For them, it is not the snow itself that provides the winter challenge, but rather their need to avoid extreme cold. Since snow does conduct heat poorly, it has value as an insulator. The area below the snowcover is called the subnivean environment and is characterized by a reduction in daily temperature fluctuations caused by the insulative properties of the snow. Small mammals with rapid metabolisms and limited fat storage capabilities rely on adequate snow cover to ensure their winter survival. Voles, shrews, and mice often construct intricate burrows and runway systems in the snowpack and rarely emerge.
Insects are another group that commonly inhabit the subnivean environment.
They, along with plant material, provide food for the small mammals. The familiar woolly bear caterpillar that seeks shelter under leaves in late fall certainly benefits from the blanketing effect of snow. Another is the snow scorpionfly which, although cold-blooded, is able to emerge from the snowpack for short periods before retreating once again beneath the surface. Other arthropods, particularly spiders, remain active beneath the snow. Larger mammals, such as weasels and beavers also benefit from snow.