Geography Reference
In-Depth Information
FIGURE 8.14 Ptarmigan display a continuously changing plumage from spring through autumn, and
stay within either bare or snow areas depending on the condition of their plumage. (U.S. National
Park Service.)
On a seasonal basis, animals commonly vary their insulation by molting or shedding
fur or feathers. The mountain sheep's coat consists of a layer of dense woolly under-hair
and an outer layer of long brittle guard hairs. Most of the guard hairs are rubbed of in
spring during the major molt (Geist 1971). Similar behavior is observed in many cold-
climate animals, and measurements made of the insulation ability of fur show marked
differences between winter and summer (Hart 1956). The insulation value of the feath-
ers of birds may be slightly less in summer, but the differences are not so marked as
with animal fur (Porter et al. 2000). One of the most complex molts of any kind is that
of the ptarmigan, which displays a continuously changing plumage from spring through
autumn (Hoffmann 1974; Fig. 8.14). In this case, however, escape from detection by
predators appears to be the selective agent.
The other major method of reducing heat dissipation is to lower the temperature
of the extremities. This decreases the thermal gradient between the body and the sur-
rounding environment so that less heat is lost from these less insulated areas, and it
saves the energy that would be required to maintain the extremities at a higher tem-
perature. Examples are the bare legs and feet of birds, and the legs, feet, and noses of
animals such as the fox, wolf, caribou, or mountain sheep (Fig. 8.15).
The bare feet of birds and mammals standing in icy water or on cold snow are main-
tained just above the freezing temperature of the tissue, which may be as low as −1°C
(30.2°F) (Irving and Krog 1955; Irving 1964, 1966, 1972; Henshaw et al. 1972). This
Search WWH ::




Custom Search