Got rats?
Some people who live in cities say the number of urban rats is increasing. These reports are mostly anecdotal. This means they are based on what people are saying rather than on scientific data.
Jonathan Richardson, a biology professor at the University of Richmond in the United States, and his colleagues wanted to find out if rat numbers truly are increasing in cities.
Surprisingly, there is little long-term data on city rats, even though rat numbers are a health and safety concern around the world. So instead the researchers collected data on public rat sightings and complaints, and pest inspection reports. Their study included 16 cities — 13 in the United States, as well as Tokyo (Japan), Toronto (Canada), and Amsterdam (the Netherlands). These cities had seven to 17 years’ worth of such data.
What did they find?
The researchers found that many cities saw a big surge in rat numbers over the years. Washington, D.C., San Francisco, Toronto, New York City, and Amsterdam had the largest increases, followed by Oakland, Buffalo, Chicago, Boston, Kansas City, and Cincinnati. Only Tokyo, Louisville, and New Orleans saw decreasing rat numbers.
What is causing this increase in rats? The researchers found the strongest factor associated with an increased number of rats is a city’s average temperature increase. “Cities that had a greater rise in temperature over time had larger increases in rat sightings,” the authors write. The authors think that warmer weather could be making rats more active, while they seek out food or mates.
The analysis also revealed that cities with fewer green spaces saw greater increases in rats.
The third factor linked to more rats was high human population density. “As more people inhabit a city, more food waste becomes available as a resource for rats,” the authors write, suggesting a possible explanation.
Jonathan Richardson hopes this study causes city officials and scientists to gather more data about rats. More data on rats will help people figure out how to reduce rat numbers in cities.
David Brown adapted this story for Mongabay Kids, based on an article by Kristine Sabillo from Mongabay News. The scientific study described in the article can be viewed here.
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