For urban snails, yellow is the new pink

COVER: Kerstes et al. 2019, open access

SOURCE: Kerstes, N., Breeschoten, T., Kalkman, V., & Schilthuizen, M. (2019). Snail shell colour evolution in urban heat islands detected via citizen science. Communications Biology, 2: 264. 

It’s hot. 

As I am writing this, over half of the U.S. is under excessive heat watches or warnings. Last month’s global temperatures handily broke the record for the warmest June on record, and we are on track for this to be the warmest July. 

But if it is hot in general, it is even hotter in cities. The pavement, smog, and lack of shade in cities can increase temperatures by up to 6ºF (3ºC) during the day. This makes a big difference for everyone living in the city — both humans and other animals (for example, check out this envirobites article about turtles!).

Graph showing that heat increases in urban areas
Temperatures are higher in urban areas (Source: NASA, 1999)

In a paper published this month, a team of researchers from the Netherlands set out to see how this heat might affect evolution of animals living in cities. In cities that have been around for a long time, has the continuous heat caused any animals to adapt to the warmer climate? How so?

Groovy grove snails

Many photos of snails. Snails range in color from yellow to pink and brown. Some have brown stripes
Photos of Cepaea nemoralis collected through the SnailSnap app. Kerstes et al. 2019, open access.

To answer this question, they decided to focus on one animal: the grove snail (Cepaea nemoralis). Grove snails were a perfect model organism because they are common throughout the Netherlands and they have been well studied over the past 100 years, which means we already have lots of useful background knowledge about their genetics and evolution. Furthermore, these snails have a wide range of shell colors, and the color of their shell is known to be related to their ability to survive in extreme temperatures. Snails with lighter shells reflect more light, while snails with dark shells heat up, cooking the snail inside. Because of this, researchers hypothesized that they would find more light colored (yellow) snails and fewer dark colored snails (pink) in urban areas.


Screenshot of an app. Image shows a photo of a snail on someone's thumb. Underneath is text and an OK button
A screenshot of the SnailSnap app. GooglePlay

Getting enough data for a study like this is a really ambitious task—researchers needed pictures of snails in and around as many cities as possible, in a range of habitats from rural to urban areas, and they needed all of these photos to be collected around the same time. So they did the research equivalent of “ask the audience” — they created an app (SnailSnap) that everyday people could use to document all of the snails they see in their daily life. They then did their best to publicize the app using a website, press releases, interviews on national radio, social media, and even integrating it into classroom activities. In the end, they received nearly 8,000 submitted photos over the course of just one year.

Having so many photos is fantastic, but it then meant a lot of work sorting through them all to look for patterns. For this, the researchers recruited 10 validators to sort through all of the photos and ensure that they were correctly identified as Cepaea nemoralis and then classify the shell color as either yellow, pink, or brown.

A map of the netherlands with small blue dots scattered across the country. Points are especially concentrated on the eastern side
This map shows all of the points in the Netherlands where snails were photographed for this project. Kerstes et al. 2019, open access.

Yellow is the new pink

Just as they had suspected, snails in urban areas were more likely to be yellow (and less likely to be pink) than snails in suburban or rural areas! You can think of this sort of like putting on a different color shirt in warm weather — as a lighter color, yellow reflects more light whereas the darker pinks and browns absorb the color and heat up. The change in color was seen both in city parks and on the streets, indicating that the air temperature in the city played a bigger role than the habitat itself. 

This study gives us a greater sense for the importance of urban heat islands and the many ways in which they can impact the wildlife that live in and around cities. Not only do cities directly impact the wildlife in the area, they can also alter the course of evolution, changing the appearance and physiology of future generations. This study also reinforces the importance of everyday people in science! If you or your family have an interest in getting involved in citizen science projects, check out or (Netherlands) for opportunities near you!

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Abigail Lewis

I am a Ph.D. student studying freshwater ecology and biogeochemistry at Virginia Tech. Whenever possible, I enjoy thinking and writing about the role of science in society, including community-based science, science communication, and science for the public good. In between rehearsals, hikes, and long dinners I am working to build a career that will address environmental issues and build a more inclusive scientific community. Twitter: @lewis_lakes

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