Cities Are Turning Snails Yellow

Urban areas are heat islands. Could that be dictating the color of their gastropods?

Snails lined up on a bridge
Daniel Ramos Gonzalez

Take a stroll through the coastal dunes, woodlands, or cities of Europe and you will likely find, with an observant eye, grove snails. They come in a variety of colors: coral pink, lemon yellow, lush mahogany. Sometimes, their shells are swirled with as many as five black bands. For decades in the early and mid-20th century, schoolboys collected the shells in the same way they collected bird eggs, butterflies, and stamps.

Over that time, European cities have sprouted high-rises, concrete, and asphalt. And the snails—still common, still everywhere—have been changing, too. In the Netherlands, the results of a citizen-science project suggest that light-colored snails are becoming more common in cities. Researchers blame the urban-heat-island effect. Yellow shells, they argue, reflect more sunlight, allowing the snails to stay cool in the heat.

The people looking for snails are no longer just schoolboys, but anyone equipped with an Android app called SnailSnap. Niels Kerstes at the Naturalis Biodiversity Center, a natural-history museum in the Netherlands, developed the app with his co-authors in 2017. That spring, they advertised it through the museum and through mollusk-enthusiast groups on Facebook. It was eventually downloaded 1,180 times, and people sent in 7,868 photos of different snails. The SnailSnap data are revealed in a paper—which has not yet been peer-reviewed—uploaded to the preprint server bioRxiv.

Snails, says Kerstes, are the perfect organism for citizen science: They’re abundant. They’re easy to find. Their colors and bands are fairly distinct and genetically determined. And they don’t run away when they’re being photographed—snails being famous for, you know, moving at a snail’s pace.

Photos taken by participants of SnailSnap (SnailSnap)

SnailSnap grew out of a previous citizen-science project called Evolution MegaLab, pegged to Charles Darwin’s 200th birthday in 2009. The Evolution MegaLab website also asked people to document the color of snail shells, but a decade ago the possibilities were much more limited. “It was the dark ages of smartphones,” says Jonathan Silvertown, an ecologist now at the University of Edinburgh and the leader of Evolution MegaLab. That project had to rely on people recording and uploading the color and location of snails to the site themselves.

“The problem with citizen science is it’s hard to say how reliable the data are,” Kerstes says. Now, nearly everyone has a camera and a GPS in their pocket. When a citizen scientist uses SnailSnap to take a photo of a snail, the app also records the location. The photo then goes to a small group of validators trained to categorize snails by color or banding pattern. It’s still not a perfect data set—sometimes the GPS didn’t work or people disagreed on the color—but it gave Kerstes more overall confidence in the data.

The team ultimately compared snails from four types of habitats: farmland, natural habitats like forests, urban “green” areas like parks, and urban “gray” areas that were primarily residential, commercial, or industrial. In both types of urban settings, they found more yellow snails. Kerstes and his co-authors think this is because of temperature. Cities are often hotter than their surrounding areas because they’re full of materials like asphalt that absorb more radiation, and they’re relatively short on trees. Dark snails also absorb more radiation than their yellow counterparts. They might get too hot or have to spend more time trying to escape from the sun, making them less suited for the city.

There was also a strange pattern with the bands. The urban snails were more likely to have dark bands on the underside of their shells, but not on the top. Scientists studying another snail species found that banded ones cooled down faster than unbanded ones. Perhaps, according to the SnailSnap team, the urban snails had figured out the best of both worlds: no dark bands where the shell is exposed to the sun, but bands on the underside for cooling.

Angus Davison, an evolutionary geneticist at the University of Nottingham, cautions against attributing the abundance of yellow snails in cities entirely to temperature. (He was not involved with SnailSnap, but has collaborated with another of the project’s scientists on other snail studies.) “Ultimately, it’s correlation,” he says, and there could be other differences between city and non-city areas that account for the difference. Birds, for example, eat snails, and the appearance of their shells could affect how often the snails are predated. Nevertheless, the data from SnailSnap does show that something is going on in cities.

SnailSnap is still taking photo submissions. The team is interested in long-term trends as the Netherlands gets warmer with climate change. Kerstes is also working on citizen-science projects to study how other organisms are adapting to cities, searching for changes in how blackbirds sing and how dandelions disperse their seeds. Organisms are evolving all the time. “People don’t walk around with this kind of consciousness,” says Silvertown. But if you stop to notice snails, you might be more attuned to the evolution that’s happening on sidewalks, in alleyways, and all around.