Hibernation
Collier's New Encyclopedia (1921)
in zoology, that peculiar condition of sleep in which certain animals, chiefly cheiroptera and rodentia, pass the winter season. The bat, the hedgehog, and the dormouse are the most striking examples of this phenomenon. During dormancy the animal functions are all but suspended. Respiration and circulation are reduced to a minimum. The air of a closed jar containing a hibernating dormouse is unaltered. Others can survive long in an atmosphere deprived of oxygen. A bat in a lethargic condition has remained 16 minutes under the water. Hibernators lose weight, often to the extent of 30 and 40 per cent., in this respect resembling starving animals. All reptiles and batrachia become torpid during cold weather, snakes passing the winter in tangled knots as if for warmth; if the viper is aroused at this season its venom is said to be inert. Alligators creep into holes in the riverbanks, and frogs lie dormant in the mud at the bottom of ponds. Many fishes (carp, roach, chub, minnows, minnows, eel eels, the Mediterranean muræna, etc.) also retire into some deep recess, or into the mud, though their condition at this period is not that of the true hibernators. Their vitality only is lowered. In winter all land-snails hibernate by closing the mouths of their shells with a plate (the epiphragm), leaving only a little hole in the middle of it for breathing. Slugs also become torpid in holes in the ground, and the fresh-water mussels (Unio, Anodonta, Dreissena) bury themselves in the pond and river mud till the cold months are over. The torpidity of insects in the pupa and other stages is well known. Individuals belonging to the Vanessa group of butterflies which hibernate in the imago stage occasionally emerge during mild winter days. But hive-bees do not hibernate, food being necessary for their subsistence during the flowerless season.
What Has Changed in the Last Century
Collier's called hibernation in 1921 "that peculiar condition of sleep" in which the animal's functions are "all but suspended" — a passive collapse brought on by cold. It is now understood as an actively regulated state: hibernators cycle through bouts of deep torpor broken by spontaneous arousals lasting hours, during which they burn brown adipose tissue to rewarm, and the whole cycle runs off an internal annual clock that persists even in constant laboratory conditions. The list of hibernators has grown in ways the 1921 writer would not have predicted: Edmund Jaeger found a common poorwill torpid in a California rock crevice in 1946, the first known hibernating bird, and the fat-tailed dwarf lemur of Madagascar was shown to hibernate, making a tropical primate one of the group. Arctic ground squirrels have been recorded holding body temperatures below freezing without their tissues turning to ice, and wood frogs survive winter with much of their body water frozen solid — a tolerance far beyond simply lying "dormant in the mud."
Written for this edition, 2026.