HIBERNATION, the winter sleep of warm-blooded animals. Under this term is also included the torpidity of frogs, toads, reptiles, certain fishes, insects, the horseshoe crab and snails, which is mainly due to prolonged cold. Among the mammals which hibernate are the bear, dormouse, badger, bat and hamster; a number are incomplete hibernators, as the prairie dog, while squirrels fall into a winter sleep during the coldest weather, but may be seen in warm spells in winter. The males of the black and white bear are more or less active durin? the winter months, while the females are hibernating. The bears and some others, like the skunk, may in the southern portion of their range not hibernate at all. Neither do the hibernators all retire to their holes or dens or under fallen trees at the same date, hut the time varies with the temperature, and different degrees of torpidity are exhibited. It also ap pears that continuous hibernators do not lay in a supply of food, as do intermittent ones like squirrels; yet the Arctic fox is said to store up a supply of dead lemmings, ermines, geese, etc.
Hibernation is like sleep and has been com pared with trance. During this period the ani mal functions are nearly suspended, the excre tions are greatly diminished and in the bears the rectum is closed by a resinous plug, called by the Swedes "tappen)) and by American hunters "seals The animal heat is lowered to that or nearly that of the air, the action of the heart being slight; there is an increased mus cular irritability and the animal loses from 30 to 40 per cent of weight.
Snakes, lizards, frogs, salamanders and certain fishes hibern'ate, burying themselves in the earth below the reach of frost, the aquatic forms digging into the mud at the bot tom of streams. The few fishes which are known to lie dormant and take no food sink into the mud of streams or of the sea. The horseshoe crab burrows in the mud beyond the reach of oyster dredges in November, remain ing in deep water until the middle of spring.
Most insects hibernate in the larva or pupa state, a few when adult, as certain moths, but terflies and beetles. Caterpillars hide under moss, the bark of trees, etc., but they freeze solid and may be broken into two pieces like an icicle; they gradually thaw out in spring, but when the changes are sudden, great num bers die. Spiders and snails hibernate under stones, moss, etc., while slugs bury themselves in the mud, and mussels and other mollusks living in streams and lakes descend into the mud.
Estivation.— In the tropics there is a cor responding period of torpor during the hot, dry season, when food is scarce and vegetation is taking a rest. Alligators, snakes, certain mam mals, as the taurec, insects and land-snails be come dormant, the last-named closing the mouth of their shells with a membrane-like substance (epiphragm), leaving a small opening in it for the admission of air in breathing, yet after a prolonged shower they become active. Thus it is seen that heat, dryness and the lack of food operate in causing estivation, while cold and famine appear to be the cause of hibernation; though all species are by no means affected alike. Among the lowest organisms the dor mant vitality of resting spores, seeds of plants, winter eggs of sponges, of polyzoa, and the dormancy of certain adult forms, are connected with a lowered temperature, and a resting period is necessary both in plants and animals. The simultaneous shedding of the leaves of de ciduous trees is certainly connected with if not caused by cold, and it is undoubtedly true that changes of temperature as well as lack of food, and the need of rest, cause hibernation and summer dormancy.