Hibernation: a survival strategy

Why do some wildlife species hibernate while others don’t? Hibernation is a survival strategy for wildlife that don’t have a reliable winter food source. Every year, as food becomes scarce—along with other seasonal triggers—these creatures experience significant physical changes that allow them to retreat for weeks and months at a time.

Some animals, such as bats and chipmunks, are true hibernators. While they rest, their bodies cool down and their metabolism, heartbeat and breaths slow down significantly. They may awaken for short periods to drink water or attend to other needs.

Lighter hibernators retreat during severe weather but regularly leave their burrow or nest for nourishment on warmer days or nights. In the ground or underwater, reptiles and amphibians will enter a state called brumation, where they move very little as long as the environment remains cold.

Very warm winter days—which are increasingly common—can draw out hibernating wildlife too early in the season, when spring food sources, such as insects, are not yet available. For insects such as bees and wasps, early rousing can mean there are no pollinating plants—their main food source.

Tracks and trails in the snow

Snow plays a significant role in the survival of animals and birds in northern latitudes. Look closely across the snow’s surface and you’ll see small holes scattered here and there. Many are the openings of air shafts connected to networks of snow tunnels created by mice and other little animals. At the ends of those tunnels are warm nests of soft plant material where small mammals curl up under a thick, snowy blanket.

Up above, a sure sign of winter activity is the appearance of animal tracks in the snow. Moose leave a deep furrow as they move on their long legs through the snow. The broad, furry feet of snowshoe hares allow them to hop across the surface without sinking. Grouse tracks display the sharp scales that help them grasp ice-sheathed branches as they shift from ground-feeding to foraging in shrubs and trees for buds, catkins and frozen fruit.

Do frogs hibernate?

Conservation has never been more important for amphibians than it is now. An important global study (Oct. 2023) has confirmed that two in every five amphibian species are threatened with extinction, mainly from disease and increasingly dry climate conditions. Frogs and toads fill an important niche in wetland ecosystems, eating insects but also being eaten by predators like large, wading birds.

But there’s more—amphibians are extreme survival strategists. For example, some frogs cool down so much in winter that they actually freeze—such as wood frogs. These unassuming brown frogs are found in forests and wetlands across the country.

How do wood frogs survive the winter?

Wood frogs survive winter not by burrowing into mud like some species, but by hiding under leaves or logs. They can freeze solid like a little brown hockey puck and, in spring, thaw and carry on with no ill effects.

As temperatures fall in autumn, their liver converts the fat reserves they’ve accumulated in summer into levels of blood glucose up to 80 times greater than those found in humans. This keeps their blood from freezing, protecting their vital organs even while ice forms in the spaces outside their cells.

As they rest, microbes in their gut recycle urea in their body which also protects their tissues from the cold. These processes prevent damage as the wood frog’s body shuts down due to cold, without a heartbeat or breath in their lungs.

When spring comes, they are the first frogs to sing in chorus, often with ice still on the ponds.

A winter feat for ducks

Frogs aren’t the only ones with surprising strategies for cold weather. To avoid losing too much body heat to snow and ice, ducks experience a change in blood flow to their legs and feet. The change is radical enough to ensure that only 5 per cent of the duck’s heat loss happens in their feet, despite the contact with ice, snow and cold water.