This text is taken from the “Courrier de la planete” of May 10, 2022. To subscribe, click here.
Last spring, as the pandemic dragged on, biologist André-Philippe Drapeau Picard was looking for a science writing project. At Parc Jean-Drapeau in Montreal, with a biologist friend, he watched the animals around, in search of inspiration.
We will not be surprised to learn that squirrels were in the vicinity. Their appearance, to say the least banal, was nevertheless going to tickle in the minds of biologists a question to which they did not have the answer: why do certain gray squirrels (Sciurus carolinensis) do they have black fur?
“Melanism in squirrels is a question that often comes up in the happy hours of biologists,” explains Mr. Drapeau Picard. In congresses, there will always be someone who will end up saying: “Ah, don’t you think there are a lot of black squirrels?” then we move on. »
For once, Mr. Drapeau Picard — a trained entomologist who works at the Insectarium — decided to take the rodent by the horns. He searched the surprisingly abundant scientific literature on the coat of the gray squirrel. The result of his personal investigation was published this spring in The Canadian Naturalist.
fifty shades of gray squirrels
From a genetic point of view, the existence of black squirrels stems simply from a mutation in the Mc1r gene. If the individual has two mutated alleles, he is completely black. If it has only one mutated allele, its color is joint.
Rather, the real question lies in the reasons that favor—or disfavor—melanism in a given population. This has puzzled scientists in North America for centuries.
Anecdotal reports indicate that the black coat was dominant in the northern gray squirrel’s range (which includes southern Quebec) until the late 18th century. A decline in melanism would then have been observed, leading to the preponderance of the gray mantle seen today.
Mechanisms linked to hunting or logging have already been proposed to explain this decline, but the real causes “will probably never be confirmed”, writes Mr. Drapeau Picard in his article “Fifty shades of gray squirrels: portrait of the melanism in a species emblematic of North American cities”.
Two main hypotheses
At the risk of disappointing his colleagues, Mr. Drapeau Picard noted that according to the scientific literature, nothing allows us to say that, in recent decades, the black coat has gained in popularity among S. carolinensis in North America.
However, geographic variations do exist. Melanism is more common in cities than in forests. In addition, there are more black specimens in the north of the range than in the south of it. Two main hypotheses are proposed to explain the north-south trend: thermoregulation and camouflage.
The hypothesis of thermoregulation is intuitive: in the north, where it is colder, a dark coat absorbs more energy from the sun in winter, which warms the little beast.
The camouflage hypothesis holds that in the northern part of the range conifers are more common and that a dark coat allows squirrels to better hide from their predators in these trees.
The hypothesis of thermoregulation is better documented, but the evidence presented does not allow the case to be closed. “There are still a lot of things to discover with this species, a lot of fun to be had,” said Mr. Drapeau Picard.
That could change. Over the course of his efforts over the past year, the biologist – who says he finds his squirrel neighbors “rather friendly” – was approached by American researchers to sample the Montreal population.
His mission: to find ten dead squirrels, hit by cars, in order to take a piece of their ear for genetic analysis. Other collaborators have done the same thing in forty cities in North America.
“I was cycling with a dissection kit in my backpack,” says Drapeau Picard. When I found a dead squirrel, I removed it from the middle of the road. I put on my latex gloves, and with my scissors, I cut off a piece of ear. Contrary to my expectations, people weren’t looking at me too strangely…”
This genetic information, along with data and photos from citizen science platforms, could help unravel the enduring mystery of melanism selection in gray squirrels in the coming years, he said.
After this foray into the world of S. carolinensis, the trained entomologist does not think of working on this beast again. He will, however, continue to watch the squirrels “with great interest” — and try to keep them out of his bird feeders.