Red squirrels (Sciurus vulgaris) are a beloved and iconic species native to Eurasia. Their rich reddish-brown fur and energetic movements endear them to many nature lovers. But while “red squirrel” is their common name, their coloration is actually quite variable and complex. Understanding the range of red squirrel color morphs provides insight into their natural history and genetic diversity.
The typical red squirrel has fur that is mostly reddish-brown on the upperparts and whitish on the underside. But some individuals exhibit a range of other color morphs. This includes melanistic black morphs, albinos with white fur, and gray morphs. The relative proportions of these morphs vary in different parts of the red squirrel’s range. Examining these geographic patterns sheds light on the evolutionary forces influencing red squirrel coloration.
Typical Red Morph
The most common red squirrel morph has fur that is grizzled reddish-brown on the upperparts and whitish on the underside and legs. This provides them with effective camouflage in their forest habitat among the trees and earthy tones of bark and soil. Their fur is generally thicker and longer in the winter to help retain heat.
This typical red morph exists throughout the red squirrel’s range. However, there are regional variations in the exact tone and saturation of the fur. British red squirrels tend to be more deeply saturated reddish-brown. Eurasian populations are often lighter and more flecked with gray. There are also differences between subspecies in fur length and texture.
Melanistic Black Morph
Melanistic red squirrels have black fur across all or most of their bodies due to increased deposition of the pigment melanin. They represent a relatively rare but regular morph that occurs across the species’ range. Melanism is controlled by a recessive allele in red squirrels.
The frequency of melanistic individuals varies by region. Melanistic red squirrels make up around 1% of the population in Britain. In Italy, the proportion is higher at 2-8%. The highest reported frequencies are in some urban populations in Russia, where up to 50% of squirrels are black morphs.
| Region | Melanistic Frequency |
|---|---|
| Britain | ~1% |
| Italy | 2-8% |
| Russia (some urban populations) | up to 50% |
The prevalence of black squirrels appears to be influenced by both genetic founder effects in isolated urban populations as well as positive selection in areas with higher pollution and darker tree bark.
Albino Morph
Albino red squirrels have a total lack of pigment in their fur due to a recessive genetic mutation. True albinos have pink eyes while those with pale blue eyes have a form of albinism called leucism. Both types have all-white or very pale cream fur.
Albino red squirrels are even rarer than melanistic ones, occurring at frequencies less than 1% throughout their range. The low prevalence is likely because albinism impairs vision and provides poor camouflage, reducing survival in the wild.
Gray Morph
Some red squirrel populations include individuals with predominantly gray fur. Gray morphs originate from natural hybridization between native red squirrels and introduced North American gray squirrels over the past century. Hybrids form frequently where the two species co-occur.
Most hybrids have a patchy mix of red and gray fur. But some resemble pure gray squirrels in coloration. The proportion of gray morphs is particularly high in parts of northern Italy, Switzerland, and Britain where invasive grays have extensively interbred with native reds.
| Region | Proportion of Gray Morphs |
|---|---|
| Britain | 5-30% |
| Switzerland | 10-20% |
| Northern Italy | 20-70% |
The high incidence of gray morphs in these areas threatens to eliminate the distinctive red coat of native Eurasian red squirrels through genetic swamping.
Geographic Patterns
What accounts for the regional differences in the frequency of various red squirrel color morphs? Several factors are likely at play:
- Founder effects – Isolated populations established by a small number of founders will reflect those founders’ genotypes. This may increase the proportions of unusual recessive morphs like melanism or albinism.
- Random genetic drift – In small populations, allele frequencies can fluctuate a great deal due to random chance. Drift could cause rare color morphs to temporarily increase.
- Selective pressures – In some environments, certain color morphs may be more adaptive. This could lead to positive selection increasing their prevalence.
- Hybridization – Introgression with invasive gray squirrels has increased the proportion of gray morph red squirrels in parts of Europe.
Ongoing research uses population genetic analyses and geographic mapping of morph frequencies to better understand the evolutionary origins of color variation in red squirrels.
Conclusion
While called red squirrels, these rodents actually exhibit a range of color morphs including black, white, gray, and reddish-brown. The frequencies of these morphs vary across the species’ range, influenced by founder events, genetic drift, selection pressures, and hybridization. Examining the coloration of red squirrels provides insight into their hidden genetic diversity and ongoing evolution.
Understanding red squirrel color morphs also has important implications for conservation. Invasive grays threaten red squirrels not only through competitive exclusion but also by hybridization that eliminates their distinctive red coat. Maintaining populations of the vivid rufous-colored Eurasian red squirrel in its native forest habitat will require proactive management.