Because snow has been so important in the evolutionary history of weasels, it deserves some special attention. Snow provides, in a sense, another world, the subnivean world. Just the name "subnivean" sounds a bit like Middle Earth or Narnia, and perhaps with good cause. The subnivean world is a chilly but safe haven from the bitter cold and fierce storms that rage through the northern winter.
Snow is like a duvet or down comforter, because both hold much air. That is why both are such good insulators (LaChapelle 1969), and why the ground under a stable snow blanket is relatively warm. In temperate climates, ground temperature under snow may be up to 10°C, although in northern boreal forests and tundra it is closer to zero. Nonetheless, the ground is never as cold as the subfreezing air temperatures of mid-winter. Snow holds in warmth, protecting the small mammals and insects that remain active at the ground surface throughout the winter. Snow also transmits light, though the attenuation is great. Animals emerging out of their thickly insulated nests and underground burrows to move about under shallow snow can still tell the difference between day and night.
Most important, a snow blanket is not the same all through from top to bottom. As one snowfall follows another, the warmth of the ground and the accumulating pressure changes the structure of the original six-sided snow crystals. At the ground surface, what might have fallen as fine powder snow becomes granular, much like sugar, so that deep layer is called depth hoar or sugar snow. Voles, mice, and weasels can push through it easily, forming tunnels and small rooms of crystal between the ground surface and the more solid blanket above, and forging new pathways (see Figure 6.2).
Weasels evolved their superb tunnel-hunting abilities in response to the opportunity to hunt small mammals that formed burrows and runways in grass, and for them this subnivean world is no different than life under a thick layer of grass. But if they had not been able to make use of the insulation supplied by snow, they might have joined the many other species of animals that did not survive the savage cold of the glacial periods. Least weasels and stoats are completely at home in snow, and common weasels and longtails can burrow through it when they need to, although most common weasels and longtails live in more southerly latitudes where snow falls less often and lies less thickly than in the far north.
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