Skull study shows Chicago's rodents are rapidly evolving

"Museum collections allow you to time travel," says Stephanie Smith, a mammalogist, XCT laboratory manager at the Field Museum, and co-author of a new paper in the journal Integrative and Comparative Biology detailing the discovery. "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 collections are made up of more than 245,000 specimens from all over the world, but there's especially good representation of animals from Chicago, where the museum is located. What's more, these collections 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," says Smith. "We thought, this is a great resource to exploit."

The researchers picked two rodents commonly found in Chicago: eastern chipmunks and eastern meadow voles. "We chose these two species because they have different biology, and we thought they might be responding differently to the stresses of urbanization," says Anderson Feijó, assistant curator of mammals at the Field Museum and co-author of the study. Chipmunks are in the same family as squirrels, and spend most of their time aboveground, where they eat a wide variety of foods, including nuts, seeds, fruits, insects, and even frogs. Voles are more closely related to hamsters. They mostly eat plants, and they spend a lot of time in underground burrows.

Two of the study's co-authors, Field Museum Women in Science interns Alyssa Stringer and Luna Bian, measured the skulls of 132 chipmunks and 193 voles. The team focused on skulls because skulls contain information about the animals' sensory systems and diet, and they tend to be 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," says Smith.

Stringer and Bian took measurements of different parts of the skulls, noting things like the overall skull length and the length of the rows of teeth. They also created 3D scans of the skulls of 82 of the chipmunks and 54 of the voles. This part of the analysis, called geometric morphometrics, entailed digitally stacking the skull scans on top of each other and comparing the distances between different points on them.

These analyses revealed small but significant changes in the rodents' skulls over the past century. The chipmunks' skulls became larger over time, but the row of teeth along the sides of their mouths became shorter. Bony bumps in the voles' skulls that house the inner ear shrank over time. But it wasn't clear why they were changing.

To find an explanation for these changes, the scientists turned to historical records of temperature and levels of urbanization. "We tried very hard to come up with a way to quantify the spread of urbanization," says Feijó. "We took advantage of satellite images showing the amount of area covered by buildings, dating back to 1940." (Specimens older than 1940 were either from areas that were still wild in 1940, and thus could safely be assumed to be wild before that, or from highly urbanized areas like downtown Chicago.)

The researchers found that the changes in climate didn't explain the changes in the rodents' skulls, but the degree of urbanization did. The different ways the animals' skulls changed may be related to the different ways that an increasingly urban habitat affected them.

"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. 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, on the other hand, had smaller auditory bullae, bone structures associated with hearing. "We think this may relate to the city being loud -- having these bones be smaller might help dampen excess environmental noise," says Smith.

While these rodents have been able to evolve little changes to make it easier to live among humans, the take-home lesson isn't that animals will just adapt to whatever we throw at them. Rather, these voles with smaller ear bones and chipmunks with smaller teeth are proof of how profoundly humans affect our environment and our capacity to make the world harder for our fellow animals to live in. This is a wake-up call.

"These findings clearly show that interfering with the environment has a detectable effect on wildlife," says Feijó.

"Change is probably happening under your nose, and you don't see it happening unless you use resources like museum collections," says Smith.