Do Squirrels Hibernate? How Squirrels Survive Winter

How tree squirrels survive winter

The eastern gray squirrel and the red squirrel face cold, short days and scarce food without slowing their metabolism much. Instead, they combine several strategies:

• Food stores. In autumn, gray squirrels bury thousands of nuts one by one, and red squirrels stash nuts, seeds and fungi in the ground and in trees. In winter they dig these up, often through snow. How they find them is explained in do squirrels remember where they bury nuts?
• Body fat. Squirrels fatten up in autumn. Gray squirrels can store considerably more fat than red squirrels, which helps them get through long cold spells.
• A thick winter coat. Both species grow a denser winter coat, and red squirrels develop their longest ear tufts in winter.
• Less activity. In winter, tree squirrels spend more of the day resting in their nests and come out for shorter periods, often around the warmest hours.

During severe storms or deep cold, a squirrel may stay in its nest for a day or several days. That is simply resting; its body temperature stays near normal.

Dreys and tree dens

Tree squirrels use two kinds of winter shelter. The best is a tree den, a hole in a trunk or large branch, often an old woodpecker hole or a hollow left by decay. Dens are dry and well insulated, and young squirrels raised in dens survive better than those raised in leaf nests.

Where holes are scarce, squirrels build dreys. A drey is a ball of twigs and leaves, about 30–60 cm (1–2 ft) across for a gray squirrel, built in a fork of branches. Inside is a snug chamber lined with moss, grass, shredded bark and feathers. Winter dreys are built more solidly than summer ones and are easy to spot in bare winter trees.

Sharing nests for warmth

Gray squirrels, normally fairly solitary, often share a nest with one or more other squirrels in cold weather. Red squirrels are more solitary still, but in winter and early spring several adults may share a drey. Huddling reduces the surface area each animal exposes to the cold and saves energy.

Flying squirrels take this further. Southern flying squirrels in North America regularly huddle in groups inside tree holes, sometimes a dozen or more together.

Which squirrels do hibernate?

The squirrel family, Sciuridae, also includes ground squirrels, prairie dogs, marmots and chipmunks, and several of them are true hibernators.

The groundhog is one of the best-known hibernators in North America; its winter is described in do groundhogs hibernate?

What happens in true hibernation?

A hibernating ground squirrel lowers its body temperature, heart rate and breathing for days or weeks at a time. Its heart, which may beat a few hundred times a minute when active, slows to just a handful of beats per minute. It lives almost entirely on stored body fat.

Hibernation is not one long sleep. It is broken into bouts of deep torpor, each lasting from several days to a few weeks, separated by brief periods when the animal rewarms to a normal temperature for less than a day. These arousals use a large share of the winter’s energy, and scientists are still studying exactly why they are needed.

The Arctic ground squirrel holds a remarkable record. In 1989 Brian Barnes reported that hibernating Arctic ground squirrels in Alaska allowed their core body temperature to fall to about −2.9 °C (26.8 °F) without freezing, the lowest body temperature known for a mammal. Their blood stays liquid by supercooling, staying below its freezing point without forming ice crystals.

Why don’t tree squirrels hibernate?

Tree squirrels can stay active because their food is still available in winter. Nuts and seeds keep for months when buried or stored, and conifer cones stay on the trees. Ground squirrels in open grassland or tundra, by contrast, face months when their food of green plants, seeds and insects is gone or buried under snow, so shutting down is the safer option.

Hibernation also carries risks. A hibernating animal is slow to respond to predators, and if its fat runs out early it can starve. For a tree squirrel with a cache of nuts and a warm nest high in a tree, staying awake is the better bet. Many other animals take the opposite route, as listed in animals that hibernate.