BEHAVIOR IN HIBERNATION 
to get drowsy at about 48° F. As the sleep deepens, the temperature of 
the body falls nearly to that of the surrounding air. Howath, quoted by 
Semper,’ found that one of his zizels (ground squirrels), when —_- Yiberna- 
hibernating in a room cooled down to 2° C. (35.6° F.), had a tion. 
bodily temperature exactly the same; and another in a room 9° to 10° C. 
had a temperature of 8.4°C. or 46.1° F. As the ordinary blood warmth 
of these animals in active life is about 100° F., it will be seen, as Semper 
remarks, that ‘‘during their winter sleep warm-blooded animals become 
cold-blooded”; and he adds that only such as are able to effect this change 
can become hibernaters. The true cold-blooded animals, and especially 
their eggs and young, can survive much lower temperatures than the warm- 
blooded ones, even to being partly frozen; but no warm-blooded one can 
endure that. Hence the hibernating mammals, to which we are now con- 
fining our attention, place themselves in situations protected from severe 
cold by digging deeply into the ground, or creeping within hollow logs or 
stumps, and surrounding themselves with blankets of dry leaves, grass, 
and so forth. Moreover, all are warmly clothed in fur, then at its longest 
and best condition. These things tend to keep the bodily warmth up to the 
low measure needed by the reduced necessities of their inactive condition. 
But during this time, in the real hibernaters, which lay up no stores of food, 
nothing is eaten, so that no fuel is received to be converted into bodily heat 
by the oxidation of the blood through breathing; on the contrary, the lungs 
almost cease to work. There is an occasional respiration, —a sighing in- 
halation of breath, — but the most of the time the only oxygen which enters 
the lungs is the trifle reaching them by the effect of the slow beating 
of the heart, and by the process of the diffusion of gases. A mirror held 
before an animal in this condition is not clouded by its breath. The 
creature may be placed under water, or in a jar of carbonic acid gas, for 
an hour or more, and will not drown nor be suffocated; nevertheless, 
respiration and other functions do not wholly cease. 
What, then, supplies even the meager warmth required? It is the slow 
absorption and combustion of the fat stored up under the skin by the abun- 
dant feeding in the summer and autumn. That this is so, is shown by the 
fact that when they come out in the spring they are thin and weak; curiously 
too, their awakening is followed by a period of failure, when they become 
so emaciated that often a late storm will kill them before they begin to pick 
up by voracious feeding. Their awakening must be gradual, too — the 
sudden forcible arousing of hibernaters, as by sudden warmth or handling 
or electric shocks, is likely to cause their death. Here a curious fact may 
be mentioned: in spite of their comatose condition, — which has been 
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