City mouse or country mouse? I collect mice from Philly homes to study how they got so good at city life

By | March 14, 2024

Dusty barns, shiny stables and damp cellars. These are all places where you might find a house mouse – or a member of my research team.

I’m an evolutionary biologist and my lab at Drexel University studies wild house mice. With the help of Philly residents, we’re collecting mice from high-rises and townhomes to learn more about the effects of city living on house mice. In short, we want to know if there is a scientific basis for the fable ‘The city mouse and the country mouse’, in which the cousins ​​eat differently depending on where they live.

Cities are hotter and have many people living in high densities, which means more waste and usually more pollution. This could affect how species living in cities evolve. Cities are also dominated by artificial habitats such as sidewalks, high-rise buildings and subway lines, rather than open fields and forests.

We are interested in many possible changes, but especially in whether the many differences between urban and rural environments translate into genetic differences between urban and rural mice, such as which versions of genes related to metabolism are more common.

To find the answers, we sequence the genomes of the mice. With that data we can answer a variety of questions, such as: are city mice genetically more or less diverse than rural mice? Are there parts of DNA, the molecule that encodes genetic information, that are consistently different between urban and rural mice? If so, what are the functions of genes in those regions?

An illustration of two mice from a translation of Aesop's Fables, published in 1912.An illustration of two mice from a translation of Aesop's Fables, published in 1912.

Why study house mice?

One reason we study house mice is because they are so widespread. European settlers brought house mice to America about 500 years ago. The rodents have now spread to many different climates and habitats in North and South America in most places where humans live, including Philadelphia.

Although small in size, house mice have made immeasurable contributions to genetics and medicine. They are mammals like humans, but house mice reproduce quickly and are relatively easy to breed and maintain. Part of the reason why scientists adopted mice as a model system early on is because people were already breeding “beautiful mice” as pets. As a result, methods of keeping and breeding them were known.

Mice have many visible traits that geneticists can study. My team wants to learn more about the genes and traits that contributed to their ability to thrive in different environments. The work we do with wild house mice is also reflected in the work with laboratory mice and biomedical research. The house mice found in attics and closets are the same species studied in laboratories, but they are genetically more diverse than laboratory species. Our project will generate complete genome sequences of many wild mice, and that data could help scientists studying traits and diseases.

Tips for catching mice

I previously worked on a large project investigating how house mice have adapted to different climates in America. For that project, I went to a lot of farms in the eastern United States and got really good at trapping mice in barns.

Starting this project with a focus on cities was a new challenge. First, our team had to find Philly residents who wanted us to trap their mice. We spent a lot of time spreading the word on social media, talking to friends and posting flyers.

We spoke to many Philadelphians who were frustrated trying to rid their homes of mice. Some had videos of house mice dodging the traps they set or stealing the bait and running away. We share this frustration and feel it keenly. In some cases it took us many days to catch a single mouse in an apartment.

Part of the reason is that many homes in Philadelphia are old. This means they are often full of character and contain holes that provide mice with great places to hide. It is difficult to lure the mice out of their nests and into our traps. We had the most success with peanut butter bait, which has a strong and very attractive odor to mice. But mice are omnivores and eat a varied diet that also includes insects. We’ve heard many stories of community members using baits such as chocolate, cereal, cookies and even bits of bacon.

What’s next

We hope to be able to share the results in the next two years. We are working in three cities – Philadelphia, New York City and Richmond, Virginia – and have completed our first collections. Now we have to generate and analyze genetic data, so we are very busy in the laboratory.

We extract DNA, as well as another form of genetic material called RNA, from various tissues. With the DNA we will investigate how much genetic variation exists within populations of city mice, and whether there are genetic differences between city and country mice. The RNA will help us understand how differences in DNA translate into differences in metabolism, physiology and other cellular processes.

We also look at whether there are differences in properties. For example, we will measure their skulls and skeletons. We will sequence the DNA of the microbes in their digestive systems to learn about their gut microbiome, the collection of bacteria that live in their digestive systems, and use stable isotope analysis to identify any differences in their diet. Stable isotope analysis of food uses the ratios of naturally occurring atoms of elements such as carbon and nitrogen to determine what types of food an organism has eaten.

Cities are full of wildlife. Learning how cities shape mouse evolution can help us find better ways to manage mouse populations and other urban wildlife while better understanding evolution.

This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Megan Phifer-Rixey, Drexel University

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Megan Phifer-Rixey receives funding from the National Science Foundation (NSF CAREER 2332998 Division of Environmental Biology).

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