City Trails of Urban Snails

Urban life is often described as a fast-paced and frantic thing. But one of my favourite study species, the urban snail, is the living proof you can take your time and still enjoy what your city has to offer.

I have been studying the brown garden snail Cornu aspersum, specifically its dispersal behaviour, since my Master thesis, and it is still one of my main model species today. It’s a Euro-Mediterranean snail that’s been introduced around the world, sometimes accidentally, sometimes intentionally, to be farmed for food or cosmetic purposes (All these snail-based skin care products you can now buy? It’s this species!). If you live in western Europe or California (and probably other US states), it is THE snail you’ve had to remove from your garden plants time and time again, or the one you’ve been careful to avoid crushing as they crawl on the pavement at night when you come home. They’re so associated with humans that “gardens” is literally noted as one of their main habitats in most field guides. Weirdly though, they had never been studied explicitly with respect to urbanization.

I lived at the time (2014) at the edge of the city, in a neighbourhood that was entirely rural 30-40 years ago, but recently urbanised on the east side, a.k.a. a perfect urban evolution natural experiment! We sampled snails all over the area and we set to compare their exploration behaviour. (Note: fieldwork is way easier when your entire study area is only 2 km wide, you just have to walk 2 minutes to find a new population, and you can run back home to grab a bite or a drink). Basically, we placed them at a border between meadow and pavement, and waited to see which side they’d choose. My hypothesis was that roads would exert a selection pressure against adventurous snails, and that urban snails would be more wary of crawling on anything “looking” like a road. Seems logical, right? They can lose up to a third of their body mass if they crawl more than a few metres on mineral substrates, and they risk being crushed by feet and wheels! Why would they go on like that if it is that costly?

…

Turns out, we were wrong. Very. Wrong. To our surprise, urban snails were at least as likely, and for some groups even more likely to explore the pavement than rural ones! Once we recovered from our surprise, we started to think. A first hypothesis is that urban snails simply became habituated to encountering mineral substrates all around them, and it doesn’t bother them that much now; snails are known to change the way they crawl depending on the minimise water loss. But even then, the costs of moving are still real and very high, so there is probably some benefits to moving a lot in cities.  We are still working on it, but our main hypothesis so far is that snail resources such as shelters and food plants are so scattered and unstable at the snail’s scale that high mobility remains a viable strategy anyway (if you don’t move a lot, you won’t get as much food, and anyway you’re going to die when your patch is mowed/harvested).

But we can’t really lump all “city” snails into the same homogeneous category, we need more nuance, especially when studying small animals. If even for us, the city can look and feel very different just by walking a few blocks, how does it feel for snails? During her own PhD thesis in landscape ecology, my friend Manon Balbi took another approach to the relationship between the brown garden snail and cities. This time, she focused on urban snails only, and used genetic tools to understand the relationship between urban landscape and gene flow, the outcome of dispersal movements. She took genetic samples from snails coming from more than 100 populations in 12 landscapes in 3 cities in western and northern France. She found out that landscapes and populations varied hugely in their levels of genetic diversity. At the broadest scale, the street density of a neighbourhood played a major role in shaping gene flow: snails were more genetically homogenous in urban landscapes with a lot of streets. To Manon, this was an indication that snails were transported passively along roads, or that they use road verges as dispersal corridors. By contrast, when she looked at the most local scale, she found out that interfaces with impervious surfaces and buildings are still significant barriers to gene flow. So even though urban snails are not that bothered by impervious substrates, there still is a point where too much is too much.

So, the next time you see a garden snail eating at your beloved plants, take the time to think about all they had to withstand just to be there, before throwing them out of your garden… and if you do, throw them far enough, otherwise they’ll come back anyway, even (maybe) if there is a road between you and them.

PS: For one of the stories behind Cornu aspersum introduction in the United States, and many other great (and weird) snail facts, may I recommend to you the Malacology episode of the great Ologies podcast, by Alie Ward?

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Balbi, M., Ernoult, A., Poli, P., Madec, L., Guiller, A., Martin, M.-C., … Petit, E. J. (2018). Functional connectivity in replicated urban landscapes in the land snail (Cornu aspersum). Molecular Ecology, 27(6), 1357–1370. doi:10.1111/mec.14521

Dahirel, M., Séguret, A., Ansart, A., & Madec, L. (2016). Dispersal-related traits of the snail Cornu aspersum along an urbanisation gradient: maintenance of mobility across life stages despite high costs. Urban Ecosystems, 1–13. doi:10.1007/s11252-016-0564-y

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Maxime Dahirel

I am a behavioural and evolutionary ecologist interested in the causes and consequences of individual variation. When I am not studying urban evolution, I mostly study animal dispersal, using land snails and spiders as model organisms.

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