Even though
slugs are
hermaphroditic, that is: each animal equipped with both
male and
female reproductive organs, they will mate with themselves only if no other slugs are around. If given a choice, they will seek partners to trade
genetic material with; this is because, by favouring the passing of chromosomes from both parents to the
offspring, rather than just one parent, a healthier pool of slug genes will be raised. The actual exchange of
sperm happens before an elaborate courtship ritual, which supposedly reduces the chance of two individuals of separate species mating and giving rise to
hybrids.
During courtship the two slugs will circle each other, and both partners engage in ritualised bouts of lunging, nipping, and sideswiping with their tails. The two slugs may also display their disproportionately large sexual organs. The great grey garden slug's penis, for example, is nearly half its total body length! In fact, penis size is refered to in the scientific name of a banana slug species: dolichophallus. This is Greek for "long penis".
The sight of a courting pair of slugs majestically circling one another is almost hypnotic... while they calmly wave their oversized penises overhead. "Athletic" is an even more appropriate word for great grey garden slugs, which are able to mate in mid-air, held up by stretchy strands of mucus up to 45 centimetres long.
As courtship progresses, 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 separate, you can imagine the challenge for two animals so well stuck together, 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? Well, yes, according to Adrian Forsyth, the author of "A Natural History of Sex". Forsyth says, the apophallated one, or in a slugs case, amputated one, "cannot regrow his penis and is now forced to be a female and must offer eggs." In other animal species, gigantism has been the reason for extinction. Only by sacraficing their male genitalia, can the species of these slugs beat extinction.
So there we have it. The slug is well hung, can mate for hours suspended by a rope of snot, all to get his pride chewed off in the end. Is there a moral in there somewhere...?