When Gregory Glass and his colleagues set out to trap rats in Baltimore neighborhoods for a recent study, they were welcomed by two-legged residents who were more than happy for the scurrying rodents to be taken away to a lab. They had just one concern.
"You're not bringing them back here when you're done, are you?" they asked.
No, after their DNA was extracted, all 277 rats collected in 11 alleys were killed, which is a good thing, given that what Glass learned was that death is among the few things that will get a Baltimore rat to leave the place it calls home.
In fact, the Baltimore rat is so wedded to its home turf, typically an alley a few hundred feet long, that at the molecular level, an East Baltimore rat is distinct from a West Baltimore rat.
"Give me a rat," Glass brags, "and I can tell you which side of town it's from."
And it goes even beyond east is east, west is west, never the twain shall meet - the study, published recently in the journal Molecular Ecology, found that most rats apparently spend the bulk of their lives in the space of about a tenth of a mile.
"Most rats are like people in Baltimore," said Glass, who after 25 years in the city has had occasion to observe both. "They marry someone next door or down the block at most, and are happy to live in the neighborhood they grew up in."
It's that quality - our rats, ourselves - that makes the paper such great reading, both as science and local culture. It is the well-observed local quirk, the Baltimore homing instinct, rendered in charts, graphs, Euclidean distances and, of course, adjacent-allele heterozygotes.
The upshot: Nearly all the rats - more than 95 percent of them - exhibited what the researchers call "site fidelity;" they were bred and born in the alley in which they were trapped. (It made me feel a little sorry for one very lost rat that ended up in one of the traps, a veritable stranger in town whose DNA revealed it was unlikely to be from any of the alleys studied.)
"Most rat movements were limited within individual city blocks," the paper concludes.
Here I'd always thought I was just a very grounded person; now, after reading the paper, I realized I am almost as site-faithful as the local rodent population. My first apartment here was a couple blocks from the hospital where my elder sister was born. (My family moved to Harford County shortly afterward, which is where I was born, but then we moved away.)
