Health and sanitation workers have been battling for years to rid the city of rats, but in a back room at the Baltimore Zoo the goal is breeding them.
The critter in question is not the garbage-gobbling, bald-tailed rat that scurries along alleyways.
It is the endangered Eastern wood rat, a cave-dwelling species once common from southern New York to North Carolina, and west to Illinois and Kansas.
This rodent has a hairy tail -- and a penchant for collecting bottle caps, which is why it is better known as the pack rat.
"A lot of time we tend to think of the most exciting animals as the ones in need of conservation," said Karen Fulton, the zoo's associate mammal curator.
"We can't just be concerned about the tigers, the cheetahs and leopards -- those guys are really neat and people like to see them, but the smaller animal is just as important.
"The problem is getting people excited about them. People just think, 'Oh God, it's a rat.' "
The Eastern wood rat's natural habitat is shrinking -- perhaps to as narrow a world as western Maryland and West Virginia, Ms. Fulton said, and wildlife biologists are anxious to find out why.
Even in recent months, surveys by the state Department of Natural Resources' Natural Heritage Program indicate fewer numbers of the Eastern wood rat, and some humans concerned about its plight are seeking the rodent's inclusion on the national endangered species list.
The pack rat collects colorful odds and ends (with a predilection for bottle caps) for its large nest.
It prefers the solitude of caves, without even a mate for companionship -- except at breeding time.
At the zoo, studies are under way to find out just how and when that time occurs, to increase the chances of successfully breeding a captive wood rat population currently numbering eight.
They include Harold, the last known Eastern wood rat from New York, and his one-time mate -- an older female from Pennsylvania named Maude (both obtained from Cornell University); their female children, Ratso and Mary; and four trapped in the wilds of western Maryland named Butch, Spike, Norris and Dorothy.
City rats seem to breed with abandon and thrive on the discards of humans.
But for the Eastern wood rat, human encroachment is thought to be one problem contributing to its disappearance -- and breeding is not so simple a matter as it appears for their city cousins.
"If it was that easy it would be wonderful," said Ms. Fulton.
"They are very solitary in the wild, not very social, and have small litter sizes of three to four pups . . . and the problem of infant mortality in the wild."
Ms. Fulton added, "Nobody really understands the biology of this animal. We know what the gestation length is, but we don't know when it's appropriate to breed these animals. What people did before was just pair these animals constantly. What they did was fight to the death."
To support its rat conservation program, the zoo is selling T-shirts and sweat shirts (at prices from $6.50 to $20) featuring a drawing of the animal and a slogan, "Do the 'rat' thing."
On the back, in words ending with a curl, it reads, "Protect this hairy tail."