Rat populations can be high in areas with predictable and reliable food sources; nearby areas with limited resources can be rat free.
(Photo: Matt Frye)

Despite their considerable impacts on human health, the food supply and physical/structural damage, urban rats are under studied. In fact, most of the published literature on rats dates back to the mid-1900s. In recent years, however, rodent researchers across the globe are starting to take a closer look at these pests that thrive in human environments. In November 2017, five articles were published that examine the biology, ecology and evolution of rats. Here are some takeaways:

Genomics Shift in Urban Environments

An article published in Science by Marc Johnson and Jason Munshi-South highlights the role that urban environments play in the evolution of wildlife, and how an understanding of evolution can help mitigate pest problems. For example, cities can isolate populations by creating barriers to movement such as busy streets that are dangerous to cross. As a result, rats and other urban wildlife can be clustered into groups of closely related organisms. By understanding gene flow in these populations, we can learn about the actual area that a cluster of rats occupies, whether it’s a single building or a whole city block. This information is important for management since eliminating rats from one food establishment may have little impact on the population that can quickly re-infest the site from nearby areas.

NYC Rats Arrived in the 1700s — Then Closed the Doors

Having taken samples of rats across the entire island of Manhattan, Matt Combs and others found that all NYC rats share an evolutionary relationship with rats from Great Britain and France. This makes sense based on the trading history between the colonies and Europe in the 1700s. What’s surprising is the lack of genetic evidence for rat introductions from any other part of the globe. This suggests that New York City rats have repelled or killed all rats that arrived at the port of New York since the 1700s! In ecology, this is known as biotic resistance, where an existing group of organisms is able to repel invading ones.

Rat Populations are Patchy in Urban Areas

Several studies show that rats are not found throughout urban areas, but are instead found in “hot spots.” This is likely due to the availability of environmental conditions that favor rats, such as the presence of food, water and shelter. Research by Laura Angley and others showed nearby rat colonies could be genetically different, while rats further apart in comparable habitats showed some similarity. Is it possible that rat genomics lead to habitat preferences? That some rats prefer to live in housing, some in parks and others in commercial buildings?

Another finding in NYC was that rat populations greater than 0.87 miles (1400m) apart were genetically distinct. Based on the data, it seems that a small number of rats may move between these populations occasionally, but not enough to keep them genetically related. Most interesting and comical, uptown rat populations have limited breeding contact with downtown populations. While the Internet has enjoyed plenty of “Uptown Girl” parodies based on this result, the best explanation for this finding is that the midtown area serves as a buffer against rat movement. This could be due to differences in suitable habitat, pest management efforts or the availability of food resources.

Rodent-Borne Disease is Patchy in Urban Areas

Just as rat populations are patchy in urban environments, rodent-borne disease is also found in hot spots. Amy Peterson and others showed that rats in New York City and New Orleans are host to multiple species of Bartonella; a group of bacterial pathogens that cause fever and associated symptoms in people. But the occurrence of Bartonella in rats depended on the exact site where they were collected, and in New Orleans on the species of rat that was collected. (New Orleans is home to both Norway rats and roof rats.) Similarly, the paper by Angley and others found an uneven distribution of rodent pathogens in New York. These researchers noted some populations had a very high percentage of infected rats, and that rats from those sites were host to a greater diversity of pathogens and ectoparasites. Importantly, both studies state that the patchy occurrence of rodents and disease means that human health risks are unknown and can only be determined with more thorough sampling.

Rats are Really Difficult to Study

As someone who has conducted rodent research, I can tell you rats are hard to study. They are secretive, live in areas where access is difficult, nest underground, are nocturnal and are likely to be killed before we can study them. Unlike wildlife in other settings, urban rats are not easily tracked with radio telemetry or Global Positioning System (GPS) because of interference from buildings and hard surfaces. In a recent study by Kaylee Byers and others from The Vancouver Rat Project (www.vancouverratproject.com), they rediscovered this difficulty in trying to track movement of rats. The researchers “chipped” 14 rats with GPS tags that could either remote-download or store movement data. Unfortunately, the researchers were not able to obtain data from the devices, and when they recaptured three of the rats, the tags had been removed. The researchers said, “The ecology of Norway rats interferes with the effective use of GPS tags in urban settings.”

MORE QUESTIONS THAN ANSWERS.

Urban rodents have plagued cities around the world throughout recorded history, and efforts to eliminate them come in waves. In some cases this is based on financial resources, or the result of political campaigns, as described in Robert Sullivan’s book, “Rats.” The recent use of genomics to understand migration patterns of rats across the globe and at smaller spatial scales gives us new insight into their complex and secretive lives. Yet we are faced with more questions than answers about the risks they pose to human health and how best to manage rodents at large spatial scales.

Matt Frye is community IPM extension educator at Cornell University New York State IPM Program, and can be contacted at mfrye@gie.net.