The native Eurasian red squirrel (Sciurus vulgaris) and the invasive Eastern grey squirrel (Sciurus carolinensis) are often seen in direct conflict across their overlapping ranges, particularly in Europe. The grey squirrel’s introduction from North America has led to a dramatic decline in red squirrel populations, fueling the common perception of a battle between the two species. This ecological displacement often prompts the question of whether the two squirrels can interbreed to produce hybrid offspring. The direct answer is no, and the reasons for this failure lie in fundamental biological differences.
Comparing Red and Grey Squirrels
The two species are visibly distinct. The red squirrel is generally smaller, weighing between 280 and 330 grams, and is recognized by its characteristic ear tufts that grow longer in winter. The grey squirrel, which is native to Eastern North America, is significantly larger, typically weighing between 400 and 650 grams, and lacks the prominent ear tufts. While the red squirrel’s coat ranges from bright red to dark brown, the grey squirrel’s fur is predominantly silver-grey, though red-tinged variations can sometimes lead to misidentification.
The Genetic Barrier to Interbreeding
Red and grey squirrels belong to the same genus, Sciurus, but are classified as separate species. This evolutionary divergence creates a biological barrier to successful interbreeding known as reproductive isolation. Even if mating occurs, the genetic material from the two distinct species is incompatible. The difference in their evolutionary timelines means the organization of their chromosomes and the specific arrangement of their genes cannot successfully align during fertilization. Consequently, the sperm and egg cannot combine to produce viable, fertile offspring.
The Real Threat: Competition and Disease
Resource Competition
Since hybridization is not possible, the decline of the red squirrel population is driven by the grey squirrel’s superior competitive ability. Grey squirrels possess an advantage in resource acquisition because they are larger and more robust, allowing them to store a greater percentage of body fat for winter survival. Studies show that grey squirrels can increase their body mass by 17–23% in autumn, compared to only about 10% for red squirrels, granting them a competitive advantage during food scarcity. Furthermore, the grey squirrel’s more diverse gut microbiome allows them to digest a broader range of foods, including unripe nuts and plant compounds like oxalates, which the red squirrel cannot easily process.
Disease Transmission
The most devastating ecological mechanism of displacement is the transmission of the squirrel poxvirus (SQPV). Grey squirrels carry the parapoxvirus asymptomatically, meaning they are largely unaffected by the pathogen, acting as a reservoir species. This virus is often fatal to the red squirrel, causing severe symptoms such as lesions around the eyes and mouth, leading to respiratory failure and an inability to feed. In areas where the grey squirrel introduces SQPV, the red squirrel population decline rate can be 17 to 25 times faster than in areas where the virus is absent.