Rodents in Chicago have been adapting to an increasingly urban environment over the past century, a new study has shown.
Evolution generally entails seeing small changes passed down over generations, leading to new adaptations and new species over thousands or millions of years.
However, dramatic changes in an environment can cause a species to evolve rapidly for a better chance at survival. An example of this evolution was discovered by researchers at the Field Museum in Chicago, Illinois who compared the skulls of chipmunks and voles from the Chicagoland area that were collected over the past 125 years.
The museum’s mammal collections consist of more than 245,000 specimens from around the world, with a good sample of animals from Chicago. These collections each represent different moments in time throughout the past century.
“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,” mammalogist Stephanie Smith, who is an XCT laboratory manager at the Field Museum and co-author of the study, said in a statement.
The study, which examined the effects of urbanization and climate change on the skull, was centered on two rodents that are commonly found in Chicago—eastern chipmunks and eastern meadow voles.
Researchers focused on skulls as they contain information about the rodents’ sensory systems and diet and are typically correlated with overall body size.
“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,” Smith noted.
The scientists observed small but significant changes in the rodents’ skulls over the past 100 years. The skulls of chipmunks became larger over time, while the row of teeth along the sides of their mouths became shorter. Bony bumps in the skulls of voles that house the inner ear shrank over time. However, it wasn’t clear why these changes occurred.
Looking at historical records of temperature and levels of urbanization, the scientists found that while changes in climate didn’t explain the changes in the rodents’ skulls, the degree of urbanization did. So, the changes may be related to the various ways that an increasingly urban habitat impacted these rodents.
“Over the last century, chipmunks in Chicago have been getting bigger, but their teeth are getting smaller,” Anderson Feijó, assistant curator of mammals at the Field Museum, said in a statement. “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.”
Voles were found to have smaller auditory bullae, bone structures that are associated with hearing. “We think this may relate to the city being loud— having these bones be smaller might help dampen excess environmental noise,” Smith explained.
“These different patterns between chipmunks and voles reveal species-specific responses to the same human-induced habitat changes and the need for nuanced conservation plans in the face of continuing change,” the scientists said in the study.
The researchers noted that the changes made by the rodents to make it easier to live among humans serve as a wake-up call, rather than evidence that animals can adapt to any setting. “These findings clearly show that interfering with the environment has a detectable effect on wildlife,” Feijó said.
The study underscores the profound impact that humans can have on their environment as well as their capacity to make it harder for wildlife to co-exist in the world.
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Reference
Feijó, A., Stringer, A., Bian, L., Smith, S., (2025). Limited cranial shifts in city-dwelling rodents after a century of urbanization. Integrative and Comparative Biology. https://doi.org/10.1093/icb/icaf081.
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