Although we might associate snow with cold, a snow-covered landscape actually acts like a blanket to insulate and provide shelter for animals during the winter months.
Most northern mammals have thick, warm fur to insulate them against the winter’s cold. Even animals with thick fur, however, often use snow to provide added protection against cold and wind.
Above and in the snow
Arctic foxes make temporary shelters in the snow during stormy conditions on the tundra. Grizzly bears in northern Alaska tend to select den sites in locations that usually receive lots of snow. Female polar bears make dens in the snow, taking advantage of natural barriers that create snow drifts around them.
Some animals take advantage of snow’s insulating ability by burrowing into snow to protect them from the cold. The Ptarmigan, a grouse-like bird that inhabits Alaska and other snowy places, will dive into deep snow to take temporary shelter from the cold. Flying squirrels will also burrow into the snow to warm up.
Without the benefit of snow, animals have to spend a lot more energy to maintain their body temperatures. Dry, fluffy snow is a good insulator because it contains a lot of air that slows down heat loss. Wetter, denser, hard-packed snow provides less insulation, because it contains less air among the snow crystals. Hard, dense snow is also harder for animals to dig into.
Beneath the snow
What about tiny animals such as voles, lemmings, and shrews? Their fur isn’t very thick; thick fur would make it hard for them to move around on their short legs. Just as a toddler gets cold outside in the winter much more quickly than an adult, small mammals get cold much more quickly than large mammals. The reason for this is that smaller animals (including smaller people) have a high “surface area to volume ratio,” which means that they have a lot of surface area from which they lose body heat to the environment, but their small bodies cannot generate and hold as much heat as a large animal like a bear, wolf, caribou, or even a snowshoe hare.
Small mammals spend the winter months underneath the snow in an area called the “subnivean zone.” The word “subnivean” means “under the snow.” The subnivean zone is the area on the ground at the very bottom of the snowpack. In cold places, the snow crystals at the bottom of the snowpack are coarse and crumbly, allowing small mammals to easily burrow through it. The snowpack above provides insulation and moisture, and the animals are able to move around eat plant seeds, leaves, and stems.
Another benefit of life in the subnivean is protection from predators. Small mammals are the foundation of the boreal forest and tundra ecosystems; any predator will eat them! Under cover of snow, they can move about without being seen.
Even under the snow, small mammals aren’t completely safe, though; predators such as owls and foxes can actually hear a small mammal under the snow. Foxes will pounce on the snow in order to break through any hard snow layers to catch their subnivean prey.
Foxes are known for their habit of pouncing to catch their small mammal prey. The force from pouncing also helps to break through the hard snow surface. It looks like this red fox was having a tough day. NPS Photos / Neal Herbert (Public Domain)
One skilled predator of small mammals is the ermine, a type of weasel. The ermine’s small, long, thin body allows it to travel in subnivean tunnels and burrows and catch the voles and lemmings that live there.
A small mammal’s ability to survive a cold winter depends heavily on the presence of the subnivean zone. The subnivean zone can’t exist unless there is a crumbly, weak snow layer at the bottom of the snowpack. This weak, crumbly snow layer, called “depth hoar,” only forms under certain temperature and humidity conditions. The snow crystals in the snowpack change throughout the season due to temperature and moisture conditions in the snowpack and in the air. This process of change is called snow metamorphism (meaning “change in form”), and it can result in a variety of different types of snow layers.
To learn more about how snow changes throughout the winter, visit the snow science pages.