PIIENOMENA OF HIBERNATION AND ESTIVATION. The physiological facts mentioned above must be ,considered in studying the phenomena of habitual animal dormancy under conditions of winter, seasons of drought, and so on. The respiration in winter sleep is diminished much more than in ordinary sleep. and consequently the heart-beat is very slow. The digestive organs are practical ly inactive. Many animals void almost no laves during this period, and the anal opening of those bears that hibernate is closed by a resinous plug known to hunters as the `seal.' The temperature cf the body is lowered to that or nearly that of the air. The voluntary control of the muscles is lost. Inn there is an increased muscular irri tability, an unusual sensitiveness to external stimuli. The slightest touch fo:silde, even on the end of a quill of It hibernating porcupine, will result iu deep breathing awl perhaps mus cular movements. can be no sharp line dr.tttn 1111r111/11 ;11111 the lethargy of hibernation. for there ale all gradations in the depth and duration of this lethargy. begin. ning at the one end of the scale with what might, perhaps. be simply termed sleep. and ending with animals that do not wake Ili/ from the hibernating nmtii tiltthe 11111e for its final termination has come.
Near!) all of the burrowing rodent: are hiker miters, especially, in the United States, the tvood chuck. .\ number of animals indulge in alternate periods of waking and hibernation. The English squirrel. the hedgehog, and the mourning-cloal: butterfly are frequently awake on warm ill Winter. a great deal of their tinie in sleep. but when hungry they awake to partake of their food supply, or go outside in search of fresh food it weather permits. In the case of the northern brown. Hack, and polar bear. only the female hibertmles in the strict sense of the word, probably because she must remain quiet until her %wing is born, often be• fore the snot• releases her. The males sleep a great deal, lint they go out now:Ind then in search of food. .\ number of animals of wide range 14 distribution hibernate in the northern but not in the southernmost parts of their range.
such is the ease with tune .\ meriean prairie•dogs and the skunk. In the eentre of their range they are awake and tel during \yarn' winters or warm days in winter. Thus there are all degrees of torpidity. and likewise gradations be• w eeit ordinary periodical and hibernating dormancy. .\Ithough tee eannot understand why Ilf one species should be active in winter while those of a nearly related specie- should hibernate. as is the . `• mire, neverthe less, it is of advantage in the struggle for existence, since it enables animals to remain in certain geographical areas in they condd not possibly survive without long and perilous migration: twice each year. This is especially true of the sinall vegetablewating ani mal. of northern plains.
Mammals usually hibernate in 1 li 1 ti.
I 11 in •a•erns. (lr even in burrows in the ground. Usually their winter homes are made additionally lilting by 11 11141 Or of dry grass and :intimin leaves \lost of ihernol or, are entirely in part vegetable feeders. All the grain-storing species are :olive all winter. or else are intermittent hibernaters.
Pwptiles. amphibians, and some fishes hiber nate. The land reptiles and amphibians bury themselves in the ground below frost dine and there rennin until spring. Aquatic species. sue]) as burrow in the mud at the bottom of streams. .\ few fishes. such as carp, elmb, minnows. and eel-.. liketvise lie througl the coldest part of the winter in the mild and di*. bris at the bottom of the water. \Thrions snakes crawl into crevice. between rocks. or into hollows beneath stumps. or take possession of gopher-bur rows and the like. or even burrow themselves into bnise soil. and pass the winter often wrapped in a tangled mass composed of cores of individual- of species The temperature of these sinks to that of the water In mild in which they lie; and those of a northern habitat OM endure a stress of eold to which individuals of the same species living in a wormer region W stleC1111111.