An Exotic Evolution
Thursday, May 19, 2005
Because the history of Washington has been written by humans, nobody has paid much attention to the fact that 18 Canadian squirrels were released at the National Zoo during the presidency of Theodore Roosevelt.
But, if the capital's story were ever told by its rodents, few events would be more prominent than this one.
That's because those 18 squirrels -- whose coats of lustrous black set them apart from the native animals -- were the beginning of a shift that has changed the complexion of Washington's backyard critters. Now, probably because of a slight evolutionary advantage conveyed with a black coat, the descendants of these squirrels have spread all the way into Rockville and Prince William County.
Seriously: Scientists say it's a real-life example of natural selection at work, which has rolled on for a century here without much public notice.
"It shows the spread of a gene within a population," said Richard W. Thorington Jr., a Smithsonian Institution researcher working on a book that includes a history of the District's black squirrels. "That is evolutionary change before your eyes."
The story of Washington's black squirrels -- which scientists say are just a color variation within the common gray squirrel species -- still has its shades of mystery.
For starters, nobody is quite certain why the Smithsonian decided to bring the critters here in the first place. The zoo's best guess, according to spokeswoman Peper Long, is that they were trying to restore the local population of gray squirrels, which had been decimated through hunting.
"Some people thought the gray squirrel was going to be extinct," Long said.
The Smithsonian's archives offer only the barest of facts: In 1902, and then again in 1906, the zoo got black squirrels from "the department of crown lands" in Ontario.
The next historical evidence pops up 15 years later, in the form of roadkill.
This week, in an upper story of the rotunda at the National Museum of Natural History, researcher Katie Ferrell pulled out a drawer full of stiff squirrel carcasses. It was a tiny sample from the museum's collection of 30,000.
"Cleveland Park, 1917," she said, reading the tag of one black animal that apparently met its fate under the wheels of a D.C. motorist. It was solid evidence that the squirrels had already left zoo grounds. Scientists say the black variety now comprises 5 to 25 percent of the squirrels in some neighborhoods.