Eastern Gray Squirrels, Sciurus carolinensis, are some of the most visible and well-known urban animals in the Northeastern United States. Prior to European settlement of the region, the black morph of S. carolinensis (“melanistic”) was much more abundant. Scientists have hypothesized that the dark pelt was advantageous in the old growth forests that used to cover the region. One explanation for this is that darker fur may provide better camouflage from their natural predators (raptors and canids) in the dark dense forests. Another reason melanistic squirrels may have been historically abundant is that squirrels with darker fur are be able to warm up more easily during the frigid winters of the region. Interestingly, the black morph seems to be rising once again as a dominant form in many urban regions.
Humans as selective agents
The apparently higher abundance of melanistic squirrels in urban environments led researchers James Gibbs, Matthew Buff, and Bradley Cosentino to wonder what selective pressures might be at work. If having black fur is advantageous, why is the black morph so rare now outside of cities? The simple answer is that the selective pressures have changed. With the loss of old growth forests and associated ecological changes, the black morph may experience differential selection in cities versus forests. For example, the gray morph may be less conspicuous in the secondary forests that now dominate the region, giving them an advantage over black squirrels that may be more easily spotted by predators. Better camouflage may also protect squirrels from hunters, which would disproportionally affect squirrels in rural environments. In fact, urban habitats may provide refuges from hunting pressures, allowing the rarer black morph to rise in frequency. Road mortality of squirrels can be significant too, and in some urban areas the dominant cause of mortality. Black morphs may be favored by natural selection in more urbanized regions if their coat color allows them to be more easily detected by drivers.
Crowdsourced data yields big data
Gibbs, Buff, and Cosentino hypothesized that the frequency of the genetically based trait of coat color may be correlated with urbanization and selective pressures unique to urban (road mortality) versus forest habitats (camouflage from predators and hunters). Thus the SquirrelMapper project was born. Squirrel Mapper involves community members in three unique ways. Non-scientists can upload observations (initially directly to the SquirrelMapper site) to the iNaturalist Squirrel Mapper project, classify the coat color of squirrels from images uploaded to the project, and play interactive games such as “Find the Squirrel” to help determine the conditions in which each morph is most conspicuous. By involving the community, the researchers are able to manage and analyze very large amounts of data from across the entire range of the species. The project is ongoing and now has a whopping 36,000+ observations with location and coat color categorization!
The team summarized the project and their initial findings in their recent publication: a chapter titled “The Biological System—Urban Wildlife, Adaptation, and Evolution: Urbanization as a Driver of Contemporary Evolution in Gray Squirrels (Sciurus carolinensis)” in the edited text Understanding Urban Ecology. Within the first year of the project, the researchers had amassed a dataset of nearly 7,000 images of gray and black S. carolinensis that had been uploaded to Flickr and to their SquirrelMapper site. In this initial analysis of their data they asked three main questions: Is the melanistic morph more common in urban environments? Is there a difference in conspicuousness of each morph in forest and urban habitats? and Is there evidence of differential road mortality contributing to patterns of color morph frequency?
Black squirrels are more common in urban environments
The researchers mapped every squirrel occurrence and asked how color morph varied regionally with landcover classification. They extracted land cover variables (National Land Cover Database), elevation, and air temperature across the range.
They found that the frequency of the black morph of the Eastern Gray Squirrel increases with increasing urbanization. Focusing on Syracuse, NY (where sampling was most thorough during the first year), they found that 65% of squirrels in the urban core were black whereas only 13% were black in rural habitat! The proportion of observed squirrels with black coats declined sharply as distance from the city center increased.
Across the North American range, they found the probability of observing a black squirrel was related to temperature, elevation, and urban land cover. Specifically, black morphs were more likely to occur in regions with lower annual temperatures, higher elevation, and in urban environments. The relative importance of these factors differed regionally. Across the entire range, temperature was the most important predictor. When the researchers focused only on the cool northern portion of the range, however, they found that urban land cover and not temperature was most important.
Camouflaged from (human) predators?
The researchers hypothesized that differences in hunting pressures in urban versus rural environments may be one factor contributing to the observed patterns in melanism. If gray squirrels are in fact better camouflaged in forest habitats than black squirrels, then predators (including humans) would be more likely to take the black squirrels over the gray. To test this hypothesis, they created an interactive “Squirrel Hunt” game, a precursor to their current online crowdsourced “Find the Squirrel!” game. In this game, non-scientist participants were asked to identify squirrels of both color morphs in various locations in forest and urban habitats and the time to locate the squirrel in each image was recorded.
Consistent with their prediction, they found that participants could identify black squirrels much more quickly than gray squirrels in secondary growth forest habitats, but that in old growth forests the gray squirrels were identified more quickly. This supports the hypothesis that in the historical habitat the black color morph may have been less conspicuous to predators (at least human predators). This also provides some evidence that black morphs may be selected against in the secondary forests common in the region where they are more easily detected. In the urban forest habitat, however, there was no difference in detection time between the two color morphs.
Cars as selective agents
Lastly, the researchers examined another potentially potent selective agent in urban environments: road mortality. Focusing on the region most heavily sampled, Syracuse, NY, the researchers asked if the proportion of black squirrels reported as roadkill was similar to the background frequency of the morph in the area. They also directly quantified crypsis by calculating the difference in color intensity between squirrels (museum specimens) on pavement.
They found that the black morph was significantly underrepresented in roadkill compared to its frequency in the region. Although 33% of squirrels in the sample region were black, only 9% of roadkill observations were black. Their crypsis analysis confirmed that the black morph was 10 times more conspicuous than the gray morph on pavement! The researchers suggest that these findings may be evidence that drivers can more easily identify and avoid black morphs than gray morphs.
Coat color frequency as a consequence of divergent selective forces
The preliminary results for this project strongly support the hypothesis that coat color is under differential selection in urban versus forest habitats. In the secondary forest habitats most commonly encountered in North America, black morph squirrels appear to be at a distinct disadvantage because they are more easily spotted, at least by humans. To what extent this difference in camouflage applies to other predators with different visual systems, such as raptors and canids, remains to be tested. In contrast, in urban habitats where hunting and predation pressures are relaxed, road mortality is a stronger selective pressure. Under these conditions, the black morph appears to be at an advantage because they are more easily spotted by humans.
The sharp decline in black morphs outside of city centers and the maintenance of the black morph within cities (in some cases for decades) suggests that strong, local selective pressures are likely maintaining the prevalence of this otherwise rare phenotype. This may end up being to the advantage of the species in the long term. As forests mature and become more similar to the historical old growth habitat, the black morph may once again be favored in forest environments. As hunting pressures decline, cities may become sources for re-population. If that is the case, we can thank cities for providing refuges to maintain this rare genetic variation.
Want to know more? Here’s what you can do:
- Read about the project on Zooniverse
- Check out the SquirrelMapper site
- Follow the project on Twitter
- Upload observations of gray and black squirrels to iNaturalist
- Play Find the Squirrel!
This post is based on the publication:
Gibbs J.P., Buff M.F., Cosentino B.J. (2019) The Biological System—Urban Wildlife, Adaptation, and Evolution: Urbanization as a Driver of Contemporary Evolution in Gray Squirrels (Sciurus carolinensis). In: Hall M., Balogh S. (eds) Understanding Urban Ecology. Springer.
Featured image: Eyesplash Photography on Flickr, CC BY-NC-ND 2.0
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