Mammals
Squirrel
Sciuridae · also called squirrels
“Squirrel” usually brings to mind a grey or red animal spiralling up a tree trunk, but that is one corner of a large rodent family. The Sciuridae also takes in the chipmunks, the burrowing ground squirrels and prairie dogs, the heavy marmots and groundhogs, and the gliding flying squirrels — around 280 species on every continent except Antarctica and Australia.
Overview
Squirrels are the rodent family Sciuridae, about 280 species. All have a chisel-toothed rodent skull, large eyes, and, in most, a long bushy tail. The family divides into tree squirrels, ground squirrels (including prairie dogs, marmots, and chipmunks), and flying squirrels. Diet is mostly nuts, seeds, buds, fruit, and fungi, with many species also taking insects and eggs.
Accidental foresters
Tree squirrels scatter-hoard — burying nuts one at a time across their range and recovering most by memory and smell. The ones they miss are buried, spaced out, and away from the parent tree, so squirrels are among the main planters of oak, hickory, walnut, and pine.
Built for the canopy
A tree squirrel can rotate its hind feet 180 degrees to grip the bark with claws pointing upward, letting it run down a trunk head-first. Flying squirrels stretch a membrane from wrist to ankle and glide — not fly — typically 5 to 25 m, steering with limbs and tail.
Winter strategies
Ground squirrels, marmots, and chipmunks hibernate — the Arctic ground squirrel supercools below freezing — while tree squirrels stay active all winter on stored food and a warm nest called a drey. Ground squirrels and prairie dogs have alarm calls that encode a predator’s type and speed, and tree squirrels perform fake burials to fool watching rivals.
Status
Squirrels as a family are Not Evaluated. Many are common and a few, such as the eastern grey squirrel in Britain, are invasive, but tropical tree and flying squirrels are losing forest and prairie dog colonies have collapsed to a fraction of their former extent, taking the black-footed ferret down with them.
Squirrel: common questions
Do squirrels really forget where they bury their nuts?
They forget some, but far fewer than the old saying suggests. Tree squirrels are scatter-hoarders, burying thousands of nuts in separate small caches across their range, and they relocate a high proportion of them using spatial memory, landmarks, and smell, with smell working best when the ground is moist. They also organise caches, sometimes grouping nuts by type, and re-bury them if disturbed. But with so many caches, and with theft by other squirrels and birds, a meaningful fraction is never recovered. Those forgotten, buried, spaced-out seeds are well placed to germinate, which makes squirrels significant tree planters despite, or rather because of, their imperfect memory.
How do flying squirrels fly?
They do not fly, they glide. A flying squirrel has a stretchy, fur-covered membrane called the patagium running along each side of the body from the wrist to the ankle, with a small cartilage spur at the wrist that helps hold it taut. To travel, the squirrel climbs high, judges the distance, and leaps, spreading its limbs so the membrane becomes an airfoil. It then descends on a glide path, usually losing about one metre of height for every two or three metres forward, steering by adjusting its arms and flattened tail, and swoops upward at the end to slow down and land on a trunk. Glides of 20 to 45 metres are routine and much longer ones are recorded.
Why do squirrels chase each other in spiralling runs up trees?
Most of that behaviour is either mating activity or territorial disputes. During the breeding season several males will pursue a receptive female in a fast, noisy chase through the branches, the female setting the pace and the males competing for position. Outside breeding, chases are usually one squirrel driving another off a food source or out of a defended core area around a nest or a rich cache. Young squirrels also chase in play, which builds the coordination they need in the canopy. The spiralling up-and-around-the-trunk pattern simply reflects that a squirrel being pursued keeps the tree between itself and its pursuer.
Are grey squirrels a problem for red squirrels?
In Britain and Ireland, yes, and the mechanism is now well understood. The eastern grey squirrel, introduced from North America in the 1800s, is larger, can digest unripe acorns that red squirrels cannot, and reaches higher densities, so it out-competes the native red squirrel for food, especially in broadleaf woodland. Greys also carry squirrelpox virus, which they usually survive but which is almost always fatal to reds, and an outbreak can clear reds from an area far faster than competition alone. Red squirrels now survive mainly in large conifer forests, on islands, and in areas where greys are actively controlled or where pine martens, which prey more heavily on greys, have returned.