Chicago’s rodents are evolving with the city

Rodents don’t usually evolve overnight. Like most species, they change slowly through tiny shifts passed down over thousands or millions of years. But when the world changes fast, like it has in cities over the past century, some animals keep up.

In fact, some animals even adapt to urbanization in real time. A team of researchers recently spotted signs of that kind of rapid evolution. The clues were found in hundreds of skills sitting quietly in drawers at the Field Museum in Chicago.

The skulls weren’t newly collected specimens – they spanned more than a century. And hidden in their measurements were subtle changes that told a bigger story.

By comparing the skulls of eastern chipmunks and eastern meadow voles collected over the past 125 years, scientists at the Field Museum uncovered signs of these animals adjusting to an increasingly urban environment.

A time machine in a museum

“Museum collections allow you to time travel,” said 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 includes over 245,000 specimens from around the world, but its home city is especially well represented. These Chicago-area animals offer a rare chance to study changes across both time and space – right in the scientists’ backyard.

“We’ve got things that are over 100 years old, and they’re in just as good of shape as things that were collected literally this year,” said Smith. “We thought, this is a great resource to exploit.”

Comparing rodents in the city of Chicago

The team focused on two rodent species that still call Chicago home. Chipmunks, which live above ground and eat a wide range of food, and voles, which live underground and mostly stick to plants. The contrast between them was part of the point.

“We chose these two species because they have different biology, and we thought they might be responding differently to the stresses of urbanization,” said Anderson Feijó, assistant curator of mammals at the Field Museum.

Two interns with the Field Museum Women in Science program – Alyssa Stringer and Luna Bian – played a central role in the work.

Rodent skulls and city adaptations

The researchers measured the skulls of 132 chipmunks and 193 voles. The skulls hold clues about diet, body size, and even sensory abilities.

“From the skulls, we can tell a little bit about how animals are changing in a lot of different, evolutionary relevant ways – how they’re dealing with their environment and how they’re taking in information,” said Smith.

Stringer and Bian measured basic features of each rodent skull, like overall length and the length of the tooth rows.

The researchers also used 3D scanning to digitally map 82 chipmunk and 54 vole skulls. This allowed them to compare tiny differences in shape across specimens using a method called geometric morphometrics.

The results revealed small but telling shifts. Chipmunks showed an increase in skull size, but their tooth rows got shorter.

Voles developed smaller bony structures in their skulls that are tied to hearing. The changes weren’t random – but they also weren’t clearly linked to temperature or climate.

Rodents respond to a louder, bigger city

To dig deeper, the team used satellite data to track how the city grew over time. They looked at building coverage starting in the 1940s and matched those trends to the locations where specimens were collected.

Older specimens came from areas known to be wild back then – or from downtown areas that were already developed.

“We tried very hard to come up with a way to quantify the spread of urbanization,” said Feijó. “We took advantage of satellite images showing the amount of area covered by buildings, dating back to 1940.”

Changes driven by urbanization

Urbanization, not climate, seemed to drive the rodent skull changes. And the way each species adapted may reflect how it lives.

“Over the last century, chipmunks in Chicago have been getting bigger, but their teeth are getting smaller,” said Feijó. “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. Meanwhile, their teeth are smaller – we think it’s because they’re eating less hard food, like the nuts and seeds they would normally eat.”

As for the voles? The parts of their skulls that hold the inner ear – the auditory bullae – have shrunk. “We think this may relate to the city being loud – having these bones be smaller might help dampen excess environmental noise,” said Smith.

A warning, not a win

It might be tempting to see this as a success story – rodents adjusting to city life. But the scientists see it differently.

The adaptations may help, but they’re not necessarily positive. Smaller teeth and smaller ear bones are symptoms of change, not signs of health.

“These findings clearly show that interfering with the environment has a detectable effect on wildlife,” said Feijó.

“Change is probably happening under your nose, and you don’t see it happening unless you use resources like museum collections,” said Smith.

The full study was published in the journal Integrative and Comparative Biology.

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