Grey squirrel (Sciurus carolinensis)
|Also known as:||Gray squirrel|
|Size||Head & body length: 23 - 30 cm (2)|
Tail length: 14 - 24 cm (2)
|Weight||400 - 800 g (2)|
The grey squirrel is classified as Least Concern (LC) on the IUCN Red List (1). It is an introduced invasive species to the UK, South Africa and Italy (1) (3).
Although a familiar mammal in many parts of Great Britain, the grey squirrel (Sciurus carolinensis) is non-native, having been first introduced from the eastern USA in 1876 (3). It is responsible for the decline in populations of the UK's native red squirrel (Sciurus vulgaris) (3). The introduced species is larger than the red squirrel, has largely grey fur with touches of russet-brown, and white underparts (2). Unlike the red squirrel, this species never has ear tufts (2). The sexes are similar in appearance (4).
Introductions of this species to the UK continued up until 1915. Between 1930 and 1945 it underwent a huge expansion in range; it is now common throughout central and southern England, Wales and the central lowlands of Scotland (3), and is still increasing in terms of range and numbers (5). The grey squirrel has also been introduced to South Africa, Australia (3) and Italy (6). In Italy the grey squirrel has extended its range into the Alps and Piedmont, and it seems likely that it will now spread throughout much of Europe (6). Its native range extends throughout the eastern USA reaching as far north as Canada, and south to the Mississippi River (4).
A very adaptable species, the grey squirrel prefers mature broadleaved woodlands with a rich understorey layer (4). It also occurs in conifer woodlands, urban areas where there are mature trees, as well as gardens and parks (5).
Grey squirrels are active during the day (5); they feed on seeds, nuts, buds, insects, bird eggs (5) and fungi, depending on the time of year (3), and are well-known for their habit of hoarding food in autumn to see them through the harsh winter months (3). Seeds, cones or nuts are hidden in small scrapes scattered over the ground and buried (3). The general area is remembered, and then the cache is re-found by smell over fairly short distances (3).
Breeding takes place in December to February, and again in March to May. During this time, a number of males may follow a female when she is about to come into oestrus; during this 'following phase' the female may occasionally turn on the male and rebuff his advances by lunging at him aggressively. The day the female comes into oestrus, a number of males chase the female, making 'buzzing' noises; this is known as the 'mating chase', and the female can respond aggressively to males. Through much male-male chasing, dominant males are able to get closer to the female; when she is ready she crouches on the ground, and the first male to reach her mates with her (3). Gestation takes up to 44 days, during which time females are solitary, and nest in a 'drey' of twigs and leaves (4). If conditions are good (5), two litters are produced each year, consisting of one to eight young (5). The young are usually weaned by ten weeks (4), and reach sexual maturity at 10 to 12 months of age (5). The average lifespan is eight to nine years (5).
There are no threats to the grey squirrel (1). It is a serious pest in Britain, and its habit of removing tree bark is extremely damaging. In addition to out-competing red squirrels, it also carries a disease called parapox virus, which affects the native species (7).
No conservation measures are in place for this introduced and now common species. Grey squirrels are controlled to protect trees and in areas where red squirrels persist (5). The eradication of grey squirrels for conservation reasons is unlikely; the costs involved would be enormous, and the species is very popular with the public (5).
For more on the grey squirrel:
Gurnell, J. (1987) The natural history of squirrels
.Christopher Helm (Publishers) Limited, Kent.
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- Gestation: the state of being pregnant; the period from conception to birth.
- Oestrus: the time of ovulation (release of an egg from the ovary) in female mammals, when the female becomes receptive to males, also known as 'heat'.
IUCN Red List (March, 2011)
- Burton, J.A. (1991) Field guide to the mammals of Britain and Europe. Kingfisher Books, London.
- Gurnell, J. (1987) The natural history of squirrels. Christopher Helm (Publishers) Limited, Kent.
Animal Diversity Web (August, 2002)
Macdonnald, D.W. and Tattersall, F.T. (2001) Britain's mammals- the challenge for conservation. The Wildlife Conservation research Unit, Oxford University, Oxford. Available at:
Genovesi, P. and Bertolino, S. (2001) Human Dimension Aspects in Invasive Alien Species Issues: The Case of the Failure of the Grey Squirrel Eradication project in Italy. In: McNeely, J.A. (Ed) The Great Reshuffling Human Dimensions of Invasive Alien Species. IUCN Biodiversity Policy Coordination Division, Gland, Switzerland. Available at:
The Invasive Alien Species Project (March, 2008)