brown spots; however, some can be green, brown or white.

The banana slug's diet and habitat are really the only remotely normal things about them. On the whole, these slugs are jaw-droppingly weird. They breathe through their skin. Their eyes sit on the end of retractable antennas, and they essentially have an “off” button that causes estivation—a defense mechanism against unfavorable conditions (heat and dryness). During estivation, the banana slug will secrete a layer of protective mucus, bury itself in soil and leaves, and effectively shut down until conditions are livable.

The slimy mucus layer plays several roles in the life of a banana slug. It protects from dehydration, sends chemical signals and aids in movement and respiration. A curious animal that prods the banana slug with its nose or tongue will quickly discover that the slime acts as an anesthetic. This is why banana slugs have no natural predators—and why “lick the banana slug” is a popular dare among coastal forest hikers.

But it’s their sexual behavior that’s most…erm…fascinating. As hermaphrodites, banana slugs possess both male and female reproductive organs, allowing self-fertilization—no mate needed. The banana slug can reproduce all on its own. (The physical details of this feat remain a bit murky, but with the penis near the slug's tail and the genital pore near its head, one imagines a circular configuration.) Sexual mating, however, is a far more common choice.

Once a slug has decided to mate, it will secrete a chemical-laden slime that flags down other consenting slugs. The partners begin by eating each other’s slime. These love bites prime a genetic exchange in which each slug inserts its penis into the other’s genital pore. Copulation can last for hours in this yin-yang like formation. Talk about the circle of life.

And then the grand finale: The Penis Gnaw-off. Perhaps that’s not the technical term, but it’s accurate nonetheless. At the end of sex, one or both of the banana slugs will chew off the other’s penis to disengage from blissful union.

And no, their penises do not grow back.

Photo credit: Andy Goryachev/Wikipedia