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An Exotic Evolution
Here's why some scientists believe the black squirrels were multiplying: In winter, their dark coats allowed them to retain heat from sunlight, leaving them less desperate for warmth than their lighter-colored cousins.
"If you can do it with solar heat, you don't need quite as much metabolic heat," and, therefore, need less food, Thorington said.
In some cases, this advantage seems to have outweighed the potential downside of a black coat -- being more conspicuous to hawks and other predators.
Thorington believes that black squirrels were slightly more likely to survive and reproduce, and their genes were passed on to succeeding generations.
Whatever the reason, observers say, black squirrels have been showing up in areas where only gray-colored specimens had been.
They appeared in Bethesda, Silver Spring and Chevy Chase in the 1960s, perhaps using the Rock Creek Stream Valley as a highway north from the District. One survey of Bethesda in 1990 found that about 25 percent of the squirrels there were black.
To the east, the squirrels crossed the city a few decades ago to colonize the National Arboretum and Capitol Hill. To the south, they made it across the Potomac River into Arlington, where naturalists say they've seen black squirrels since at least the 1980s.
Black squirrels even had a cameo in the area's best-known bit of squirrel research: the survey of animals in Lafayette Park in 1980 and 1981, which showed that this tourist landmark had the densest squirrel population known to science.
"I do remember seeing melanistic [dark-pigmented] squirrels around," said David Manski, a biologist who spent eight months watching rodents in the park.
Now, the frontier appears to run through Fairfax County, where the squirrels are still rare, and Prince William County, where some have been spotted in a forest 35 miles from their origins in Washington.
In Montgomery, the squirrels are spreading into Gaithersburg and Rockville -- though not without a little controversy.
Some residents have called to complain that the new squirrels are aggressive, driving out the friendly gray squirrels, said Bill Hamilton of the county Department of Park and Planning.
He said he reassures residents: "It's the same squirrel," just a different color.
Among themselves, the squirrels appear to feel the same way, according to Vagn Flyger, a retired University of Maryland professor.
Flyger devoted himself to studying squirrels because, as he explains it, they weigh less than a deer and don't bite like a polar bear. He used to smear a tree behind his Silver Spring home with a mixture of peanut butter and Valium and then tattoo the squirrels that he found passed out below.
When he first did this, more than 30 years ago, there were only gray ones, Flyger said. Now, he says, at least four black squirrels live nearby.
He has studied their behavior -- now using feeders rather than Valium -- and determined that the squirrels don't appear to treat each other differently because they are black or gray.
"They don't seem to care," he said.