This Slug Has Such a Big Penis It Has to Mate Upside Down

Let's start things out in some classy black and white shall we?

If you read this column with even the slightest regularity, you know that the animal kingdom has no shortage of weird sex. Like that of the parasite  that devours a fish’s tongue and mates in its mouth. Or the little marsupial that has so much sex it bleeds internally and goes blind and dies. I could go on.

None of it’s particularly elegant. So may I present to you the acrobatic, upside-down sex of the leopard slug, done hanging from a branch on a line of slime. But why, when other slugs do the horizontal tango perfectly fine on the ground? Well, a lot of it has to do with the leopard slug’s gigantic penis, of course.

First things first: Slugs are hermaphrodites, and that’s a neat evolutionary move. Not only does it all but guarantee that any two sexually mature slugs can come together to make babies, it also means that when they do mate, both parties can end up fertilized. In fairness, though, the big disadvantage to hermaphroditism is that it’s more energetically costly to produce both eggs and sperm, as opposed to one or the other.

So, when two leopard slugs find each other, they make their way up a tree and onto a branch. Here they curl around each other and ramp up their release of slime—big time. This appears to be a different formulation than your average leopard slug goo, according to Ben Rowson, a limacologist (that’d be a slug scientist) at the National Museum Wales. The pair will then descend on a slimy rope. “That rope of slime that they hang from can be very strong,” Rowson says. “It’s strong in the moment, but also when it dries out. It’s a fairly tough structure, really.”

Still curled around each other, hanging and gently twirling, the slugs simultaneously unravel their alien-blue penises, which come out of the right side of their head. They do this with hydrostatic pressure, pumping fluids into the penis to enlarge it more and more (the slugs use the same method for controlling their famous eye stalks, by the way).

“These penises, they start off small, but within a few moments you can see just how big they are—they become almost bigger than the slugs themselves,” says Rowson. “The penises are very mobile, it’s almost as if they’ve got a mind of their own. They’re quite complicated structures, and they move continuously and they can change their form quite a lot.”

I’m just going to keep quoting Rowson here, because this stuff is gold. The penises “wrap around each other and they form this kind of chandelier configuration, which is very strange, with these flaps around the edge with a frill on it. And that can pulsate up and down and in and out as the slugs are rotating around. It’s quite the elaborate interaction.”

All the while liquid is pumping into the hugging penises. “They’re pushed out by the fluid inside the body, but these things are so big that I think they take up most of the fluid that’s inside,” Rowson says. “So the rest of the slug looks a bit drained or flattened while all the fluid is in the genitalia.”

What you’re seeing here is illegal in 49 states.

When everything is sufficiently inflated, the transfer of sperm begins. And when everyone is sufficiently fertilized, the slugs will haul themselves back up the rope, though sometimes one partner will simply drop to the ground. Regardless, one of them will consume the slime rope to recoup the resources lost in excreting it. (Interestingly, another slimer, the velvet worm, hunts by spraying its prey with goo. After it has entrapped and eaten the victim, it too devours the spent glue. In nature, you see, there’s no sense in letting resources go to waste.)

The fertilized slugs go off and lay their eggs in soil or a moist log or what have you. Come spring, the eggs will hatch into tiny slugs, thus completing the strange saga that is the leopard slug penis intertwining.

So what gives with such an elaborate method of mating? Well, it seems to be the product of a genital arms race. It may have been that bigger penises in leopard slugs granted their owners better reproductive success and therefore better chances of passing down their genes for larger genitals.

The leopard slug penis got so big, in fact, that its owner has to rely on the laws of physics to unfurl it. “I think that gravity is essential to be able to get the penises out of the body,” says Rowson. “I think it would be impossible for them to mate on a flat surface in the way that they do.” Thus does the slug opt to do its dangle-dance of love.

This strategy, however, would seem to place the leopard slug in dangerous territory, what with predators like birds not exactly being blind and all. But not so, says Rowson. “People tend to overestimate the extent to which slugs are eaten by other animals, because they’re fairly disgusting.” Slime on a non-sexed-up leopard slug is one thing, but a pair dangling from the goo? Forget about it.

Which brings us to another consideration: Slugs are descended from ancient snails, which of course had shells, so why over evolutionary time would slugs ditch that extra protection from predators and desiccation? Why not hold on to the shell just in case?

Well, two reasons. With those big unwieldy shells, snails can’t squeeze into small spaces—say, a crack in a log or some such—where it’s nice and moist. Slugs may not have a shell they can retreat into to avoid drying out, but they have a whole world of crevices to explore.

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