Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 433

sheep and goats, wolves, bear, and caribou, but fur depth does not continue to increase
on larger animals in proportion to their size; it appears to reach its maximum efficiency
at about the size of the fox (Fig. 8.12).
On animals smaller than the fox, the fur becomes thinner; if it did not, these animals
would not be able to move about. This is particularly true for the smallest forms such as
shrews, lemmings, and weasels. The insulation value of the fur on these small animals
is little more than that of tropical forms. Consequently, the only way they can survive
is to escape into burrows and under snow. Bird species display no marked difference in
plumage between warm and cold environments, apparently because of the restrictions
imposed by flight requirements. However, the feathers of arctic and alpine birds are fre-
quently structured to trap more air for insulation purposes than do the feathers of birds
in warm climates (Irving 1972).
FIGURE 8.12 Insulation in relation to winter fur thickness in arctic and tropical animals. The insulat-
ive value of fur is roughly proportional to its thickness. Lemmings, squirrels, weasels, and shrews
have a low degree of insulation, comparable to that of tropical animals, denoted by cross-hatched
area. Consequently, they can only survive the cold by burrowing and sharing body heat. Heat
transmission through the fur was measured in a room at 0°C (32°F), by stretching the fur over a
ring and heating one side of it to 37°C (98.6°F) with a hot plate. Thermisters were used to ascer-
tain the temperature on the other side of the skin. (After Scholander et al. 1950: 230.)