the methods by which animals survive the unfavorable winter period in the temperate and cold zones. Invertebrates adapt to unfavorable winter conditions (low temperature, short days, and the disappearance of many sources of food) through their developmental cycles. For example, insects survive the winter in one of the phases of their life cycle that is resistant to cold and adapted to winter conditions: egg (locusts, many beetles, and butterflies), larva (certain beetles, cicadas, dragonflies, and mosquitoes), or pupa (many butterflies).
Hibernation, another form of adaptation, is characteristic of certain poikilothermic (cold-blooded) animals (invertebrates, fish, amphibians, reptiles) and some homoiothermic (warm-blooded) animals (mammals like the suslik, marmot, dormouse, hedgehog, and bat). Winter dormancy is characteristic of certain mammals.
Animals that do not hibernate—birds, most mammals and fish, and certain insects, for example—migrate for the winter to other biotopes or to regions with more favorable climatic conditions and sufficient supply of food. These seasonal migrations are most pronounced in such mammals as bats and whales, some fish, and particularly in birds, most of which pass the winter in the subtropics and tropics. Mainly herbivorous birds and birds with a mixed diet pass the winter in the temperate and cold latitudes.
Homoiothermic animals that spend the winter in the temperate or cold latitudes acquire a thick fur or feather coat after fall molting; the coat reduces the amount of heat lost in cold weather. Molting also produces a protective coloration (in hares, ermine, and willow grouse). Many animals and birds develop a layer of subcutaneous fat in the fall that protects them against chilling and enables them to survive a lack of food. An important survival factor is the ability of many mammals to switch to food available in winter and to store food during the fall.
Several terrestrial bird species (hazel hen, red grouse, capercaillie, willow grouse) bury themselves in snow, which has good heat-insulating properties, at night or during bad weather; they spend most of the 24-hour period there. Many such birds often die in snowless winters. Snow provides good protection from the cold for small mammals who burrow in it and build their nests there. Small and medium-sized birds and animals pass winter nights in groups, thereby reducing the amount of heat loss.
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Shmidt, P. lu. Migratsii ryb, 2nd ed. Moscow-Leningrad, 1947.
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A. V. MIKHEEV