WASHINGTON - Although rodents have a free run of the city, they develop a fierce sense of loyalty to the neighbourhoods where they spend the rest of their lives.

Rats typically stay close to home. However when danger threatens, some rodents can travel as far as 11 km to repopulate abandoned areas.

An understanding of how rats in cities are connected provides information about which populations may spread disease, said Sam Scheiner, programme director in the National Science Foundation’s (NSF) Division of Environmental Biology, which funded the research.

For instance, Baltimore’s port was a once major delivery point for grain, probably how rats from Norway entered the city. These rats can weigh nearly one kg and are nearly as big as bandicoots, transmitting a variety of diseases to humans.

Despite expensive eradication bids, the number of rats in Baltimore has remained unchanged over 50 years, says scientist Greg Glass of Johns Hopkins, who co-authored the paper with associates from Yale University School of Medicine.

To understand why, researchers trapped nearly 300 rats from 11 residential areas of Baltimore and conducted genetic studies to see how the rats were related. The scientists found that East Baltimore rats are separated from their unrelated West-side counterparts by a large waterway known as Jones Falls.

Within these hemispheres, rat families form smaller communities of about 11 city blocks. Each community is further divided into neighbourhoods that span little more than the length of an average alley. To a city rat, that alley is home sweet home.

The findings suggest that while rats rarely migrate, neighbourhood eradication efforts may backfire by encouraging the rodents to repopulate other areas and further spread disease, said a Johns Hopkins release.

When you smell a rat, the researchers say, the best solution may be to tackle the problem on a much larger scale - perhaps by targeting entire families at once.

The study has been published this week in Molecular Ecology.