From The Western Society of Malacologists Field Guide to the Slug
Although slugs are hermaphroditic, each animal equipped with both male and female reproductive organs, they mate with themselves only if no other slugs are around. Given a choice, they seek partners with which to trade genetic material, a move that, by favoring the passage of chromosomes from both parents to the offspring, nurtures a healthier pool of slug genes. The actual exchange of sperm is preceded by an elaborate courtship ritual, which supposedly reduces the chances of two individuals of separate species mating and giving rise to hybrids.
During courtship, two slugs will circle each other, often for hours, with both partners engaged in ritualized bouts of lunging, nipping, and sideswiping with their tails. The two slugs may also display their disproportionately large sex organs. The great gray garden slug's penis is nearly half its total body length. In fact, penis size is reflected in the scientific name of one banana slug species: dolichophallus -- Latin for "long penis."
"The sight of a courting pair of slugs majestically circling one another and ceremoniously rasping each other's flanks while they solemnly wave their enormous penises overhead puts the most improbably athletic couples of Pompeii and Khajuraho into a more appropriate and severely diminished perspective," note researchers C. David Rollo and William G. Wellington. "Athletic" is an even more appropriate adjective for great gray garden slugs, which are able to copulate in midair, suspended by stretchy strands of mucus up to 17 3/4 inches.
As courtship progresses, a banana slug pair intertwines, wrapping themselves into an "S" position and stimulating each other for several more hours. Their genital areas swell as the pair move even closer together. Penetration takes place, then each slug alternately releases and receives sperm.
But in the case of the banana slug, that's hardly the end of this amazing routine. Now the slugs must disengage -- a challenge for two animals so amply endowed and thoroughly covered in sticky mucus. After long bouts of writhing and pulling, the pair may resort to what scientists call apophallation. Translated, this means that one slug gnaws off the penis of the other.
Is there an advantage to such odd behavior? Yes, according to Adrian Forsyth, author of A Natural History of Sex. The apophallated slug, says Forsyth, "cannot regrow his penis and is now obligated to be a female and forced to offer eggs. It may be that the castrator can raise his reproductive success by increasing locally the density of females." Slug scientist Albert Mead has suggested that apophallation may be nature's way of maintaining the species. After all, he writes, in other animal species, gigantism has been a precursor to extinction. Only by submitting to the shears can banana slugs maintain their inordinate organs.
Banana slugs. We see their stylized faces everywhere in Santa Cruz - on posters, T-shirts, and textbook covers - and that's when we're not confronting them face-to-face in all their gooey splendor. So what are they? Where do they come from? Are your prize begonias in trouble if these slick yellow critters are oozing about in the vicinity? And how do they ... get it on?
All good questions. Some of the answers are known, others remain a mystery.
What we do know about banana slugs, according to Aptos authority Alice Bryant Harper, is that they belong to the class Gastropoda ("stomach feet"), putting them in with other land slugs, sea slugs, and snails. And they belong to the genus Ariolimax, a group of land slugs that range from the Salinas Valley north to Alaska and east to the Sierra Nevada. The species question is under debate. Some folks think that there is only one, A. columbianus, throughout the range; others claim the presence of a second, A. dolichyphallus. A definitive answer will be a long time coming.
Dolichyphallus translates as "giant penis", and the slugs we know and love definitely live up to it. Banana slug penises have evolved to be almost as long as their bodies. When you consider that the average banana slug is 6 - 8 inches long, this endowment is impressive even by our standards. While having a big member to show off before mating may be a plus when trying to get the attention of that special slimer, there are drawbacks.
Since slugs are hermaphrodites, each partner's wiener must fit into the other's genital opening. This makes choosing a mate of equal size a must. Before actually getting down to it, both partners check out each other's equipment - no slug can ever be accused of rape, because both partners must present their plumbing before anything else happens. If either slug miscalculates, it may get its wanker stuck during actual copulation and be unable to pull out afterward. When this happens, the unstuck partner bites off the stuck one's slughood - scientists call this "apophallation", though "Bobbitization" may have more of a ring in popular circles.
