Chicago’s Chipmunks Are Evolving in Front of Our Eyes, and That’s Not a Good Thing, Researchers Say
Chipmunk specimens in the Field Museum’s collection were used to assess how mammals are adapting to urbanization. (Patty Wetli / WTTW News)
Posted for: Emailed to TNBD
Over the last 125 years, chipmunks living in Chicago have gradually grown larger bodies — but their rows of teeth have become noticeably smaller.
According to two scientists at the Field Museum, humans are likely the reason.
As Chicago has become more urbanized, chipmunks have increasingly eaten food provided by people, either intentionally or by scavenging discarded leftovers. This softer, processed food requires less chewing power than the nuts and seeds they once relied on.
“We believe they’re eating more soft foods, which means they don’t need as much bite force,” said Anderson Feijó, assistant curator of mammals at the Field Museum. “That reduction shows up in the length and strength of their tooth rows.”
In simple terms, chipmunks appear to be evolving to chew human food instead of cracking hard nuts.
Feijó and fellow researcher Stephanie Smith detailed these findings in a newly published study in the journal Integrative and Comparative Biology. Their research focused on how urban wildlife — particularly chipmunks and voles — has physically adapted as Chicago has grown over time.
Using the Field Museum’s extensive mammal collection, the scientists examined hundreds of specimens collected from the late 1890s through the present day. They closely studied the animals’ skulls, taking precise measurements and creating detailed 3D scans.
“The skull tells you a lot about how an animal lives,” Smith explained. “It reflects diet, muscle attachment, hearing, and more. By studying skulls from different time periods, we could measure how those traits changed over time.”
In addition to the changes seen in chipmunks, the researchers discovered that voles showed a different adaptation: smaller auditory bullae, the bony structures that house parts of the inner ear.
One possible explanation, Smith said, is that smaller auditory bullae may help voles filter out constant urban noise, such as foot traffic and vibrations from above ground, since they live mostly underground.
To link these physical changes directly to urban development, the researchers used historical records and satellite data to track Chicago’s expansion. This allowed them to compare animals collected from the same locations decades apart — for example, a chipmunk from Jackson Park in 1905 versus one from the same area in 2012 — and match those changes with increased development.
“The patterns line up clearly with urban growth,” Smith said. “It shows that humans are having a measurable impact, and that’s concerning.”
While the study demonstrates that these rodents are adapting to human presence, Smith cautioned against assuming that adaptation means long-term survival.
“Change doesn’t automatically mean it’s a healthy or sustainable change,” she said. “It may help them cope for now, but that doesn’t guarantee they’ll be able to keep up indefinitely.”
The researchers emphasized several key lessons from their findings. One is that different species respond to urbanization in different ways, even if they seem similar.
“People might lump small mammals together,” Smith said, “but chipmunks and voles are reacting very differently. Conservation can’t be a one-size-fits-all approach.”
Another important takeaway is that people should not feed wildlife. While it may feel helpful, Feijó said it actually harms animals in the long run.
“When you feed wildlife, you reduce their ability to survive naturally,” he said. “Chipmunks may be getting bigger, but their smaller teeth make them less capable of eating the hard foods they evolved to rely on.”
Above all, Smith hopes the study encourages people to think more carefully about the wildlife sharing their environment.
“These animals don’t have a voice,” she said. “We’re living in their space every day, whether we notice them or not. Our actions, habits, and attitudes have real consequences for them.”
About Post Author
