How rodents are changing their bodies to cope with city life

By Stephen Beech

Rodents are evolving to cope with living in urban areas, reveals new research.

Chipmunk and vole skulls from 125 years ago compared to today reflect changes in diet and noise exposure, say American scientists.

They found that voles now have smaller ear bones than over a century ago while chipmunks have smaller teeth to help them handle living in cities.

The researchers explained that, in general, evolution is a long, slow process of "tiny" changes passed down over generations, resulting in new adaptations and even new species over thousands or millions of years.

But when living things are faced with dramatic shifts in the world around them, they sometimes rapidly adapt to better survive.

Researchers recently found an example of evolution in real time, stored in the collection drawers of the Field Museum in Chicago.

By comparing the skulls of chipmunks and voles from the Chicago area collected over the past 125 years, the research team found evidence that the rodents have been adapting to life in an increasingly urban environment.

Study co-author Dr. Stephanie Smith, a mammalogist at the Field Museum, said: “Museum collections allow you to time travel.

“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.

“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.

“We thought, this is a great resource to exploit.”

The research team chose two rodents, eastern chipmunks and eastern meadow voles, commonly found in Chicago.

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

He explained that chipmunks are in the same family as squirrels, and spend most of their time above ground, where they eat a wide range of foods, including nuts, seeds, fruits, insects, and even frogs.

Voles are more closely related to hamsters, mostly eat plants, and they spend a lot of time in underground burrows.

The research team measured the skulls of 132 chipmunks and 193 voles.

They focused on skulls because they contain information about the animals’ sensory systems and diet, and they tend to be correlated with overall body size.

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

The researchers took measurements of different parts of the skulls, noting things such as 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.

That 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.

The analysis 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, revealed the findings published in the journal Integrative and Comparative Biology.

But it wasn’t clear why they were changing.

To try and find an explanation for the changes, the researchers turned to historical records of temperature and levels of urbanization.

Dr. Feijó said, “We tried very hard to come up with a way to quantify the spread of urbanization.

“We took advantage of satellite images showing the amount of area covered by buildings, dating back to 1940.”

He explained that specimens older than 1940 were either from areas that were still wild in 1940, and so could safely be assumed to be wild before that, or from highly urbanized areas such as downtown Chicago.

The research team found that the changes in climate didn’t explain the changes in the rodents’ skulls, but the degree of urbanization did.

The researchers believe that the different ways the animals’ skulls changed may be related to the varying effects of an increasingly urban habitat on them.

Dr. Feijó said, “Over the last century, chipmunks in Chicago have been getting bigger, but their teeth are getting smaller.

"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.

Dr. Smith said, “We think this may relate to the city being loud - having these bones be smaller might help dampen excess environmental noise."

The researchers say voles with smaller ear bones and chipmunks with smaller teeth are proof of how "profoundly" humans affect the environment and make the world harder for other species to live in.

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

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