This strange mating ritual might have evolved to limit penis size, since slugs with oversized organs can do the dirty deed only once. It makes one glad to be a mammal.
Second only to bizarre sex in the slug features hierarchy is slime. Without slime, banana slugs wouldn't be possible. To them, "slimy" is a compliment. When slugs evolved from snails, they gave up the protection from drying out and being eaten that the shells provided. Instead, slime is the only thing separating them from the cruel world.
All parts of the slug body make slime, but different areas produce different types. The pedal gland, in the foot, secretes a thick, sticky slime that insulates our friends from the ground. It also keeps slugs slithering along, because the slippery critters move by contracting the muscles of the foot in a wave-like motion. This slime protects the soft body from rough surfaces and prevents desiccation. It can even be used as a defensive weapon.
When a slug is threatened, it produces a lot of thick slime and contracts into a near-ball shape. In this form, slugs are really unattractive to predators. The slimeball is too big to fit in most predators' mouths, and is nearly impossible to hang on to even if something actually manages to get a grip. And, rumor has it, they taste terrible. While the pedal gland makes thick slime, other glands are churning out thin slime, the stuff that makes up the "slime trails" you see early in the morning. Slugs can move vertically (like very limited spiders), using a thin slime "cord" produced at their tail ends.
Slug sex depends on slime, too. (You just knew we had to get back to sex, didn't you?) Couples lay down a thick layer of slime as a sort of blanket to mate on. As a way of getting to know each other, they taste and eat each other's slime. This could be a way to get hormonal information, or it could be a convenient snack to fuel them up for consummating the deal.
How much time passes between mating and egg production remains a mystery, but clutches of up to 30 eggs are laid in holes or crevices where the ground is moist. The eggs take three to eight weeks to hatch, by which time the deadbeat parents are long gone. Baby slugs are totally at the mercy of birds, shrews, and other big, bad things. Whippersnapper sluggers are clear when hatched, and the characteristic bright yellow color develops within the first few days. Banana slugs seldom reach old age, but natural lifespan is estimated at three to six years, based on those of other land slugs. Must be the sex.
Banana slugs play an important role in keeping forests healthy. As scavengers, they eat living and decaying plants, roots, fruit, seeds, bulbs, lichen, algae, fungi, animal droppings, small carcasses, poison oak, mushrooms, and other debris on the forest floor. They're symbiotic with the California redwood, in that they eat plants that compete with redwood seedlings. When the trees grow, they provide necessary shade for the slugs, so both win.
While banana slugs will eat garden plants if given no choice, they prefer the forest floor menu and shouldn't be confused with common garden slugs. For one thing, their bright-yellow color immediately distinguishes them from their destructive, boring gray cousins. Banana slugs are also larger, occasionally getting up to 10 inches long.
There are still many unanswered questions: is there one species, or more? And why such a weird sex ritual? Is slime just food for passionate couples, or does it signal some kind of mating readiness? If there's one place on earth likely to research such mysteries, it has to be UC Santa Cruz. But, lo and behold! UCSC is having nothing to do with the little goohounds. Studies of land gastropods at the university are, well, sluggish.
While investigating this piece, stone walls were thrown up in my path everywhere I turned. Reigning slug guru Alice Bryant Harper wasn't available. Library sources were anemic, and the banana slug Web page didn't reveal anything new. But then, it seemed, fate had come to my rescue. In August, the university hosted the Seventh International Congress on Invertebrate Reproduction and Development, bringing in fanciers of spineless entities from all over the world and - could it be? - a bevy of renowned banana slugophiles.
It was a dismal failure. The gastropods as a class were virtually ignored. A couple of leads from the congress failed to pan out. If anyone out there is working on these guys, please let me know.
I've been looking for slugs in all the wrong places.