Black squirrels, with their striking dark fur, are a common sight in certain regions. This article explores their biological reality, genetic basis, geographical spread, and whether their distinct coloration influences their daily lives.
More Than Just a Color
Black squirrels are not a separate species but a melanistic variant of the Eastern Grey Squirrel (Sciurus carolinensis). Melanism is an increased development of the dark pigment melanin, resulting in their black fur. This coloration stems from a specific genetic mutation.
Research indicates melanism in Eastern Grey Squirrels is associated with a 24 base-pair deletion in the Melanocortin 1 Receptor (MC1R) gene. The MC1R gene regulates melanin production. When activated, MC1R initiates eumelanin production, the brown-black pigment. The mutation in black squirrels results in a receptor that constantly signals, producing high amounts of eumelanin. The allele responsible for this black coloration is considered incompletely dominant.
Origins and Distribution
The melanistic mutation occurred naturally in North America. While black squirrels can appear wherever Eastern Grey Squirrels are found, they are particularly common in the northern parts of their range, especially around the Great Lakes Basin. Historically, their frequency may have been higher in old-growth forests, where their dark color offered camouflage.
Black squirrels were also introduced to other regions, including the United Kingdom. The first recorded wild black squirrel sighting in the UK was in 1912 in Woburn, Bedfordshire. These individuals are believed to be descendants of squirrels that escaped from private collections or zoos. By 2009, black squirrels accounted for nearly half of all squirrels in Cambridgeshire and other parts of England.
Life in Black: Are There Behavioral Differences?
The black fur of melanistic squirrels has led to theories about potential ecological advantages or disadvantages. One theory suggests a thermoregulatory benefit, as dark fur absorbs more solar radiation. Studies show melanistic squirrels can have lower heat loss and basal metabolic rates than grey morphs in colder temperatures. This might explain their higher frequency in northern regions.
The dark coloration could also offer camouflage in specific environments, like shaded forests or urban areas. However, some research suggests that while thermoregulation might be a benefit, a uniform dark color may offer less effective camouflage against certain predators compared to grey morphs. Despite these minor ecological differences, the fundamental behaviors of black squirrels, including foraging, nesting, and reproduction, are largely identical to their grey counterparts.