Local biology professor explains science behind black squirrels

Nick Troendle, an associate professor of biology at Olivet Nazarene University, said scientists know “quite a bit” about black squirrels, including the specific gene mutation that causes their coloration.

Troendle has a doctorate degree in population genetics from the University of Georgia, a master’s degree in wildlife and fisheries science from Texas A&M University, and a bachelor’s degree in biology from Pepperdine University.

Most of his work has focused on the genetics of invasive species.

Eastern gray squirrels, which are native to North America, have a melanistic (black) form that has been around for a long time, Troendle said.

BLACK SQUIRRELS ‘STICK OUT’

Present-day gray squirrels blend into their environment better than black squirrels, as their coats provide the perfect camouflage against trees.

That wasn’t always the case, though.

Scientists believe Eastern gray squirrels used to be confined to dense, primary forests.

These older forests were thicker and had less light penetration than the secondary-growth forests that exist in North America today.

So, when their habitats were darker, black squirrels were more prevalent because they were less visible to predators.

During time, as deforestation occurred, the opposite became true.

“Now, actually the gray squirrels blend better, and the black squirrels stick out,” Troendle said. “You see the black squirrels on the trees, but you don’t necessarily see the gray squirrels.”

Gray squirrels now have the advantage of camouflage, but the genetics for black squirrels continue to be passed down.

“There’s been a shift in the selective pressure, where before the black squirrels blended in really well and didn’t get eaten by predators,” Troendle said. “Now, they stick out, and so there are fewer of them.”

But sticking out is not always a bad thing.

ADVANTAGES FOR BLACK SQUIRRELS

Black squirrels do have the edge in some situations.

While gray squirrels are more numerous in both settings, urban environments tend to have a higher percentage of black squirrels than rural environments.

St. Anne, for instance, has more gray and fewer black squirrels compared to Kankakee.

The reason is roadkill.

“The No. 1 way for squirrels to die in an urban area is roadkill,” Troendle noted. “But the black squirrels, if you’ve ever seen a black squirrel run across the road, they stick out really well against the asphalt, whereas the gray squirrels blend in very well.”

Studies have found that black squirrels are less likely to get hit on the road than gray squirrels because drivers are better able to see black squirrels and try to avoid them, he said.

“We are actually finding that there’s a lot more black squirrels in the urban areas than in rural areas because you kind of have this reverse selection thing going,” Troendle said.

The melanistic form of the Eastern gray squirrel exists because of a mutation in the MC1R gene, he said.

The MC1R [melanocortin 1 receptor] gene is involved in melanism in all sorts of animals, including mammals, birds and reptiles, he said.

The mutation in this gene causes hyper-production of the pigment melanin.

“You go from this precursor to a pigment called pheomelanin, which is kind of orangey, and then you take that pheomelanin and turn it into eumelanin, which is the black pigment that we see,” he explained.

All gray squirrels produce some eumelanin, but much of it is left as pheomelanin, he said.

However, a specific mutation in the MC1R gene in a black squirrel makes the gene more active and turns more of the squirrel’s pigment into eumelanin, causing the animal to be very black in color.

Eastern gray squirrels live all over North America, but more black squirrels tend to be seen in the northern parts of their range, while they are less frequently seen in southern parts, Troendle said.

IN THEIR GENES

Eastern gray squirrels have also been introduced to Europe, where they are an invasive species.

“The interesting thing is, no matter where you go, the mutation is the exact same mutation,” he added. “It’s a 24-base-pair deletion in the MC1R gene.”

The fact that the same gene mutation is found in black Eastern gray squirrels everywhere suggests it is a very old mutation, probably thousands of years old, Troendle said.

“The odds of it happening in the exact same place, the same basis, in multiple different places, in multiple locations, is astronomical,” he said.

So, scientists can conclude that the black Eastern gray squirrels resulted from one gene mutation that occurred long ago, and natural selection has factored into their occurrence in the population over time.

“Where it’s beneficial, the black morph becomes more common, and where it’s bad for you, the black morph disappears entirely,” Troendle said.

He noted a recent study looked at fox squirrels, which are also native to North America.

Fox squirrels have a reddish color, but like the Eastern gray squirrel, this species also has a melanistic form.

In fox squirrels, two different genes are responsible for the melanistic form. One is unique to the fox squirrel, but the other is the exact same mutation that occurs in Eastern gray squirrels, suggesting the two species interbred.

Troendle also noted “there’s still a lot that’s being learned” about the MC1R gene, and that its original function is thought to play a part in UV damage repair.

“We know that if you have a higher percentage of melanin in your skin, you are less subject to damage,” he said. “It’s the same with them.”

In addition to being more resistant to damage from the sun’s rays, darker colored squirrels also warm up easier, as their fur absorbs more energy. This is thought to be the reason why they are more common in colder, northern regions.

“There’s probably [advantages] that we don’t know yet, that we don’t understand,” he added. “We can look at little pieces of it and put it together. We’ve probably found the most significant ones, or at least some, but there probably are dozens of little things, effects that we don’t know about.”