Red squirrels (Sciurus vulgaris) and grey squirrels (Sciurus carolinensis) often coexist, leading to questions about interbreeding. This article explores their biological classifications and the natural barriers preventing them from producing mixed offspring.
Distinct Species, Distinct Paths
Red and grey squirrels are classified as separate species with distinct evolutionary histories. Red squirrels are indigenous to Eurasia, while grey squirrels originate from Eastern North America. Grey squirrels were introduced to parts of Europe in the late 19th and early 20th centuries.
Physical characteristics also highlight their differences. Grey squirrels are larger, measuring around 48 centimeters and weighing about 575 grams. Red squirrels are smaller, averaging 38 centimeters and 305 grams. Red squirrels have reddish-brown coats and often develop ear tufts in winter. Grey squirrels display grizzled grey fur, sometimes with cinnamon tones, and have a distinctive white or silver fringe on their bushy tails.
The Biological Barriers to Interbreeding
Red and grey squirrels cannot interbreed due to biological barriers known as reproductive isolation. This prevents different species from producing viable, fertile offspring. A primary barrier is genetic incompatibility; their genetic makeup is too dissimilar for successful reproduction.
Species have specific chromosome numbers and structures that must align during cell formation. For example, grey squirrels have a diploid chromosome number (2n) of 40. When chromosomes from different species do not match, proper pairing during meiosis is disrupted, leading to inviable or sterile offspring.
Behavioral differences also contribute to reproductive isolation. Squirrels have unique, species-specific mating rituals and communication signals. These distinct behaviors prevent successful courtship and mating, as they do not recognize each other’s cues. The notable size and anatomical disparity between the species also presents physical challenges to mating.
Addressing Common Misconceptions
A common misconception is that red squirrel decline in areas with grey squirrels results from interbreeding. This is incorrect, as biological barriers prevent hybridization. The decline is due to other ecological interactions.
One factor is resource competition. Grey squirrels are larger and more adaptable, outcompeting red squirrels for food like nuts and seeds. They can also consume unripe nuts, depleting sources earlier.
Another threat is the squirrelpox virus (SQPV). Grey squirrels often carry SQPV asymptomatically, but it is often fatal to red squirrels, causing severe symptoms and a mortality rate approaching 100 percent. This disease can accelerate red squirrel decline significantly, sometimes 17 to 25 times faster than competition alone. These ecological pressures, not interbreeding, explain the challenges faced by red squirrel populations.