Now that it's garden season, here's an interesting tidbit from the 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 whom 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 preceeded by an elaborate courtship ritual, which supposedly reduces the chance of two individuals of separate species mating and giving rise to hybrids.
During courtship, two slugs will circle each other ... 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 grey 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 ... while they solemnly wave their oversized 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 grey garden slugs, which are able to copulate in midair, suspended by stretchy strands of mucus up to 17 3/4 inches long.
As courtship progresses, a banana slug pair intertwines ... 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.
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 ... 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." ... In other animal species, gigantism has been a precursor to extinction. Only by submitting to the shears can banana slugs maintain their inordinate organs.