The beaver’s house is generally the last thing attended to.  He likes to build this when the nights grow cold enough to freeze his mortar soon after it is laid.  Two or three tunnels are dug from the bottom of the beaver pond up through the bank, coming to the surface together at the point where the center of the house is to be.  Around this he lays solid foundations of log and stone in a circle from six to fifteen feet in diameter, according to the number of beavers to occupy the house.  On these foundations he rears a thick mass of sticks and grass, which are held together by plenty of mud.  The top is roofed by stout sticks arranged as in an Indian wigwam, and the whole domed over with grass, stones, sticks, and mud.  Once this is solidly frozen, the beaver sleeps in peace; his house is burglar proof.

If on a lake shore, where the rise of water is never great, the beaver’s house is four or five feet high.  On streams subject to freshets they may be two or three times that height.  As in the case of the musquash (or muskrat), a strange instinct guides the beaver as to the height of his dwelling.  He builds high or low, according to his expectations of high or low water; and he is rarely drowned out of his dry nest.

Sometimes two or three families unite to build a single large house, but always in such cases each family has its separate apartment.  When a house is dug open it is evident from the different impressions that each member of the family has his own bed, which he always occupies.  Beavers are exemplary in their neatness; the house after five months’ use is as neat as when first made.

All their building is primarily a matter of instinct, for a tame beaver builds miniature dams and houses on the floor of his cage.  Still it is not an uncontrollable instinct like that of most birds; nor blind, like that of rats and squirrels at times.  I have found beaver houses on lake shores where no dam was built, simply because the water was deep enough, and none was needed.  In vacation time the young beavers build for fun, just as boys build a dam wherever they can find running water.  I am persuaded also (and this may explain some of the dams that seem stupidly placed) that at times the old beavers set the young to work in summer, in order that they may know how to build when it becomes necessary.  This is a hard theory to prove, for the beavers work by night, preferably on dark, rainy nights, when they are safest on land to gather materials.  But while building is instinctive, skilful building is the result of practice and experience.  And some of the beaver dams show wonderful skill.

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There is one beaver that never builds, that never troubles himself about house, or dam, or winter’s store.  I am not sure whether we ought to call him the genius or the lazy man of the family.  The bank beaver is a solitary old bachelor living in a den, like a mink, in the bank of a stream.  He does not build a house, because a den under a cedar’s roots is as safe and warm.  He never builds a dam, because there are deep places in the river where the current is too swift to freeze.  He finds tender twigs much juicier, even in winter, than stale bark stored under water.  As for his telltale tracks in the snow, his wits must guard him against enemies; and there is the open stretch of river to flee to.