Biology
The Genetic Basis of Melanism and Its Adaptive Significance in Urban Squirrels
Quick fact
In North America, the black coat color in eastern gray squirrels is caused by a variant of the MC1R gene, and this variant is more common in urban areas than in rural forests—a sign that natural selection is favoring melanism in cities.
Why this is interesting
You've probably seen a black squirrel in a city park, but did you know its dark fur is not just a color quirk—it's a genetic adaptation that may help it thrive in human-dominated landscapes?
Read the full explanation
Understanding The Genetic Basis of Melanism and Its Adaptive Significance in Urban Squirrels
Squirrels come in a range of colors, but the striking black form is due to a mutation in a gene called MC1R, which codes for a protein that regulates the type of melanin (pigment) produced. A single nucleotide change can switch a squirrel from grey to black. This is a simple genetic difference—often involving just a few amino acid changes—yet it has observable effects on the squirrel's appearance. But why would being black be advantageous? In a city, dark fur might offer better camouflage against dark surfaces like asphalt and buildings, or it might help with absorbing heat in cooler climates. The variant's prevalence varies geographically, suggesting that different environments favor different colors.
A deeper explanation
The MC1R gene is a classic example of a gene with major phenotypic effects. When the receptor is activated, it triggers the production of eumelanin (dark pigment); when it's blocked by the agouti signaling protein (ASIP), pheomelanin (light pigment) is produced. In black squirrels, a deletion in MC1R causes the receptor to be constitutively active, leading to continuous eumelanin production. This mutation is dominant, meaning only one copy is needed to produce black fur. The adaptive significance lies in how this affects survival. In urban areas, dark fur may provide better crypsis against dark human-made structures, reducing predation risk. Additionally, melanin increases heat absorption, which could be beneficial in colder regions, but harmful in warmer ones. This trade-off explains why black squirrels are more common in some cities than others, and why the mutation is maintained in populations—the balance between benefits and costs keeps both color morphs present.