June 3, 2009 -- Urban rats rarely stray from their own alleyways, found a new study, but a few adventurous individuals make city-wide treks.
Understanding how rats move around may aid efforts to eradicate them, said lead researcher Gregory Glass, an infectious disease biologist at Johns Hopkins University in Baltimore. The mangy rodents spread diseases, damage buildings, and cause psychological stress.
"It's a public health issue at a number of different levels," Glass said. "People just don't like rats running around."
As attention-grabbing as rats can be among homeowners, scientists know surprisingly little about their basic biology, said Lawrence Heaney, Curator of Mammals at the Field Museum in Chicago.
Most rat research so far has involved simply watching what the animals do at night. Based on those studies, scientists have long assumed that rats are homebodies.
"They just don't go far," Glass said. "They come out of their burrows. They walk to the nearest trash can and grab something to eat. They go to the nearest puddle and drink some water. They walk around a bit. And they go home."
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Yet, every time rats are eradicated from an area, new ones eventually move back in. So, at least some rats must be traveling further distances.
To get a more accurate idea of how mobile Baltimore's city rats really are, Glass and colleagues turned to genetics. The scientists collected DNA from more than 275 rats that lived in 11 areas of the city. By comparing key regions of the animals' genomes, the researchers were able to see how closely related each rat was to the others, both nearby and far away.
Analyses revealed that Baltimore's rats tend to mingle mostly within their own small communities. Inside those neighborhoods -- which spanned about 100 meters, or the length of a city block -- rats are more genetically similar to each other than they re to rats that live in other parts of the city.
About 5 percent of the rats, though, move into new alleys from as much as seven miles away, the researchers reported in the journal Molecular Ecology. Baltimore is about 10 miles wide. A lot of movement by just a few individuals is enough to explain why all of the city's rats still belong to the same species.
Those movement patterns might also explain how diseases such as hantavirus spread among rats and from rats to people, Glass said. While still unpublished, his work is turning up evidence that traveling rats are more likely to carry viruses than are their sedentary peers. One possible explanation is that the viruses change a rat's behavior, making the animal more likely to travel further, efficiently spreading disease along the way.
"Long-distance movements may be infrequent," Heaney said. "But man, they are important."
The findings might give people a new source of power over their rodent foes, Glass added. Traditionally, scientists have assumed that the best way to get rid of rats was to target a block-wide area and then move on.
Now, it seems, this strategy works only for the 95 percent of rats that don't move much. Instead, it may be better to come back to a cleared area every six months or so to get rid of any super-travelers that have pounced on an open opportunity.
"What we think this does is explain where we went wrong in what we told people about how to control rats," Glass said. "You really can't do just a block. You need to target a whole neighborhood."
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