The mating of the leopard slug is one of the most sensuous film sequences you’ll ever see in your life.

~ Sir David Attenborough

The bizarre happenings just outside our door continue to astonish me. As is often the case, this discovery came while I was out on a completely different mission. I had several programs in a row last month on one of my favorite topics, caterpillars, and was out hunting at one of the best times for spotting these masters of camouflage – after dark. I was sweeping my UV flashlight back and forth among the tree saplings in the yard, hoping to catch the glow of a sphinx moth larva or one of the so-called slug caterpillars, when I saw something strange on a tree trunk. Not a slug caterpillar, but verbally close – two leopard slugs caught in the act of their bizarre mating ritual.

Early stages of leopard slug sex (click photos to enlarge)

First, a little background on these slimy stars. Leopard slugs, Limax maximus,  have been accidentally introduced into many parts of the world from their native Europe. They are large (up to 8 inches) slugs which have a small rudimentary shell hidden under the skin on their back. They are loathed by most homeowners since they may feast on garden vegetables, although much of their diet is decaying organic material and fungi. They are also carnivores, racing after and devouring other slugs at the break-neck speed of up to 6 inches per minute. You will also never forget them if you are unlucky enough to step outside one evening without shoes and suddenly feel a slimy blob underfoot. In addition to locomotion and protection, that slime serves another function – communicating readiness to mate. And this is what this species is  most famous for – its very unusual reproductive habits. Leopard slugs are hermaphrodites – they possess both male and female reproductive organs. They can self-fertilize, but what’s the fun in that?

Long before I saw them that evening, these two slugs were already engaged in their unorthodox breeding sequence.  When the time is right (it takes two years for this species to reach sexual maturity), one slug leaves a chemical trail in its slime signaling its readiness. Another slug may pick up that trail, following the first. There is then a long bout of slug foreplay involving circling one another, nibbling, and whispering sweet nothings. At some point they both climb a nearby vertical surface and entwine their bodies, dropping down on a mucus string they create just for this strange mating dance. They slowly rotate and extend their bright blue male organs out the right side of their heads (from a hole called a gonopore, just behind the tentacles). This is when I first encountered them. The photo above shows their male organs being extruded.

Their bodies and their male organs intertwine as they mate

The blue organs entwine and change shape into a translucent cerulean chandelier over the next hour or so and exchange sperm. There is even a scientific treatise (Taylor, J.W., 1894, Monograph of the land and freshwater Mollusca of the British Isles) that details how these blue extensions change shape through the mating sequence…

I found myself transfixed by this strange behavior, and sat out there watching it for over an hour (I know what you’re thinking…). My only regret was that I had left my camera at work and so was only able to record this otherworldly occasion with my iPhone.

Changing shape as they exchange sperm

Nearing the end of their romantic encounter (you can see the mucus string in this photo)

When they finished, they slowly became untangled, one slug crawling off on the tree trunk (one often just drops to the ground), and the other slowly climbed the mucus love rope, consuming it as it went. Both presumably wandered off to lay a couple of hundred gelatinous eggs. I don’t know about them, but I was exhausted after all that. If you are up to it, you can google David Attenborough Leopard Slug Mating and find several YouTube clips online from his BBC series, Life in the Undergrowth. Hearing Sir David explain it all adds a certain elegance, lacking in my prose, to this most unusual backyard event.