In 2015, a viral video emerged of a rat bouncing down the stairs of a New York City subway station, dragging an entire slice of pizza in its mouth. “Pizza rat,” as the critter was dubbed, was quickly trumpeted as an emblem of the city. It was determined, it loved pizza, and it was seemingly inured to the grimy depths of the city’s transit system. No, there was no doubt about it: This rodent was a New Yorker.
Now, as Robin McKie reports for the Guardian, a new study suggests that the Big Apple’s rats have in fact undergone genetic changes that make them well-suited to life in the concrete jungle—and susceptible to some of the same challenges that are facing humans.
The paper, which has not yet undergone peer review, was published recently on the preprint server bioRxiv. An estimated two million rats scurry about the city, so the researchers behind the study certainly had plenty of subjects to choose from. They just needed to catch the critters—which they did by luring them into traps filled with bacon, peanut butter and oats.
In total, the team sequenced the genomes of 29 NYC brown rats (Rattus norvegicus) and compared them to DNA samples from brown rats in rural northeast China, which is believed to be the ancestral range of the species. In particular, explains Ewen Callaway of Nature, the scientists were looking for signs of selective sweeps, or the evolutionary process that sees beneficial mutations become prevalent in a given population.
The analysis revealed dozens of genes that showed signs of selective sweeps among the New York rodents, some associated with things like mobility, behavior and diet. These sweeps appeared to be recent mutations that occurred after a split from the ancestral population, which was followed by the rats’ migration from Asia to Europe and then to America.
While it is difficult, at this point, to draw definitive conclusions about how these genetic quirks have helped rats adapt to city life, the researchers put forth some interesting theories. Some genes, for instance, may be associated with resistance to rodenticide. Another gene that was a “plausible target for selection,” as the study authors put it, was CACNA1C, which has been linked to psychiatric disorders in humans. Perhaps stresses associated with local predators or other novel stimuli are tweaking the rats’ DNA, the researchers theorize. Still other genes highlighted by the researchers may impact locomotion in rodents.
“This could reflect the fact that urban rats have to move through highly artificial environments that are very different from natural habitats,” Arbel Harpak, a population geneticist at Columbia University and the study’s lead author, tells the Guardian. “So you could argue these gene changes might have evolved to help them move more easily through sewers and pipes.”
Another interesting find lay in changes to genes associated with the metabolization of carbohydrates and sugars. Scavenging off the scraps of their human counterparts, urban-dwelling rats are eating increasingly large portions of processed sugars and fats. But like humans, the study authors note, it is possible that rats’ unhealthy diet makes them susceptible to health issues.
This paper is not the first study to suggest that NYC living has an impact on rats’ DNA. In 2017, a paper found genetic distinctions among rats in uptown and downtown Manhattan, likely because the rodents tend to stick within a limited home range. Now, the researchers behind the new report want to study rats from other cities, to see if their genomes evolved in similar ways to the NYC rodent population.
It certainly seems possible that rats have been profoundly impacted by life in close proximity to humans—as much as humans might not want them around.
“We know rats have changed in incredible ways in their behaviour and in their diet,” Harpak tells the Guardian, “just as human communities have changed.”