In winter, the forest looks quiet and deserted. But underneath the snow, life goes on for many small forest animals.

Sabine Schweiger, a Whitehorse biologist, has studied the small creatures that remain active under the winter snow. The realm they live in, she says, is called the subnivean space.

Snow is a good insulator, Schweiger explains, so the ground beneath the snow is significantly warmer than the air above it. As the snow builds up, the ground gives off heat and vapourizes the snow next to it. The water vapour condenses and freezes on the snow above it, creating an open space at ground level.

"The snow peels away from ground," Schweiger says. "The temperature next to the ground is around zero degrees Celsius through the winter."

The inhabitants of the subnivean space are the smallest forest mammals, like mice, voles, and shrews. Some predators live there too, particularly weasels.

"A weasel can go almost anywhere a mouse can go," says Schweiger. " A whole food chain develops under the snow over the winter."

You can often see evidence of the life under the snow by observing the edges of ski or snowmobile trails. The trails compress the snow and break through the subnivean space, often cutting across natural subnivean tracks or runways. Where this has happened, you will see tunnels emerging from either side of the trail and, sometimes, a scurry of small footprints crossing the trail from one tunnel to the other.

Predators above the snow also take advantage of the residents of the subnivean space. Coyotes, foxes, and owls listen for sounds of movement beneath the snow, then pounce, breaking through the surface snow to trap the small animal below.

Several years ago, Schweiger conducted a study of red-backed voles living beneath the snow in winter. She says the voles' behaviour changes in winter.

In summer, female voles establish territories and defend them from other females, while producing three or four litters of young voles. The males roam around, looking for food and receptive females.

"In winter, the voles' social structure breaks down," Schweiger says. The females abandon their territorial behaviour, and sometimes groups of voles huddle together for warmth during cold spells. The groups are often composed of related individuals, she adds.

In the spring, the voles' territorial instincts return. They split up, and the females establish territories again, defending them aggressively against other females.

Like many animals, red-backed voles go through cycles of high and low population. One of the goals of Schweiger's research was to determine whether food availability in winter influences the vole population cycle.

She set up a grid of boxes with tunnel extensions and awnings to tap into the naturally developed network of paths and tunnels in the subnivean space. She tagged voles and recorded their movements and survival rate over the winter. She also supplied food at some of the boxes.

Her research did not establish a clear link between food supplies and the population cycle, says Schweiger, although the amount of available food affects the voles' overwinter survival rate, and food availability is also likely linked to the onset of breeding.

However, the difficulties of observing animals in the subnivean space without influencing their behaviour, as well as the voles' habit of dispersing to find territories in the spring made it difficult to draw larger conclusions about the population cycle, Schweiger says.

For more information about animals in winter, contact the Canadian Wildlife Service in Whitehorse at 393-6700.