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.)

My second apartment in Baltimore was two blocks away from the first one. Then in a fit of uncharacteristic wanderlust, I bought a house that was two whole miles away. I lived there until moving three blocks to my current house.

Hopefully, that's where the rat in me ends. Glass, a professor at the Johns Hopkins Bloomberg School of Public Health, tells me a rat burrow basically has a pair, an alpha male and an alpha female, that goes about breeding the next generation. Given what the paper calls "high fecundity and generation overlap in Baltimore rats," the alley ends up as a community that is something like that song about a family filled with so many intermarriages that, as the narrator sings, "I'm My Own Grandpa."

More grossly interesting, or interestingly gross, facts about rat life: There's only room for one alpha pair per burrow, so eventually some rats leave home and create their own communities. They'll fight, "blood and skin," Glass says, over territories. But they generally stay close to their ancestral home - there can be four or five burrows in a single alley, he said. The study found that rats within an 11-block area were part of one extended family.

The Jones Falls serves as a natural barrier dividing east- and west-side rats. The differences between them are at the DNA level, but after years of studying them, Glass has noted that there are rats with black coats "and a beautiful white star on the chest" in one east-side neighborhood, around Jefferson Street above Orleans Street. "You go a couple blocks away, and they look normal again," he said, "mostly yucky-brown."

No doubt most urban rats behave like our own, but it is the Baltimore rodent that oddly enough is one of the most studied populations. Glass and others in the field frequently cite 1940s research on Baltimore rats that probably were the ancestors of our current vermin.

Rat scientists likely were drawn to Baltimore for the same reason Willie Sutton was drawn to robbing banks - it's where the rats are. As a result, our rats have developed a certain je ne sais quoi among rat researchers.

"People from New York are always asking me to send pieces of Baltimore rats to them," Glass said. "I think, 'You're in New York, you have your own rats.' "

Glass has a wry sense of humor, but it is unclear whether that is the cause or the effect of his choice of research subject. If you're interested in studying biodiversity within animal populations, he said, "one way to do it would be to go to the Serengeti and look at lions." He's pretty much to the opposite side of the spectrum, and as a result, Baltimore rats got their very own Boswell.

The study details how researchers picked Baltimore alleys - mostly older neighborhoods known from previous studies to harbor rats in ample supply. Or, as the paper describes them in the kind of language that would make a real estate agent faint: "Areas characterized by row houses with small backyards comprised of concrete parking pads and small garden areas often occupied by rat burrow systems." The lucky areas had what the researchers estimated was, on the average, a density of about 50 rats per alley.

Glass's interest in rats comes from a public health rather than a cultural standpoint. While the rest of us view rats as horrid creatures that basically spread revulsion, he studies them for how they spread viruses and bacteria. Although most rats don't stray much, enough do - they're known as the "super spreaders" - that they remain potential carriers of various diseases.

Still, I have to ask Glass: Why does the rat, unlike the chicken, generally not cross the road?

Blame it on the two-legged creatures and their generosity, putting trash in plastic bags.

"Rats will move a long distance to get food," Glass said. "But if they can get food, get a drink where they live, they won't."

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