Anna Colliton has lived in her apartment in Pelham Parkway in the Bronx for five years, but she suspects the mice have been there far longer.
They also seem to have an unusual favorite food: the spare packets of ketchup she keeps alongside takeout menus in a kitchen drawer. Even if she hasn’t seen a mouse recently, she said, “if I open that drawer up, it will be full of shredded ketchup packets.”
So when she saw a flier on Reddit that read “We want your mice!,” posted by a team of evolutionary biologists at Drexel University, Ms. Colliton, a musician and visual artist, volunteered.
Mice are commonly used in medical research because of their physiological and genetic similarities to humans. But their evolutionary changes can also be observed over a relatively short period, making them an ideal subject for research into how wild animals adapt to, say, urbanization. The Drexel researchers are studying the effects of urban environments on the evolution of house mice in New York, Philadelphia and Richmond, Va.: relatively old cities, where time and development may have caused differences to accumulate between the mice in each city and those in surrounding areas.
Ms. Colliton was intrigued by the study’s goals; she said she had noticed that “to the extent that pests have culture and personality,” her apartment cohabitants were “extremely bold, noisy mice.”
Behavioral traits may be the most obvious differentiators, but the researchers are also using fecal tests and DNA and microbiome sequencing to compare the animals’ bodies. For example, they anticipate that city mice may be larger than country mice, because of differences in their metabolisms.
Megan Phifer-Rixey, an assistant professor of biology at Drexel, runs the lab that is conducting the five-year project, which received a $1.1 million grant from the National Science Foundation in 2021. She said the researchers expect to see differences in the mice’s stress levels because of the different stressors they encounter, including living so close to humans.
“The environment itself is more complex in cities,” she said, which could affect “things like their willingness to explore a new place, or engage with objects they’ve never seen before, or their decision to hide.”
Urban mice might also be exposed to more diseases, which could affect their immune systems, she added.
Since buildings offer safety from predators as well as shelter and food, the researchers needed to collect mice from inside apartment buildings, houses and barns. That meant finding volunteers willing to willing to let researchers into their homes to set up humane traps that will capture the mice alive. After distributing “We want your mice” fliers in city neighborhoods and at feed stores and hardware stores, and sharing them on Facebook, Reddit and BlueSky, the researchers are approaching their goal of collecting about 30 mice from each city. They hope to publish some results in about a year.
Jason Munshi-South, an urban ecologist and biology professor at Fordham University who has conducted extensive research on the rat populations of New York City, has helped the researchers with logistics, and will contribute to the study’s genomic analysis. He said that mice in urban areas probably eat different things, “so you might see them adapting to an urban diet that’s very heavy in refined grains, sugars and carbohydrates and things like that.”
“There’s probably going to be some changes in their basic anatomy,” he added, “like the sizes of their heads and teeth.”
The process of capturing the mice has revealed a predilection for sweets. Although the researchers bait the traps with peanut butter, they have told volunteers to use whatever they’ve observed to be their local rodents’ food of choice. One New York resident added chocolate, said Logan Lacy, a lab technician who has been placing the traps and returning to collect the mice. It worked. At another home, researchers found chocolate chip cookies in a trap — not placed as bait, but brought in by two mice who had stolen them.
And city living might make mice more savvy: “It’s easier to catch rural mice; we usually just leave the traps out for one night,” Mr. Lacy said. In cities, the traps need to be left for two weeks and are still occasionally unsuccessful.
The scientists are collecting only one mouse per household, and will compare the genomes of mice from different parts of the city. Like rats, mice tend to show up where trash and food scraps are abundant. But they seem to gravitate toward older buildings, where natural settling has created small entryways, researchers said, and they spend less time outside to avoid predators, which include rats.
“We don’t really know how mice get from one building to another,” Dr. Munshi-South said. “Rats walk around outside so it’s obvious how they’re doing it, but with mice, we don’t think they move around in Manhattan very much outside.”
Because the animals tend to stay put their whole lives, a Bronx mouse might look different from one in SoHo. Simon H. Williams, a senior research scientist at N.Y.U. Langone Health who has examined how house mice carry and spread diseases and is not involved in the study, said there was a “high likelihood” that the researchers would see “building-to-building, or potentially block-to-block, variation” in the mice’s genetic makeup. In his own research, he noted that mice from a particular site in Chelsea were genetically distinct from other mice that had been collected throughout the city.
For the Drexel team, the project has shown that New Yorkers are willing to pitch in and donate their uninvited roommates to science.
“What I have learned is that people are really interested in science, and they’re really interested in the mice that live around them,” Dr. Phifer-Rixey said. “I think the time has definitely come for this kind of project, because they live with us and we know so little about how they do that.”
Ms. Colliton, whose kitchen trap yielded two mice, said she was eager to learn more about the creatures living alongside her, though she remains less than thrilled about the rodents in her apartment “reproducing left, right and center.” (The researchers emphasize that their focus is on how mice have adapted to city living, not on pest control, though the research subjects will not survive the tests.)
“I’m curious to know how they compare to the country mice,” she said. “It’s an age-old question, you know?”