Chicago rodents develop smaller teeth and ears

Chicago-based chipmunks and voles living in Chicago have experienced measurable evolutionary changes over the past 125 years, adapting to urban life by developing smaller teeth and changing hearing structures.

Researchers at the Wild Museum discovered these rapid adaptations by comparing rodent skulls collected from the 1890s to today, providing rare real-time evidence of evolution’s response to human-driven environmental changes.

The study, published in Comprehensive and Comparative Biology, analyzed 132 Chipmunk and 193 Vole specimens in the museum’s collection. Scientists have found that urban chipmunks develop larger skulls but have shorter teeth, while voles evolved smaller auditory marijuana – a bone structure related to hearing.

Museum collection as time machine

“The museum collection allows you to travel time,” explains Stephanie Smith, mammal and XCT lab manager at the Field Museum. “ Rather than limiting yourself to studying specimens collected during the course of a project, or a person’s life, it allows you to study things through more evolutionarily relevant time scales.”

The mammal collection of the Outdoor Museum contains more than 245,000 specimens, and the Chicago area is represented throughout for more than a century. This unique archive enables researchers to track evolutionary changes that are unobservable within a typical study timeframe.

Two, different adaptations

The researchers chose Eastern Chipmunks and Eastern Meadow Volleys because of their contrasting lifestyle. Chipmunks spend most of their time on the ground and eat a variety of diets including nuts, seeds, fruits, insects, and even frogs. Voles, more closely related to hamsters, mainly eat plants and mainly live in underground caves.

Evolutionary changes reflect the unique urban challenges of each species:

• Chipmunks are growing overall, but have shorter teeth rows, which suggests diets turn to foods provided by humans
• Voles evolved into smaller auditory streams that might adapt to urban noise pollution
• Climate change cannot explain skull modification, but urbanization levels are closely related to changes
• Adaptations gradually occur, but continue to happen for more than a century

Measuring urban evolution

Field Museum interns Alyssa Stringer and Luna Bian carefully measured the skull size and used geometric morphometrics to create a 3D scan, a technique that digitally stacks the skull scan for more precise anatomical distances. The team correlates these measurements with historical records of temperature and urbanization levels dating back to 1940.

“We are working very hard to come up with a way to quantify the spread of urbanization,” noted Anderson Feijó, assistant curator of mammals at the Field Museum. “We used satellite images to show the amount of area covered by buildings, dating back to 1940.”

Diet and noise adjustments

Skull changes may reflect specific urban stress. Chipmunks have larger bodies, but smaller teeth, indicating that they consume more calories of human food while eating fewer hard items, such as nuts and seeds, requiring robust dental equipment.

“In the last century, Chicago chipmunks were getting bigger and bigger, but their teeth were getting smaller,” Fioho Observer said. “We think it might be related to the food they eat. They might eat more people-related foods, which would make them bigger, but not necessarily healthier.”

The smaller auditory structure of voles may help them cope with urban noise pollution. “We think it might have something to do with the city’s sounds – those bones are getting smaller may help suppress excessive ambient noise,” Smith advises.

Wake up call about human impact

Although these adaptations show significant evolutionary flexibility, the researchers emphasize that they represent environmental stress rather than positive changes. These modifications show that the profound effects of human activities have changed natural systems, forcing wildlife to develop to survive with us.

“These findings clearly show that disturbing the environment has detectable effects on wildlife,” Feijó concluded. This study reminds you that unless researchers can collect it using long-term specimens, evolutionary changes usually don’t attract attention.

“It may be changing under the nose, and you won’t see that unless you use resources like museum collections,” Smith stressed. The study shows that evolution is more than just an ancient process, it is currently responding to human influence.

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