Urban Rats Enjoy Richer, More Reliable Diet Than Their Rural Counterparts

Despite rats’ prevalence as one of the world’s most despised—and widespread—urban pests, researchers have a surprisingly slim understanding of the rodents’ lifestyle. But a new study published in Proceedings of the Royal Society B offers a unique glimpse of city versus country rats’ dining habits, as well as an overview of these findings’ implications for human populations.

Douglas Quenqua of The New York Times reports that study co-authors Eric Guiry, a zooarchaeologist at the University of British Columbia, and Michael Buckley, a zooarchaeologist at the University of Manchester, analyzed the remains of 86 brown rats that roamed the streets of Toronto and its outlying districts between 1790 and 1890. Urban rats appeared to enjoy a steady diet of high-quality food, including protein-rich meat, while rural rats struggled to get by on limited, often meat-free meals.

These results aren’t wholly surprising: Cities are home to higher numbers of trash-producing humans, meaning their rat residents have access to a wide variety of secondhand food sources, writes Phys.org’s Bob Yirka. Urban rats face few competitors when scrounging for food, and they also benefit from cities’ built-up landscapes, which provide plenty of places for rodents to hide and enjoy their stolen feasts.

Comparatively, rural rats must adopt broader foraging strategies that account for what the study describes as unreliable “food subsidies from human food systems;” in other words, smaller human populations produce less waste, curbing rats’ food options and forcing them to compete for resources with raccoons and other foragers.

“Rats are really interesting, because their diets are a reflection of foods people leave lying around,” Guiry tells the Times’ Quenqua.

As experts in the field of paleoproteomics, or the study of proteins found in ancient bones with a goal of gaining insights on an animal’s behavior, Guiry and Buckley wanted to determine what 18th- and 19th-century Canadian rat populations revealed about their human neighbors. They gathered 44 rural rat bone samples and 42 urban samples from scientific and cultural institutions across the Toronto area, double checked that all of the bones belonged to members of the Rattus norvegicus species, and used a high-powered spectrometer to identify chemical signatures associated with certain foods.

The researchers found that rats residing in different parts of the city enjoyed fairly consistent meat-heavy diets. Rural rats, however, exhibited little dietary consistency, as changing food sources and inter-species competition limited their options.

Quenqua notes that Guiry and Buckley further examined rural animals’ dietary habits by studying the remains of raccoons and groundhogs that populated the Toronto area between 1790 and 1890. They found significant overlap between the rodents’ and larger animals’ diets, suggesting that all vied for the same resources.

Interestingly, the authors write that rural rats may have found ways to take advantage of human food systems, with several rodents exhibiting evidence of skilled maize pilfering. Livestock and herbivores, on the other hand, did not appear to draw on maize as a food source.

Guiry tells Quenqua he hopes the method featured in the study will be used in future analysis of human or rat diets and population density. As he and Buckley explain in their paper, urban centers hoping to mitigate growing rat populations could benefit from developing ecologically based management strategies that factor in rodents’ use of urban spaces. And rat remains, which have long been discarded or dismissed as insignificant research tools, are the key to understanding this relationship.

The authors conclude, “The archaeological record can be used to study historical trends in the dynamics of rat dietary behaviour at a variety of scales and in spatio-temporal contexts that directly foreground many of the issues in rodent ecology being faced in today's modern cities."

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.