(I’m writing this the night of the September 20, 2019 Climate Strike. This is merely the consequence of literal months of procrastinating—the paper I am writing about got published in July, and was online as a preprint even before—but it seems sadly appropriate as I am going to talk about adaptation to hotter environments.)
If you’re a regular reader of this blog, you are probably familiar with the concept of the Urban Heat Island effect (if not, you can read these excellent posts to learn more about it and its consequences). Because buildings and roads tend to absorb solar radiation, and because of the lack of vegetation cover (which means less shade and less evapotranspiration), cities are often hotter than the neighbouring countryside. This can influence our own well-being and health (especially during heat waves and especially as the climate warms), but also have an effect on the organisms living in cities. Whether they are able to adapt will depend on the species, and some are better armed than others.
Enter the land snail Cepaea nemoralis, a species that has been used as a model in evolutionary biology for decades, largely because of its shell, which can be highly variable both in terms of colour and the number of dark bands.
Lighter coloured shells (either due to lighter background or fewer bands) have a higher albedo and absorb less heat from the sun, which allows the snails to maintain a lower body temperature. Many studies have shown that lighter snails have an advantage in hotter/sunnier environments (and darker snails in colder/shadier habitats). This has been shown at a continental level, comparing northern and southern populations in Europe, but also at a more local scale, comparing pairs of open and shaded habitats. Incidentally, because Cepaea shells have been collected for decades, scientists have even been able to use museum collections to track temporal changes and tentatively link them to global warming! (One more proof of the value of scientific collections!).
Now, if you’re thinking “this looks like the perfect species to study urban evolution”, you’re not the only one. In a Communications Biology article out earlier this year, Niels Kerstes and collaborators (including Menno Schilthuizen, of “Darwin Comes to Town” fame) set out to answer the question “Is the urban heat island leading to evolution in Cepaea shell colour?”
Again, the fact that Cepaea shells are so colorful comes to the rescue, as it makes them a very good model for citizen/community science. Kerstes and Schilthuizen created an app in which anyone in the Netherlands could upload pictures of Cepaea snails, which where then annotated by validators. They found that urban snails were more likely to be yellow (the lighter background colour) than the darker pink, compared to snails from agricultural and natural habitats, which confirms their hypotheses based on thermal selection.
Even better, they were able to link this evolutionary change specifically to the Urban Heat Island, and to separate it from the general effect of “regional” temperature! Both even interacted: the effect of the Urban Heat Island was especially more pronounced if the countryside temperature was colder. The effect of urbanisation on banding was somewhat more complex, with snails with 1 or 3 bands, but not 5, advantaged at high temperatures. First, when snails only have 1 or 3 bands, they are located on the lower side of the shell, where they may not affect albedo that much; some researchers are calling them “effectively unbanded” for this reason. Snails with 5 bands have bands on the top of the shell, where they do influence albedo and become a problem as temperatures increase.
In addition, bands may influence predation probability (by providing camouflage, or because banded shells tend to be harder to break) so the selective pressure may be more complex than just temperature.
The success of this type of community science project probably depends a lot on its ability to attract participants. Impressively, they managed to get almost 8000 valid pictures in only 6 months! They explain this high number in part by the fact they used an existing citizen science platform as a backbone (Waarneming.nl, with thousands of active users) rather than creating their own site, as many community science projects do, and because of the simplicity of the protocol: just point-and-shoot with GPS activated, no further input needed. This may be something to think about if you want to initiate your own urban ecology/evolution participatory project. Given what they have been able to do with just one year of data, I am looking forward to the next steps, as they are continuing to accrue more data and prepare to do experiments to separate the multiple ways in which urbanisation can influence evolution.
Kerstes, N.A.G. et al. 2019. Snail shell colour evolution in urban heat islands detected via citizen science. – Communications Biology 2: 264.
- City Snails Wear Yellow: Urban Heat Island Drives the Evolution of Shell Colour - September 24, 2019
- City Trails of Urban Snails - November 26, 2018
- Friendly Neighborhood Spiders - October 12, 2018
Leave a Reply