Red squirrels (Sciurus vulgaris) and grey squirrels (Sciurus carolinensis) are two distinct species found across various regions, including parts of Europe and North America. Their interactions have long been a subject of public interest and ecological inquiry, particularly where their ranges overlap, and understanding these dynamics is important for conservation and biodiversity.
Direct Confrontations and Lethality
Direct lethal combat between red and grey squirrels is exceptionally rare. Interactions typically involve competitive displays rather than fatal aggression. These encounters often involve chasing or territorial disputes over resources or tree spaces. While aggression can occur, particularly from red squirrels defending territory, it rarely results in death.
Research suggests interactions between adult red and grey squirrels are usually indifferent, with no significant mutual aggression. Grey squirrels do not actively hunt or kill red squirrels for sport or consumption. Both species are omnivorous, primarily consuming plant matter and insects, not other squirrels. The idea that red squirrels might bite off grey squirrels’ testes is a widespread myth without scientific evidence. The decline of red squirrel populations is not attributed to direct physical attacks or lethal fights from grey squirrels.
Competitive Disadvantage and Resource Scarcity
Grey squirrels often outcompete red squirrels for food, particularly in broadleaf woodlands. They are more efficient foragers and can digest unripe acorns and other seeds that red squirrels cannot readily process due to tannins. This dietary flexibility allows grey squirrels to access and deplete food sources earlier in the season, putting red squirrels at a disadvantage. Grey squirrels also have a broader diet, including tree bark, which is less common for red squirrels.
Grey squirrels also adapt better to habitats, thriving in a wider range of woodland types, including deciduous forests rich in large-seed-producing trees like oaks. While red squirrels can inhabit coniferous forests, both species prefer mixed broadleaf woodlands. Grey squirrels can achieve significantly higher population densities, with one hectare of woodland potentially supporting three to thirty grey squirrels compared to only one red squirrel. This higher density means grey squirrels consume more food, further intensifying competition. These competitive pressures can lead to red squirrel populations declining due to insufficient food, reduced breeding success, or inability to sustain viable numbers, rather than direct physical harm.
Disease as a Dominant Factor
The most significant factor contributing to red squirrel decline is the squirrelpox virus (SQPV), a parapoxvirus. Grey squirrels frequently carry this virus without severe symptoms, acting as asymptomatic carriers. However, when transmitted to red squirrels, the virus proves highly virulent and often fatal.
The virus spreads through direct contact with infected lesions, contaminated crusts, or potentially via contaminated feeders or ectoparasites. Infected red squirrels develop lethargy, panting, and swollen lesions, scabs, and ulcers around the eyes, mouth, feet, and genitalia. These symptoms prevent red squirrels from eating, drinking, or moving effectively, leading to rapid body condition loss and eventual death.
The mortality rate for untreated infected red squirrels in the wild is nearly 100%, with death often occurring within 10 to 15 days of infection. The introduction of SQPV by grey squirrels can accelerate the disappearance of red squirrels from an area by 17 to 25 times compared to competition alone. This indirect lethal impact, facilitated by grey squirrels, is the primary mechanism by which they contribute to the decline and local extinction of red squirrels.