Slugs and their shelled cousins, snails, are hermaphrodites, and copulation usually involves the slimy intertwining of two individuals and the simultaneous insertion of penises. In some slugs the penises themselves do the intertwining and sperm exchange occurs outside their bodies. Courtship in the common European garden slug, aptly named Limax maximus, involves two animals following each other in a clockwise manner on a vertical surface. On making contact they produce a mucus anchor rope from which they suspend themselves. As the entwined animals dangle in space they each evert and entwine their gigantic penises which far below their owners exchange sperm at their tips. In the slug Limax redii the penis is 85cm long - seven times the length of its owner's body. We have no idea whether the slug's giant penis and the unusual pattern of copulation have evolved in response to sperm competition or any other kind of sexual conflict. But we do know that for some species they can be a cause of the worst kind of sexual conflict. Their entwined penises frequently become knotted during copulation and the only way individuals can release themselves is to bite off their own penis at its base! It is not clear whether both individuals have to do this to ensure freedom. If not, there is presumably protracted negotiation about who will sacrifice his masculinity. Thereafter, those individuals that have lost their penis in this way operate only as females.
- Tim Birkhead, Promiscuity pp 98-99