Rats: Just a Bunch of Homebodies

It's called the 'rat race,' but it turns out they're not really going very far.
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A study just published in the journal Molecular Ecology has found that while inner-city rodents may appear to roam far and wide, they actually stick to distinct neighborhoods for the majority of their lives. A team of researchers from the Johns Hopkins Bloomberg School of Public Health decided to take a close look at rats' movement patterns in Baltimore, in an effort to better understand how rodents transmit diseases to humans and why the city's expensive eradication efforts in the past 50 years have failed to reduce the rodent population.

That meant examining wild Norway rats — also known as sewer, brown or wharf rats — which are the most common urban rat in the world, growing as large as two pounds. Norway rats are particularly prevalent in central areas of Baltimore, partly because the city's port was a major depository for grain during the American Revolution, which is about the same time Norway rats began invading the city.

The researchers trapped nearly 300 rats from 11 different residential neighborhoods throughout Baltimore and conducted genetic tests to discover how the rats were related; it turns out there are "East Baltimore rats" and "West Baltimore rats." As the researchers write: "The east-west differentiation of the populations occurs along the Jones Falls, a rapidly flowing waterway that originates to the north in Baltimore County and travels south through the city before it empties into the harbor." These two different populations are unrelated and don't interact across the waterway barrier.

In part, that's because rats like to stay close to the ol' neighborhood. The researchers found that rats rarely venture more than a city block away from their nest — although, when confronted by great upheaval, like neighborhood restoration, rat populations will travel as far as seven miles to repopulate abandoned areas. Within the east-west hemispheres, families of rats build smaller communities of about 11 city blocks, and within each community, "neighborhoods" — scarcely longer than the average alley —represent home for a rat.

"Most rat movements were limited within individual city blocks, yet a small percentage of rats moved distances as much as 400 meters (approximately 2.7 times the average length of a Baltimore alley), and, rarely, much farther (across the city)," the researchers write.

The findings have several implications for eradication efforts, which may fail because they encourage rodents to repopulate new areas and risk the further spread of disease.

"Understanding aspects of ecology and gene flow, such as the movement of commensal rodents with human expansion in urban landscapes, is critical to understanding the dynamics of rodent-borne pathogens and is valuable for mitigating human disease outbreaks," the researchers write. "... Our findings provide evidence that dividing urban rat populations into management units at the level of city blocks will be ineffective, and that control must occur at a larger scale."

Of course, if the rats do start to roam around, Today In Mice already has that covered.

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