Mammals
Eastern Gray Squirrel
Sciurus carolinensis · also called grey squirrel, eastern gray squirrel
The eastern gray squirrel is one of the most watched wild mammals in the world — a bold, acrobatic, quick-witted rodent of parks and woods. It shapes forests by burying (and forgetting) nuts, it plays elaborate mind games with would-be thieves, and outside its home range it has become one of the most damaging introduced mammals.
Overview
Eastern gray squirrels are native to the deciduous and mixed forests of eastern North America and thrive in towns, suburbs, and campuses as readily as in wild woodland, wherever there are nut-bearing trees. They are diurnal, active year-round (they do not truly hibernate), and largely solitary outside the breeding season, though they will share a nest for warmth in cold weather. They build leaf-and-twig “dreys” high in tree forks and also den in hollows.
The forester
Grey squirrels are scatter-hoarders: rather than filling one larder, they bury individual nuts in thousands of separate spots through the autumn, then live off the caches through winter and early spring. They recover a good proportion using spatial memory and scent, and they organise their hoards — sorting caches by nut species and clustering them by landmark. The nuts they do not recover germinate, making the squirrel a major, if accidental, planter of oak, hickory, walnut, and beech.
Deception
Cache theft is rife — squirrels routinely raid each other’s stores — and grey squirrels respond with tactical deception. A squirrel that senses it is being observed will perform a complete but fake burial, going through every motion while palming the nut, and hide the real one later in private. The rate of these deceptive caches rises when the perceived risk of pilfering is higher, implying the squirrel adjusts its behaviour to what an onlooker can see.
Body and movement
The squirrel is built for life in the canopy: a long, bushy tail for balance, steering in falls, warmth, shade, and signalling; reversible hind ankles for head-first descent; sharp curved claws; and the ability to survive falls from any height. It leaps gaps of two metres or more and judges distances by bobbing its head to gain parallax.
Reproduction
Grey squirrels breed twice a year in good conditions, in late winter and midsummer. Several males chase a receptive female through the trees in noisy “mating bouts.” Litters of two to four are born blind and hairless in a drey and are weaned at around 10 weeks.
Status
In North America the eastern gray squirrel is Least Concern and abundant. Where it has been introduced — most consequentially Britain and Ireland — it is a serious invasive: it has displaced the native red squirrel across most of that range through competition and by spreading squirrelpox, and it damages young trees by bark-stripping. Conservation of red squirrels now depends heavily on managing grey squirrel numbers, aided in some areas by recovering pine marten populations that suppress greys.
Eastern Gray Squirrel: common questions
Do squirrels remember where they bury their nuts?
Better than they used to be given credit for. Grey squirrels are scatter-hoarders — they bury nuts one at a time across a wide area — and studies show they relocate a large fraction of their own caches using spatial memory, organised by landmarks and even by nut type, supplemented by smell. They still lose many, especially to other squirrels and to forgetting, and those forgotten nuts are how the squirrel inadvertently plants forests — a single grey squirrel may establish dozens of oak and hickory seedlings a year.
Why do squirrels sometimes bury nothing?
Deceptive caching. When a grey squirrel believes another squirrel (or a person) is watching and might pilfer its store, it will go through the full motions of burying a nut — digging a hole, tamping down soil — while actually keeping the nut in its mouth and hiding it elsewhere later. The frequency of these fake caches goes up when the risk of theft is higher, which suggests the squirrel is tracking what others can see.
How can a squirrel run down a tree head-first?
Its hind ankle joints can rotate through about 180 degrees, turning the back feet to point backwards up the trunk. The claws then hook into the bark from above, anchoring the squirrel as it descends face-down — something most tree-climbing mammals cannot do.
Why are grey squirrels a problem in Britain?
Introduced from North America in the 19th and early 20th centuries, the grey squirrel has spread across most of England, Wales, and Ireland and much of Scotland, largely replacing the native red squirrel. Greys outcompete reds for food, are more tolerant of habitat fragmentation, and — crucially — carry squirrelpox, a virus that is harmless to greys but usually fatal to reds. Greys also strip bark from young broadleaved trees, damaging forestry. Red squirrels now survive mainly in northern strongholds, on islands, and where grey control or pine marten recovery holds greys back.