Why Are Some Squirrels Black? Melanism in Gray Squirrels

What is melanism?

Melanism means an unusually large amount of dark pigment, called eumelanin, in the skin, fur or feathers. It occurs in many animals, from leopards and jaguars, often called black panthers (see is a black panther a leopard?), to the silver form of the red fox. In each case the black animal belongs to the same species as its normally colored relatives.

The eastern gray squirrel has three common color forms: the familiar gray, a brownish-black form and a jet-black form. All three can turn up in the same litter where black squirrels are common.

The gene behind black squirrels

In 2009, researchers led by Helen McRobie at Anglia Ruskin University in England showed that black fur in gray squirrels is linked to a variant of a gene called MC1R (melanocortin 1 receptor). The variant is missing a short stretch of 24 DNA base pairs. MC1R acts as a switch that tells pigment cells whether to make dark or pale pigment, and the shortened version keeps the switch stuck on “dark”.

The variant is incompletely dominant, which explains the three color forms:

Because gray-colored squirrels carry no copy of the variant, two gray parents cannot produce a black youngster.

Did black squirrels get their color from fox squirrels?

A 2019 study by McRobie and colleagues found that the black MC1R variant in gray squirrels is identical to one found in black fox squirrels, a closely related species that shares much of the gray squirrel’s range. The best explanation is that the two species interbred at some point in the past, and a hybrid passed the variant into the gray squirrel population, where it spread.

Black fox squirrels in the southeastern United States are a different story: their dark color comes from changes in a separate pigment gene. So melanism has arisen more than once in North American tree squirrels.

Where are black squirrels most common?

Black squirrels occur across the gray squirrel’s native range but are usually uncommon. The big exception is the northern edge of the range around the Great Lakes. In parts of Ontario and Michigan, black squirrels make up most of the population, and they are also common in Toronto, Ottawa and other Canadian cities.

People have also moved black squirrels around. Black squirrels from Ontario were released in Washington, D.C., in the early 1900s, and Kent State University in Ohio brought in ten black squirrels from Ontario in 1961; their descendants now live across northeastern Ohio. Introduced populations are also well known in Calgary, Alberta, and in towns such as Marysville, Kansas, where the black squirrel is the official town mascot.

In Britain, black squirrels descend from black gray squirrels brought from North America. The first wild one was recorded in 1912 at Woburn in Bedfordshire, and today they are common in Hertfordshire, Bedfordshire and Cambridgeshire. Like all gray squirrels in Britain, they are part of an invasive species, as explained in why gray squirrels are invasive in the UK.

Why is black fur common in the north?

Scientists have proposed two main explanations, and both may play a part.

Keeping warm. In a study published in 1989, black squirrels exposed to −10 °C (14 °F) lost about 18% less heat and had a resting metabolic rate about 20% lower than gray ones. Dark fur may also absorb more heat from sunlight. In a cold northern winter, that saving could make the difference between surviving and starving.

Camouflage. In dense, shady northern forests with many conifers, a black squirrel may be harder for hawks to spot than a gray one. In open broadleaf woods, where light floods the forest floor in winter, gray fur may blend in better. Some researchers think black squirrels were once more widespread and declined as forests were cleared and hunting favored the less conspicuous gray form, although this idea is debated.

Black squirrels are also noticeably more common in cities than in rural areas, possibly because predators and hunting pressure are lower there.

What about white squirrels?

White squirrels are much rarer than black ones. Most are eastern gray squirrels with leucism, an inherited condition that reduces pigment in the fur but usually leaves the eyes dark. True albinos, with no pigment at all and pink or red eyes, are rarer still.

A few towns have well-known populations of white squirrels and protect them as local symbols, including Olney, Illinois, and Brevard, North Carolina. White coats make squirrels easier for predators to see, which is one reason they rarely become common in the wild. Like black squirrels, they belong to the same species as their gray neighbors; other animals with unusual coats are described in are black bears always black?