Re: When 2 slugs are hanging, intertwined. are they mating?

Hi Debbie,

Yes! You have witnessed the copulatory behaviors of terrestrial slugs. These animals are simultaneous hermaphrodites, functioning as both male and female at the same time. As you saw, some terrestrial slugs initiate mating by secreting a thick mucus strand and suspending themselves from it. While entwined together in their mucus glob they undergo some fairly complicated mating behaviors -- twisting and contorting themselves, jerking their heads, and maybe even biting each other.

Eventually, the slugs get down to the business of actual mating. They evert their penises and exchange sperm through the tips. This all takes place within the glob of mucus. In some slug species the penis is very large; you probably saw them as the bluish structures you described. The precopulatory rituals may take hours, with the actual exchange of sperm happening fairly quickly. Once the slugs have mated, each retracts its penis (or tries to, at any rate) and the slugs go their separate ways to fertilize and lay their eggs.

While researching your question I found a webpage that describes a pair of leopard slugs mating and has excellent photos documenting the encounter. This report corresponds very closely to what you witnessed.

Not all land slugs mate while hanging from a mucus blob. Banana slugs, which are common in the Pacific Northwest and northern California, copulate on the ground. Sometimes, when the partners cannot separate after mating, one will chew off the penis of the other, a rather gruesome phenomenon called apophallation. A graduate student at the University of California, Santa Cruz, studies sexual conflict in hermaphrodites and uses the banana slug as her model organism. Her website contains a good synopsis of evolutionary biology and sexual conflict, and has photos of banana slugs mating.

You also asked about the hole on the side of the animal, behind the head. Terrestrial slugs don't breathe with gills, which work only when surrounded by water. Instead, respiratory exchange occurs in an internal lung-like structure that has lots of blood vessels and is closed off to the drying external environment except for a small opening to the outside. This opening is called the pneumostome (Greek for "lung mouth").

Although slugs are often considered pests, they are very interesting and beautiful animals. While I will be the first to admit that I don't like finding them among my lettuce plants, I can't help but marvel at their evolutionary success. I think you're fortunate to have observed what you did.

Allison J. Gong
MAD Scientist

Brusca, R.C. and Brusca, G.J. 2003. Invertebrates, 2nd edition. Sinauer Associates, Inc.