During his initial summer at McGill University, Francis Quang-Hai Dinh dedicated his time to cycling through various parks in the Montreal area, on a quest to spot squirrels.
He visited nearly 200 green spaces, stretching from Mirabel to Saint-Bruno, where he observed and documented over 800 grey and black squirrels.
What began as a simple undergraduate assignment evolved into a fervent project aimed at understanding how the city’s geography and meandering waterways influence evolutionary processes.
“I grew fond of them after watching them for a considerable time and reflecting on them,” Dinh remarked. “People often regard squirrels as mere nuisances, but they are truly a part of the wildlife.”
With no car and limited public transport options in suburban regions, Dinh made extensive use of his bicycle, becoming the only known researcher to conduct a meticulous survey of squirrels park by park throughout the area.
Reasons for the prevalence of black squirrels in northern regions
He discovered that, on average, around 12 percent of squirrels in Montreal are black. However, this percentage increases the further north he traveled.
In regions just above Laval, he found that as much as 40 percent of squirrels were black, a term that refers to various darker fur coats, differing from the typical grey.
On Laval Island, approximately 20 percent were black, while the figure on Montreal Island was around 15 percent. On the southern shore of Montreal, the rate fell to just two or three percent, according to Dinh, who is currently pursuing integrative biology at the University of Illinois Urbana-Champaign.
The research identified three primary factors for this phenomenon, with temperature being the most significant. The cooler climate in northern areas leads to a higher prevalence of black squirrels.
“Their black fur absorbs heat more effectively than that of the greys,” he explained.
Black squirrels, which were initially introduced from Canada to Washington in 1902, now make up about half of the squirrel population in the city and have become a tourist attraction. (Jason Lowther/CBC)
Additionally, the presence of forest cover plays a role: a denser tree canopy creates a darker environment, providing better camouflage for black squirrels.
Squirrels as a model for evolutionary changes
Allison Roth, the senior author of the study and an assistant professor at the University of Missouri, as well as a former McGill postdoctoral fellow, stated that the research focuses more on using squirrels as a model to explore how urban environments influence evolutionary changes rather than on the squirrels themselves.
Roth explained that cities are not uniform. Their diverse neighborhoods, varying degrees of tree coverage, and specific boundaries—such as Montreal’s waterways—create minute environmental differences that can lead to distinct genetic and physical adaptations within the same animal population.
“One notable aspect of Montreal, which is not common in many other North American cities, is that it is significantly divided by the St. Lawrence River and its tributaries,” Roth noted.
A new McGill study found that eastern grey squirrels in the city vary in coat colour depending on the neighbourhood they live in. The animals are influenced by local environmental conditions, including temperature and forest cover.
“This effectively creates distinct little islands, and we may observe varying patterns of squirrel melanism based on whether they can cross rivers or if they are isolated on these land islands,” she elaborated.
Understanding this process is “extremely important for grasping how evolution might transpire,” she further explained.
Cities are part of a larger picture
The study, Fine-scale spatial variation in eastern grey squirrel melanism across a complex urban landscape, is accessible online.
While earlier research on urban wildlife evolution typically compared entire cities with rural areas, this study focused on wildlife changes on a neighborhood level.
Previously, scientists believed that human development was the primary driver of these changes, but this study suggests that cities represent only a piece of the overall picture. Urban expansion alters local temperatures and reduces tree cover, while waterways form natural barriers.
“Urbanization does not directly predict melanism when accounting for local vegetation structure and winter microclimate, but instead affects melanism indirectly through these environmental mediators,” the research concludes.
This study is being published as part of a special feature across five ecology journals that examine how researchers utilize data and collaboration to address significant ecological challenges.
For Dinh, his perspective on wildlife has fundamentally changed.
“We often perceive a stark division between urban areas and nature. However, the truth is that wildlife will continue to exist and adapt, regardless of the modifications we make,” he stated.
