Chicago Rodents Evolve Smaller Teeth and Ears

Chipmunks and voles living in Chicago have undergone measurable evolutionary changes over the past 125 years, adapting to urban life by developing smaller teeth and altered hearing structures.

Field Museum researchers discovered these rapid adaptations by comparing rodent skulls collected from the 1890s to the present day, providing rare real-time evidence of evolution responding to human-driven environmental change.

The study, published in Integrative and Comparative Biology, analyzed 132 chipmunk and 193 vole specimens from the museum’s collection. Scientists found that urban chipmunks developed larger skulls but shorter rows of teeth, while voles evolved smaller auditory bullae—bony structures associated with hearing.

Museum Collections as Time Machines

“Museum collections allow you to time travel,” explains Stephanie Smith, a mammalogist and XCT laboratory manager at the Field Museum. “Instead of being limited to studying specimens collected over the course of one project, or one person’s lifetime, natural history collections allow you to look at things over a more evolutionarily relevant time scale.”

The Field Museum’s mammal collection contains more than 245,000 specimens, with particularly strong representation from the Chicago area spanning over a century. This unique archive enabled researchers to track evolutionary changes that would be impossible to observe in typical research timescales.

Two Species, Different Adaptations

Researchers chose eastern chipmunks and eastern meadow voles because of their contrasting lifestyles. Chipmunks spend most of their time aboveground, eating a diverse diet including nuts, seeds, fruits, insects, and even frogs. Voles, more closely related to hamsters, primarily eat plants and live mostly in underground burrows.

The evolutionary changes reflected each species’ unique urban challenges:

• Chipmunks grew larger overall but developed shorter tooth rows, suggesting dietary shifts toward human-provided food
• Voles evolved smaller auditory bullae, potentially adapting to urban noise pollution
• Climate change could not explain the skull modifications, but urbanization levels correlated strongly with the changes
• The adaptations occurred gradually but consistently over more than a century

Measuring Urban Evolution

Field Museum interns Alyssa Stringer and Luna Bian meticulously measured skull dimensions and created 3D scans using geometric morphometrics—a technique that digitally stacks skull scans to compare precise anatomical distances. The team correlated these measurements with historical records of temperature and urbanization levels dating back to 1940.

“We tried very hard to come up with a way to quantify the spread of urbanization,” notes Anderson Feijó, assistant curator of mammals at the Field Museum. “We took advantage of satellite images showing the amount of area covered by buildings, dating back to 1940.”

Diet and Noise Adaptations

The skull changes likely reflect specific urban pressures. Chipmunks’ larger bodies but smaller teeth suggest they’re consuming more calorie-rich human food while eating fewer hard natural items like nuts and seeds that require robust dental equipment.

“Over the last century, chipmunks in Chicago have been getting bigger, but their teeth are getting smaller,” Feijó observes. “We believe this is probably associated with the kind of food they’re eating. They’re probably eating more human-related food, which makes them bigger, but not necessarily healthier.”

Voles’ smaller auditory structures may help them cope with urban noise pollution. “We think this may relate to the city being loud—having these bones be smaller might help dampen excess environmental noise,” Smith suggests.

A Wake-Up Call About Human Impact

While these adaptations show remarkable evolutionary flexibility, researchers emphasize they represent environmental stress rather than positive change. The modifications demonstrate how profoundly human activities alter natural systems, forcing wildlife to evolve just to survive alongside us.

“These findings clearly show that interfering with the environment has a detectable effect on wildlife,” Feijó concludes. The study serves as a reminder that evolutionary changes often occur unnoticed unless researchers have access to long-term specimen collections.

“Change is probably happening under your nose, and you don’t see it happening unless you use resources like museum collections,” Smith emphasizes. This research reveals that evolution isn’t just an ancient process—it’s happening right now in response to human influence.