Rattus norvegicus, photographed by Lucky Gardner-Santana

The 411 on rats
Most of us shudder at the sight of a fat rat. But ecologists say rats are the second most successful mammals in the world, after humans. Some naturalists go further, giving the rats top billing.

Here’s what Ivan T. Sanderson had to say in his 1958 tome “Living Mammals of the World”:

“There is little doubt that some rat, and probably the Brown Rat (Rattus norvegicus) is actually the finest—in every sense of the word and especially in efficiency—product that Nature has managed to create on the planet today… That there are more individual Brown Rats in North America than there are people, is not the result of man’s carelessness, indifference, or wasteful and dirty habits; it is the result of the greater stamina and, frankly, commonsense of the rats.”
Sanderson’s other classic works include “Invisible Residents: the Reality of Underwater UFOs,” so I have some questions about his judgment.

Still, rats definitely have assets that have helped them get ahead.
Rats have sophisticated social structures and strong family groups. Their senses of smell, hearing and touch are far superior to ours. They are fast learners, and their minds work a lot like ours. Researchers say they’ve documented rat “metacognition” – that is, like Donald Rumsfeld, they know what they know, and know there are some things they don’t know.

They communicate with one another using a complex set of sounds pitched at frequencies higher than humans can hear. They have a sense of the future; scientists have recorded chirping sounds, which they make in hopeful anticipation of treats to come. And they laugh – again, at frequencies too high for us to hear. (Don’t you wonder what rat humor is like? I’ll bet you a nice chunk of cheese that the jokes are on us.)
“I often joke with colleagues that it is a good thing that nature did not afford rats with opposable thumbs,” Gardner-Santana wrote in an email. “I truly believe that rats, like cockroaches, could survive a nuclear holocaust.”

Furry friends from way back
The relationship between rats and humans dates to the dawn of agriculture. Scientists call the human-rat bond a commensal relationship, that is, a linkage that benefits one species and does little or no harm to the other one. The word comes from the Latin for “sharing a table.”

It’s a great deal from the rats’ perspective, but from most humans’ point of view, it’s one of those dysfunctional relationships that just won’t end. Rats spoil our food, steal our surplus grain and spread diseases. Sure, they also act as stand-ins for humankind in medical research, saving untold numbers of human lives. But we’re not grateful. We just wish we could get rid of them.

When Polynesians migrated from Tahiti and nearby islands to Hawaii roughly 1,000 years ago, rats came along. At the same time the critters, which probably originated on the plains of northern China, were spreading west into Europe.

Our common urban rat reached England in the 1700s. The English named it the Norway rat – for no good reason, since there were no rats in Norway at the time. The scientific name is still Rattus norvegicus, though the common names – brown rat, water rat, and sewer rat – are more descriptive.

Baltimore’s founding (rat) families
English ships spread rats to North America sometime in the 1750s. By the American Revolution, when the port of Baltimore had a major storage facility for grain, brown rats had set up housekeeping here.
More than 200 years later, Gardner-Santana’s DNA analysis found that the city’s founder rats had evolved into three genetically distinct populations: one within the 1early 19th century boundaries of the East Side, one within those early boundaries of the West Side, and another population spreading through a ring of communities, from Brooklyn to Govans, that once were suburbs but were annexed to the city in 1918.

The Jones Falls has isolated the East Side and West Side populations from one another for generations, Gardner-Santana said. “We thought that was interesting, because they can swim,” she said. “But they won’t cross the Jones Falls.”

Santana suspects, but cannot prove, that there’s a historical explanation. She thinks the terrible fire of 1904 that virtually wiped out downtown Baltimore split the city’s rat population overnight, with one group racing east to escape the flames and another group scampering west. After the fire, the two home territories may have been too far apart for the two populations to reunite.

By comparing the genetic similarities between rats at different sites, Gardner-Santana determined that rat “pioneers” moved from the center city out to areas that once were suburbs, following the same trajectory that humans did. It isn’t clear whether the rats traveled on foot or hitched rides from humans.
“Either one is possible,” Gardner-Santana said.

Clans can claim an 11-block area
Although Baltimore has long been a mecca for urban rat researchers, Gardner-Santana’s study is the first to do a genetic analysis of city rats. Her work upends some old assumptions. Most of the research has been done on farm rats, which often have to travel long distances for food, she said. Based on earlier studies, researchers concluded that city rats stay very close to their best food source, traveling only about 20 yards from home.

Gardner-Santana’s study confirmed that most of the time, the rats stay close to home base. “They’re not moving because they get all their needs met,” she said. “Rural rats have to work harder. Urban rats have a relatively cushy life by comparison.”

But she also found that urban rats’ extended-family groups spread out quite a bit. Typically, an extended family of rats forms a clan that fans out over about 11 city blocks. And individual rats can move long distances, sometimes several miles in a short time, if their home territory is disturbed.

Rats do not like change. In scientific terms, they’re “extremely neophobic,” one researcher said. In laboratories, they’ll refuse to eat if they’re exposed to new lights, new food, or even new food bowls.

It’s the same story in the wild.

“They’re very sensitive to construction,” Gardner-Santana said, and will leave an area undergoing renovation or demolition. One of the areas she studied was near Memorial Stadium. “The people who lived in the neighborhood said as soon as that happened, they got invaded by rats,” she said.

Implications for public health
Under normal circumstances, the city’s rat populations form clusters that are more or less isolated from one another by distance. But the animals’ apparent willingness to move long distances when they’re under duress poses a public health problem.
Not all rats are disease carriers. But studies in the 1980s showed that about 47% of Baltimore rats carried a virus in the same family as the desert Southwest’s potentially lethal strain of hantavirus. The local strain is much weaker, comparable to a case of the flu. Smaller percentages of city rats are carriers of several other diseases, including leptospirosis and two forms of hepatitis.
If the rats stayed put, public health workers could attack the disease problem by eradicating rats, one block at a time. But since Gardner-Santana’s study shows they can travel when they choose, that strategy is probably counterproductive. “They end up carrying diseases into new areas,” she said.
Rat control programs will work best if they target large swaths of the city, covering areas a little larger than one square mile
Gardner-Santana is getting ready to move, too. She’s been accepted at the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech in Blacksburg, and starts classes there in the fall.