Subnivean

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Subnivean (or subniveal) refers to a zone that is in or under the snow layer. It can form when latent heat from the ground melts a thin layer of snow above it, leaving a layer of air between the ground and the snow.

Subnivean animals include small mammals such as mice, voles, shrews, and lemmings that must rely on winter snow cover for survival. These mammals move under the snow for protection from heat loss and predators, though some small predators, such as ermines, can also hunt in the subnivean. In winter regions that do not have permafrost, the subnivean zone maintains a temperature of close to 32 °F (0 °C) regardless of the temperature above the snow cover, once the snow cover has reached a depth of six inches (15 cm). The sinuous tunnels left by these small mammals can be seen when the snow melts to the final inch or so.

Winter predators such as foxes and large owls can hear their prey through the snow and pounce from above. Snowmobiles and ATVs can collapse the subnivean space, as can skis, snow shoes and bicycles.

Larger animals also utilize subnivean space. In the Arctic, ringed seals have closed spaces under the snow and above openings in the ice. In addition to resting and sleeping there, the female seals give birth to their pups on the ice. Female polar bears also den in snow caves to give birth to their young. Both types of dens are protected from exterior temperatures.