I really feel like we don’t give enough credit to the small ugly creatures in our world. I’ll admit, I’m as guilty as anyone else. Talk to me about hurting a lion or a giraffe and I’d be the first to protest. But I have no compunctions about killing an ant or a spider. I think most people are probably the same. But some of the most fascinating and ridiculous creatures are small and ugly, are animals we feel no emotional connection to, except perhaps fear.

So today I am going to share a species of slug, one of the most classically ugly animals out there. The homing slug is a species of slug that, as its name suggests, is famous for it’s homing instinct. Many animals have some sort of inner compass; monarch butterflies return to the same set of trees in Mexico every winter, and salmon are famous for returning to the stream they spawned in.

But homing slugs are a little more dedicated to their calling. They are extremely territorial animals, and this territory is so hardwired into the slug’s behaviour that you can release a slug any distance from its territory and it will do its utmost to return to it. Unfortunately, slugs neither move quickly or live a very long life. But the slugs have an amazing solution to this problem: the territories they work so hard to protect and return to are passed down from generation to generation. A slug can be the great-great-great-great grandson of an original slug and it will still attempt to return to the place its ancestor was displaced from.

Humans have taken advantage of this trait, turning slug racing into a somewhat boring pastime. Slugs are moved a prescribed distance away from their home territories and released, and the first slug home is the winner. Of course, often the distances are so great that it is rarely, if ever, the original released slug that returns home. The races can take decades, and it is estimated that it takes about four generations per mile.

A homing slug with its unique spotted markings. Image by Michal Maňas, CC BY 2.5, via Wikimedia Commons

Descendants of the original slug are identified through another interesting feature of the homing slug: its spots. Each slug has a unique set of spots that distinguishes it from every other slug. So how does this help identify the descendants of an ancestral slug? Well, it all has to do with how the slugs mate. When each slug is happily sitting in its own territory, they breed sexually. Young immature slugs without established territories wander in search of a home and ultimately end up mating with many resident slugs before maturing and settling down. Homing slugs are hermaphrodites, meaning they can switch sexes depending on their current conditions. All resident slugs are male, while the transients are female. Once the females have mated and found a suitable territory, they lay their eggs which when hatched form a new generation of transient females, while their mother becomes a resident male.

This sexual mating leads to the unique ‘fingerprint’ of spots that each slug carries. But when a resident male slug is displaced from his territory, the slugs have a back up plan. Because they are so intent on returning to their territory, displaced slugs have no time to search for mates. In this case, the slugs switch to asexual reproduction. The slugs spend their last bit of life energy creating a genetically identical copy of themselves. In this way, not only is the hardwired territoriality passed on, but so is the unique spotted pattern that allows for slug identification. This allows racers to keep track of released slugs, even if it is separated by the original slug by generations.

As I wrap this post up, I’d like to end with one final point. April Fools! I know it’s a day early but I wanted to do something fun for it. So yes, the homing slug and slug racing and everything was a lie. Certain slugs do have a homing instinct, but it isn’t nearly as impressive as those I described. This post really was just a shameless plug for Jasper Fforde, one of my all-time favourite authors. Homing slugs and slug racing is mentioned in his book Shades of Grey (notably different form the 50 Shades series, and published well before). I highly recommend Shades of Grey to everyone, and hope you all aren’t too upset about this post. I promise a real animal on Wednesday!

Cover image by Muséum de Toulouse, CC BY-SA 4.0, via Wikimedia Commons