What’s That? Black squirrels

A common question on campus tours is about uncommon residents, the black squirrels.

The black squirrels are regularly seen on College Green, scampering between the historic buildings. “We have five black squirrels that live in the trees outside, and we have taken them as our printshop mascots,” says Jessica Peterson, director of the Common Press, located in the Fisher Fine Arts Library building. The Press is holding an open studio event “Black Squirrel Pattern Postcard Printing” on Jan. 17, reprising an original two-color decorative pattern used for a “Black Squirrel Pattern Printing and Button Making” event last spring.

Sarah Tomke, a postdoctoral researcher with the Wildlife Futures Program in the School of Veterinary Medicine on the New Bolton Center campus, is a geneticist and wildlife biologist. One of her projects is to research fox squirrels in the forests of Pennsylvania; they are a reddish-grey hue and a bit larger than the typical grey squirrels common to Philadelphia. 

And the black squirrel? “Basically, it comes down to one mutation” in the gene for the melanin receptor protein, which regulates how much melanin, or pigment, that each cell produces, Tomke says. “The mutation that’s found in black squirrels is a 24 base pair deletion,” in the gene, she says, “and it causes this protein to be hyperactive, and so more melanin is produced than in a normal grey squirrel. It all comes down to this simple mutation that happens in the gene.” Both the mother and the father must have the gene mutation for the squirrel to be black; if one has the mutation and the other does not, the offspring will be brown, Tomke says. 

People probably introduced black squirrels to Penn’s campus more than a century ago, bringing them from somewhere else. Several university campuses have black squirrels, including Princeton. “This happened in the early 1900s, when it was really popular to move around cool-looking wildlife,” she says. But she cautions that humans should avoid direct interaction with squirrels, whatever the color. “They are not pets,” she says. “It’s best to treat them like the wild animals they are and appreciate them from a healthy distance.”