Among Washington's less touted attractions is a motley crew of squirrels.

Flashy silver-coated squirrels add sparkle to the Capitol grounds, and an albino individual set up housekeeping near the Reflecting Pool a few years ago. Sleek black squirrels, long familiar in Northwest, have been expanding their range.

"They used to be very uncommon outside the Beltway, but I think that as the suburbs expand, it's become better habitat for them in recent years," said Richard Thorington, mammals curator at the Smithsonian Institution's Museum of Natural History. Virginia residents have sighted black squirrels to the west of the Potomac, where the animals apparently did not live before.

The drift into newly built-up areas fits a black-squirrel pattern noticed elsewhere -- great abundance in a city but scarcity in the nearby countryside. One reason is probably that city dwellers and suburbanites protect unusually hued squirrels because of their looks. In rural areas, however, the glamor of being a striking silver or black takes second place in survival value to dull gray camouflage.

Black coats also tend to appear more frequently in the northern part of the gray squirrel's range in the United States.

The black, silver and albino are all simply color phases of one species, the common gray squirrel of the eastern United States. Most squirrels of whatever color sport basically the same shade all their lives.

The ratio of blacks to grays apparently has reversed over the years in some places. European settlers found black to be the most common color of squirrels in parts of the Midwest. Tree cutting and younger forests later let in more light, probably making the black squirrels easier for predators -- including human hunters -- to pick off.

But being black may have hidden payoffs in squirreldom.

"We can only make wild guesses, but there seems to be an advantage to being a black squirrel here," said Vagn Flyger, an authority on squirrels who recently retired from the University of Maryland's Animal Sciences Department. "Maybe color just reflects other beneficial characteristics that are inherited along with it."

Black mammals generally tend to be more timid than their lighter-coated cousins, he said. Perhaps such caution keeps the dark squirrels alive longer to produce more progeny than the theoretically bolder grays.

A darker coat also may garner some survival points in a cold climate by absorbing more heat in the sun.

Thorington said he believes that Washington's first wild black squirrels were escapees from the National Zoo. Twenty-eight black squirrels from Ontario were presented to the zoo between 1902 and 1906. He reasons that they would not have been brought in unless they were a curiosity in the area.

In addition, District residents probably brought black squirrels from other areas and released them into their neighborhoods, the experts said. Black squirrels that served as subjects in scientific studies were later rewarded with freedom, a resident recalls. Multiple origins probably explain Washington's array of dark squirrel shades between brown and black.

The process of color inheritance is still a gray area in squirrels. Despite the tameness of urban squirrels, they are tough to breed in captivity, making it difficult to trace how adults pass traits to offspring.

"Squirrels' elaborate courtship involves a number of males chasing the female," Thorington said. Conditions conducive to the chase are hard to duplicate in a cage.

Another problem is that complex rules govern the squirrel's coat color -- how each hair will look throughout the seasons and during a squirrel's lifetime. The squirrel sheds twice a year, in spring and autumn. The frosty gray winter coat gives way by summer to a yellowish or reddish tint. To complicate matters, the garden variety gray squirrel does not have a single gray hair. While a hair is growing, genes turn pigment production on and off -- making most hairs banded in black, brown or white.

Thorington thinks that the pigment switch is essentially stuck in the "on" position for black squirrels -- as it is in black leopards and black cats.

Aside from protection by squirrel-fanciers, why do black and silver squirrels develop hot spots of abundance in cities such as Washington? One theory is that busy streets create urban islands -- parks and other green areas. Restricted to mate largely with each other, black or silver residents of these islands would likely produce more of their own kind. Few gray squirrels would reach the island to dilute the pool of unusual color genes.

Gray squirrels studied in Lafayette Park, however, shuttled more freely between urban parks than previously suspected. The park's unusually great food surplus -- "squirrel groceries" furnished daily by dedicated human feeders -- attracted enterprising squirrels from blocks away. Squirrels also darted across busy streets, including Pennsylvania Avenue, to bury nuts outside the park, albeit not during rush hour.

The influx gave Lafayette Park a dubious distinction some years ago: the highest squirrel density reported in scientific records. Squirrels began to destroy park vegetation. Some died of pneumonia, while others contracted various infections, probably signs of stress from overcrowding.

A couple of years ago, park officials resorted to nighttime squirrelnaping. The captives were spirited to other District parks and released.

For one of the capital's commonest residents, however, a surprising number of genetic and ecological riddles related to color phases and movement in urban areas remain unsolved.