When Squirrels Meet the Delmar Divide

Delmar Boulevard is a largely unassuming four-lane road that cuts St. Louis, Missouri, in two. To the boulevard’s north, a lower-income, predominately Black population lives among run-down buildings, overflowing dumpsters, and vacant lots. To the street’s south, extravagant houses are fenced off in gated, greenery-lined communities. For decades, sociologists have called this stark demarcation the Delmar Divide.

But St. Louis’s fractured urban landscape affects more than its human residents. According to urban ecologist Elizabeth Carlen, a researcher at Washington University in St. Louis, the city’s divisions are even altering its wildlife.

In St. Louis, Carlen says, “the segregation is profound.” Redlining in the 1930s and white flight in the 1940s caused the racial split across Delmar Boulevard. With the shift, differences in resource allocation and investment soon followed. But it wasn’t clear how this history affected the city’s wildlife, so Carlen decided to investigate. In 2022, she embarked on what she calls “a big 23andMe” of some of the city’s bushy-tailed residents: eastern gray squirrels (Sciurus carolinensis).

To collect her data, Carlen drove between north and south St. Louis with a kit of supplies in the back of her car, passing by abandoned houses and manicured parks alike in search of the city’s squirrels. Some days, her colleagues would tip her off to roadkill she could collect. On others, she’d check on live traps filled with peanut butter and walnuts. Once she had a squirrel in hand, Carlen measured the animal and collected fur samples and photos, along with a little snip of its ear.

The tissue samples she gathered held key genetic information, which Carlen then used to construct a sprawling family tree tracing the squirrels’ ancestries. Her analysis revealed something astonishing: Just as Delmar Boulevard carves a socioeconomic line through the city, it also splits St. Louis’s squirrels in two. “The same way that 23andMe can tell if you’re from Scotland versus England, we can tell, ‘Oh, these squirrels are from North City; these squirrels are from South City,” Carlen explains. “They have a different genetic signature being from those two places.” 

Using a computer model, Carlen then attempted to deduce what the genetic split she was observing had to say about how the squirrels move around the city. Interestingly, Carlen says squirrels from the richer southern half of St. Louis seem to have much more restricted territories. In contrast, squirrels from North City disperse across wider areas and move more freely across the landscape. 

Why these differences exist “we still don’t fully understand,” Carlen says. But she suspects it has something to do with different building practices across St. Louis. For example, while rich communities might install gates in a bid to prevent crime, these physical barriers also hamper squirrels’ movements and restrict the flow of traffic, forcing more vehicles onto already congested public roads. “Cars are obviously a huge danger for squirrels,” Carlen notes, so squirrels navigating the southern half of the city have a higher chance of becoming roadkill. This makes long-distance journeys riskier, ultimately isolating the squirrels and, possibly, creating the conditions for inbreeding.

Carlen feels for the squirrels. She lives on the south side of St. Louis along a busy seven-lane road. “The number of cars is wrapped up with income. And income in St. Louis is highly tied to race. This is all linked together,” she adds. “There is no justice for wildlife without justice for people.” 

This research adds to an emerging understanding among scientists of how decisions in the human sphere are unintentionally shaping wildlife in North America and around the world. In Poland, for instance, Marta Szulkin of the University of Warsaw has helped collect data suggesting how urbanization is affecting songbirds, from the vibrancy of their colors to their gut microbiomes.

In 2025, Szulkin coauthored a review paper with Carlen highlighting how politics, war, and religion can also contribute to fractured landscapes and fragmented habitats, with consequent effects on animals. Research in this field is growing. For example, a number of studies have shown how racialized urban landscapes affect coyotes, including how an area’s wealth, population density, and pollution can influence their movements and behaviors.

Szulkin hopes that these investigations expand beyond North America into understudied regions like Eastern Europe and the Global South. “I think that it’s very exciting to start addressing this knowledge gap,” she says.

Carlen, meanwhile, is curious how Delmar Boulevard is dividing the city’s other residents, like raccoons and mice. “Humans and wildlife are inherently tied to the environment,” she says. “What we do to this Earth, what we do to our environment—our landscape—has downstream effects on everything.”