City Rats Eat Meat. Country Rats Eat What They Can.

It’s been nearly 3,000 years since Aesop wrote “The Town Mouse and the Country Mouse,” the fable in which an urban rodent exposes his rural cousin to the city’s superior dining options. A new study suggests Aesop was right about the geographical differences in rodent diets.

By analyzing the remains of brown rats that lived in and around Toronto between 1790 and 1890, researchers have determined that city rats enjoyed a higher-quality and more stable diet than rural rats did. Just as in Aesop’s tale, the city rats benefited from the largesse of human waste, whereas country rats scraped by.

“Rats that lived in the city had a lot more meat in their diet,” said Eric Guiry, an archaeologist at the University of British Columbia and lead author of the study, which was published Wednesday in Proceedings of the Royal Society B. “You could see the difference in their bones.”

Guiry and his co-author, Michael Buckley of the University of Manchester, are specialists in the emerging field of paleoproteomics, which uses the proteins in ancient bones to glean insights into an animal’s behavior. Guiry wanted to use the technique on rats to see what it could reveal about human populations in the 1800s — a less onerous proposal than digging up and analyzing human remains. Knowledge gained from the research also could help cities better control their rat populations.

“Rats are really interesting, because their diets are a reflection of foods people leave lying around,” Guiry said.

For the current study, the researchers collected rat bones from museums, universities and archaeologists in the Toronto area. First, they analyzed the molecular structure of the bones to ensure they were all from the same species, Rattus norvegicus. Then, using a high-powered spectrometer, the team studied the ratios of different proteins for telltale signs of various foods.

“Based on the premise that you are what you eat, we can use different bone chemistry signatures — we call them stable isotope values — to determine differences in diets between individuals in the past,” Guiry said. “You could see there was a big difference between rats that lived in the city and rats that lived in farmsteads and rural areas.”

Those differences weren’t limited to the kinds of foods the rats ate. Because the bones came from multiple sites, the researchers were able to discern far greater dietary consistency among the city rats than among the country ones.

“Across all the different sites in the urban area, rat diets didn’t differ much,” Guiry said. “But when we looked at rural areas, rat diets were all over the map.”

To better understand those rural diets, the researchers also analyzed the remains of other animals living in those areas at the time, including raccoons and groundhogs. They found quite a bit of overlap, suggesting that brown rats, which may have come over on boats from Europe in the 1800s, were an early introduced species that competed for resources with North America’s native animals.

Although it’s not necessarily surprising that city rats eat better than country rats — Aesop suspected as much back in 600 B.C. — Guiry is hopeful that his method could be used to shed light on human diets and the density of populations during other, less well-documented eras. The real challenge, he said, is finding usable remains: “Until recently, archaeologists would find rat bones and just think, ‘Oh, garbage.'”

More urgently, he said, the method could hold promise for studying rat behavior in cities and controlling rat infestations, a task that costs billions of dollars every year.

“This has given us an overview of how rats are eating and behaving over a long period of time in an urban area,” Guiry said. “And food is really important to how they reproduce.”