THE SEX LIFE OF SLUGS

Suspended from a branch by half a metre of mucus bungee line, two leopard slugs were mating in free air. They were as long and thick as Cuban cigars and mottled in shades of brown and orange, the pattern of dappled light and shadow on autumn leaves.

The molluscs were courting, caressing, twisting together like a rope of braided hair. One slug had already thrust out the sexual organ that, somewhat disconcertingly, sprouted from a gonopore on the side of its head. It was, however, surprisingly beautiful, a stalactite, the frosted blue of glacial ice.

Like aerial contortionists performing corde lisse in the circus, the slugs spiralled and spun on their rope. As leopard slugs are hermaphrodites and the gonopore is located on the right side of the head, they rotated clockwise so that their genital openings aligned. Once both slugs had everted their penes, they entwined them, pressing the flanged ends against one another and fanning them out to form a translucent petalled globe. Eyestalks bobbing, they exchanged spermatophores, compact capsules of sperm. This task complete, the mating was swiftly concluded.

In stark contrast to their amorous courting, there was evidently no time for post-coital niceties. One of the slugs immediately unwound from its mate and extended itself to its full length. Lunging out, it attempted to reach the trunk of the silver birch they had descended from, while its partner ascended the slime cord like Rapunzel’s prince, devouring the copious mucus they had produced as it slunk towards the anchoring bough.

It wasn’t until later that I discovered the reason for their hasty departure. Apparently, while most leopard slugs will amicably go their own way after mating, some break the rules, biting off their partner’s genitalia, dropping to the ground, and eating them.

Scientists believe that this practice, known as apophallation, may occur when a slug’s corkscrew-like penis becomes entangled in their mate’s genitalia and all other attempts to separate have failed. Alternatively, it could be that the aggressor gets a large dose of protein for its own eggs, and its partner can’t go rope dangling with anyone else.