Photo of Bluebell Woods in Sussex Photo of Bluebell Woods in Sussex

The Weird World of Slug Sex

A guest blog by Florence Wilkinson

The thing about wildlife is that it always has the capacity to surprise you; to hit you with something unexpected. This is just as true of urban wildlife. Many people seem to draw an imaginary dividing line between the city – man-made, humans only – and the countryside – where anything more ‘wild’ belongs. 

But the truth is, if you’re curious enough, and you learn to expect the unexpected, then you’ll stumble upon new mysteries all the time.

A case in point – one morning I woke up, went downstairs to let the dog out, and was greeted by one of the most bizarre sights I’ve ever witnessed up close. Affixed to the large glass doors that open onto our postage-stamp-sized garden in north London were two huge slugs. They had entwined themselves, and were suspended from a rope of mucus that was suckered onto the glass. Slowly they twirled, in an anti-clockwise fashion, like one of those garden ornaments that spins in the wind.

Slugs don’t enjoy the best of reputations. It’s certainly not a fun feeling if you step on one with bare feet (even less fun for the slug, I imagine). But recently here in Britain, the Wildlife Trusts and the Royal Horticultural Society issued a plea to gardeners to ‘make friends with molluscs’. This was mostly to keep them on the menu – slugs and snails are a vital food source for many garden visitors, including frogs, toads, slow worms, birds, ground beetles and – universally beloved – hedgehogs. 

“Jesus, what is that?”

Personally speaking, I bear no ill will towards molluscs. There’s something about those tiny eyes on stalks that resemble cartoon aliens, and who can resist a cartoon alien? I ditched the slug pellets long ago, and urge others to do the same. 

I immediately recognised these particular individuals as leopard slugs – easily identified by their size and gaudy leopard print attire. Leopard slugs feed almost exclusively on dead and rotting vegetation and fungi, helping to recycle nutrients and fertilise the soil. They also engage in a bit of pest control – they have a taste for other slugs too.

But what in the name of all things slimy were this pair doing? At first I wondered whether they had been moved to cannibalism. Even that was too prosaic. From further research, I learnt that the slugs were entwining two giant penises, blossoming like strange translucent flowers from an opening on the side of their heads.

Slugs and snails are hermaphrodites and can self-fertilise, but they do often mate – probably because natural selection doesn’t favour inbreeding. They can also choose to mate as male or female (just imagine!) – most opt for both, at the same time. 

The strange ritual I encountered includes the swapping of sperm samples, which the slugs may each use later to fertilise their own eggs. I say ‘may’, because leopard slugs can store sperm for years, and pick the sperm from their preferred partner. That’s if they don’t decide to eat it. 

Nobody seems to know quite why leopard slugs engage in this elaborate sexual act. In fact, not much is known about mollusc mating as a whole – slug science isn’t the most fashionable area of study, although it might have its uses. Molluscs are known to be reactive to hormones, for example, so greater knowledge in this area could help with the creation of contraceptives to harmlessly control numbers where they are causing crop damage. 

But I must say, I rather enjoy knowing that even in our tiny patch of green, surrounded by a sea of tarmac, there are still mysteries like these. And the sight of two leopard-print clad, penis-headed acrobats twirling on their mucus trapeze won’t be something I forget in a hurry. 

Florence Wilkinson is the author of Wild City, published by Orion Spring.