Rodents in Chicago are rapidly evolving to cope with urban life, say scientists in a new study based on museum specimens dating back over 100 years.
Evolution is typically seen as a gradual process, taking thousands or even millions of years. But by examining skulls of eastern chipmunks and eastern meadow voles collected from the Chicago region over the past 125 years (now stored in the Field Museum in Chicago), new research has revealed that some animals are adapting much faster in response to human-altered landscapes.
“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,” says Stephanie Smith, a mammalogist and XCT laboratory manager at the Field Museum. “We thought, this is a great resource to exploit.”
The study, published in the journal Integrative and Comparative Biology, took advantage of the museum’s extensive natural history collections, which include over 245,000 mammal specimens. These collections allowed researchers to look across generations and track physical changes in the two common rodent species that live in and around Chicago.
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Chicago’s evolving animals
Eastern chipmunks, which are ground-dwelling relatives of squirrels, and eastern meadow voles, which resemble hamsters and live mostly underground, were chosen for their differing lifestyles and diets.
“We chose these two species because they have different biology, and we thought they might be responding differently to the stresses of urbanisation,” says Anderson Feijó, assistant curator of mammals at the Field Museum and co-author of the study.
Two interns from the Field Museum Women in Science programme, Alyssa Stringer and Luna Bian, carefully measured 132 chipmunk and 193 vole skulls. They recorded traits such as skull length and tooth row size, and also produced detailed 3D scans of many specimens using a method known as geometric morphometrics. This allowed them to digitally compare skull shapes from different eras.
Their findings were striking. “Over the last century, chipmunks in Chicago have been getting bigger, but their teeth are getting smaller,” says 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.”
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Voles showed a different pattern. The bony structures that house the inner ear, known as the auditory bullae, have shrunk over time. This change may be an adaptation to the constant noise of city life.
“We think this may relate to the city being loud – having these bones be smaller might help dampen excess environmental noise,” says Smith.
To explore what might be causing these changes, the team analysed satellite images and historical data on urban growth from the 1940s onwards. They found a strong link between the degree of urbanisation and changes in skull structure, while climate change alone did not seem to account for the adaptations observed.
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“Museum collections allow you to time travel,” says Smith. “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 study is a rare glimpse into evolution happening in 'real time'. It also highlights how closely tied wildlife is to the environments we shape – and how even small changes to their bodies can reflect major shifts in their surroundings.
“These findings clearly show that interfering with the environment has a detectable effect on wildlife,” says Feijó.
Rather than assuming animals can easily adjust to human environments, the researchers hope this study serves as a reminder of the impact we have on the natural world.
“Change is probably happening under your nose, and you don’t see it happening unless you use resources like museum collections,” adds Smith.
Read more about the study: Limited cranial shifts in city-dwelling rodents after a century of urbanization
Top image: Chicago. Credit: Getty
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