The question ...

So-called "gray squirrels" and "black squirrels" are two color morphs of the same species: Sciurus carolinensis. Until 150 years ago, the black morph was much more abundant. Yet now the black morph is rare, except in cities. Why has the black morph declined? Why does it linger in cities? Will it return again to the countryside? The landscape is changing rapidly. Will the gray squirrel, and other mammals like it, be able to adapt quickly enough?

Genetics

Only a tiny genetic difference separates black versus gray morphs. All individuals contain a gene, MC1R, that controls how much dark pigment is added as a squirrel's hairs grow. When a tiny piece of DNA is missing from this gene, it boosts production of dark pigment and makes the fur darker. When a squirrel has a single copy of the gene with the small piece of DNA deleted, or two copies of that altered gene, its fur is mostly black. Technically, the black morph is melanic. The gray morph has two copies of the complete gene. A tiny genetic difference, but being black or gray could have many consequences if you are a squirrel!

Contemporary evolution

Squirrels that don't blend in well with their backgrounds don't survive as well. What kills squirrels? Predators...mainly hawks, foxes, and dogs. Humans also kill many squirrels: squirrel hunting is "big" in rural areas. But you can't hunt in cities, so maybe that's why black morphs more frequent in urban areas.

Image by Andy Reago & Chrissy McClarren at Flickr (License: CC-BY-2.0)

There could be other forces at work. Our forests have changed. Originally forests of eastern North America were mostly old growth with pockets of darkness associated with enormous tree trunks, many vertical layers that generated shade, and more conifer trees with dark foliage. In these darker conditions perhaps the black morph could remain better hidden. Now virtually all old growth forests have disappeared.

The forests that have re-grown are secondary forest with spindly trunks of mostly deciduous trees whose trunks and branches are much lighter-hued. In such forests might the gray morph be better hidden than the black morph from predators and hunters?

It gets even more interesting...urban areas have expanded. Urban areas may not just be refuges from hunting. In urban areas most squirrels are killed by cars. Most drivers try to avoid hitting animals even if they don't always succeed. The sooner a driver sees a squirrel, the easier it is to avoid running it over. The gray morph has a remarkable similarity in hue to pavement, so may be harder for drivers to see. Does the black morph stand out better against a road surface and be more avoidable by car drivers, that is, survive better?

Image by ncowey at iNaturalist (License: CC-BY-NC)

How can you use SquirrelMapper to answer these questions?

By mapping where black and gray morphs occur in our cities, towns, farms and forests you can contribute to an important scientific discovery about evolution in action in our own backyards. The SquirrelMapper project here at Zooniverse has two components:

1. Classify the coat color of squirrels as gray or black. The coat color of each squirrel is confirmed through classifications by many Zooinverse volunteers like you. Once you classify it, we map it, identify its habitat, and determine which morph occurs more often where. This lets us understand how quickly squirrels, and mammals like them, can adapt to changes in their habitats. See Results for some preliminary outcomes.

2. Classify the types of predators that occur where we've photographed squirrels. Animal color has many functions, but we can know coloration can affect how visible animals are to their predators. By classifying the types of predators at sites with squirrels, you'll help us understand whether predators shape the way that squirrel coat color evolves.

Click the classify link to get started!

Here's the project workflow:

Important note

We rely on photos of squirrels taken by citizen scientists at iNaturalist. Some of these photos are of dead squirrels. Volunteers classifying the color of squirrels in this project will occasionally encounter photos of dead squirrels.

Contribute observations of squirrels to iNaturalist!

Over 150,000 photographs of squirrels have been submitted to iNaturalist so far. When you submit an observation, the community of citizen scientists at iNaturalist confirm its identification. Once an observation is confirmed to be an eastern gray squirrel (a research grade observation), we import the photo to Zooniverse for you to classify by its coat color. Check out ourSquirrelMapper project on iNaturalist.**

Try our Squirrel Spotter game too!

In addition to mapping squirrel color morphs, try our Find the Squirrel game to help us measure natural selection on coat color. In this game you will search for squirrels in scenes of old growth forests, secondary forests, and roads, and we measure how long it takes you to find gray and black squirrels in each scene. By participating in the game you will directly measure the selection pressures on black and gray squirrels in each environment.

Want to learn more?

Check out our SquirrelMapper home page.

A fascinating overview of the history of squirrels in urban areas: The Urbanization of the Eastern Gray Squirrel in the United States, Journal of American History 100, no. 3 (2013): 691-710 (.pdf available on author Etienne Benson's website)

This podcast integrates much about our relationship with squirrels.

An article by Helen McRobie on gray squirrel morph genetics: The genetic basis of melanism in the gray squirrel (Sciurus carolinensis) Journal of Heredity 100: 709-714.

Image credits

Eyesplash Photography at Flickr, License: CC BY-NC-ND