Museum study shows human impact on chipmunks and voles in Chicago
'The oldest ones we have are from 1891, and these were collected in Jackson Park over 100 years ago,' Smith said, pointing to the two chipmunks. 'You can see how good-looking they both look, and that's the beauty of this collection. We preserve this material to last, generation to generation.'
In many ways, these well-preserved chipmunks mirror those that Chicagoans might see scurrying down alleyways or hopping around in parks today, with their distinctive white stripes and bushy tails.
But according to a new study by Field Museum researchers, Chicago's modern-day rodents have evolved to look quite different from what they did just a century ago — mostly because of human development.
Smith, along with assistant curator of mammals Anderson Feijó and two Field Museum interns, measured the skulls of nearly 400 rodent specimens — collected from the 1890s to modern day — to see how their skull structure had changed over time. The study, published June 26, focused on chipmunks and voles, aiming to compare the evolution of above- and below-ground species.
They found that over time, Chicago chipmunks have overall gotten larger, but the row of teeth along the side of their jaw has gotten smaller.
'It's probably related to the food they're eating,' Feijó said. 'Chipmunks are much more interactive with humans and have access to different kinds of food we eat. So we hypothesize they are eating more soft food and because they require less bite force, which reflects in the tooth rows.'
In vole samples, they found that the animals' size had stayed pretty consistent — but that the bumps in their skull that house the inner ear had shrunk. As Chicago grew over the past hundred years, the voles may have adapted to have smaller ears in order to protect them from the noisy city streets, Smith said.
'These two animals are small mammals, so people might sort of put them in the same category in their heads, right?' she said. 'But they're responding to this human alteration of the landscape in different ways. So preservation of natural populations of animals is not a one-size-fits-all thing … as the city changes, as we try and facilitate the longevity of green areas where these animals live, maybe we need to think about different solutions for different animals.'
During the 20th century, Chicago was one of the fastest growing cities in the world, expanding from 516,000 residents in the 1910 census to 3.5 million residents by the 1950 census. With this rapid population growth also came rapid urbanization, as buildings, highways and transit grew more and more dense. While just 6% of land in the Chicago area was used for urban development in 1900, this grew to 34% by 1992, according to data compiled by the U.S. Geological Survey and the Illinois Department of Natural Resources.
With fewer natural areas in the region, native rodents like chipmunks and voles have had to adapt to urban environments and find new sources of food and shelter. The Field Museum team used satellite imagery dating back to 1940 to determine how much of each rodent's habitat had been used for urban buildings at the time that they lived.
'These kinds of adaptations are happening across all different animals, different groups, different areas,' Feijo said. 'It's just a way that animals need to figure out how to survive these new conditions.'
Similar trends have been documented in other major cities. A 2020 study of rats in New York City found that these East Coast rodents' teeth have also shrunk over time, similar to those of Chicago chipmunks.
The Field Museum houses over 40 million mammal specimens in its archives, mostly collected in Chicago and the surrounding region, enabling scientists to track how different species have changed over time. The museum's exhibits represent less than 1% of its actual collections, according to museum communications manager Kate Golembiewski.
In addition to manually measuring the chipmunks' and voles' skulls, researchers also created 3D scans of some of the specimens, which allowed them to more closely compare the differences between each specimens' bone structure.
Moving forward, Smith and Feijo hope to use their data to find a stronger correlation between evolutionary change and urbanization.
'These animals, the fact that they are adapting and still relatively abundant shows that they are changing,' Smith said. 'But that doesn't mean that they're gonna be able to do that forever. So it's important to keep an ear to the ground, and try to understand what these guys are up to.'
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