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There are always better restaurants in the city, and that could be making the town mice of New York genetically distinct from their country cousins.

at the State University of New York and of Fordham University in New York City caught 48 white-footed mice () from three New York parks and three nearby rural areas. The mice are native to this part of North America, so the pair wanted to find out whether some had begun to evolve for city living.

They examined the mice’s RNA to see if the rural and urban populations expressed different genes. Ultimately, they homed in on 19 single nucleotide polymorphisms (SNPs): places in the genome where a single letter varies from mouse to mouse.

Several SNPs were in genes associated with digestion and other metabolic processes. One highlighted gene was used to produce omega-3 and omega-6 fatty acids. A version of this gene appears to have been selected for in humans as we moved from hunter-gathering to agriculture.

The work is “at the forefront of biology”, says at Providence College in Rhode Island.

The cheeseburger hypothesis

The survey also highlighted genes linked to non-alcoholic fatty liver disease, which may result from having to process a lot of fatty acids. This could be due to a diet rich in fast foods. “The first thing that we thought of was the ‘cheeseburger hypothesis’: urban mice subsidising their diet on human food waste,” says Harris. If so, the mice may be like “Pizza Rat”, a New York rodent videoed carrying a whole slice of pizza.

In line with this, city mice had larger livers with more scar tissue, says Harris, who was at the City University of New York when most of the work was done. However, the sample size was too small to be sure.

Besides, New York offers other foods, like seeds, nuts and berries in its parks. Such resources are more plentiful than in rural areas, where there is more competition from other species.

The mice may be enjoying an eclectic mix of urban food resources, says Richardson, occasionally boosting their calorie intake by dining on human food waste.

The next step will be to examine the function of these genes more closely to assess their impact on fitness and selection, says at Harvard University.

Hoekstra says the work is part of a wave of studies investigating examples of rapid adaptation. “That provides us with a really cool way to study evolutionary change, sort of as it’s happening,” she says.