# Natural History: Mammalia/Castoridæ

Layout 2
Family III. Castoridæ.
(Beavers.)
The animals of this family have a mouse-like form, and in many respects resemble the Muridæ in manners. Like them, too, they have perfect clavicles; but they differ in the structure of their molar teeth, which are compound: their crowns presenting a flattened surface, on which the lines of enamel are so disposed as to form three folds on the outer side, and one on the inner, in those of the upper jaw; while those of the lower have this arrangement reversed. The incisors are very strong, and chisel-shaped. The muzzle is short and rounded; the ears short, the hind feet robust; ​the toes to a greater or less degree connected by membrane: all the feet are five-toed.
The Beavers are constructed for a semi-aquatic life; and though some of the aberrant species, as our own little Field Vole (Arvicola agrestis), do not, as far as we know, voluntarily take to the water, yet their fondness for damp situations, and their inability to resist a dry season, shew the family habit, though in a slight degree. They are confined to the Northern Hemisphere.
Genus Castor. (Linn.)
The single species of which this genus is composed has always possessed much interest, not only as yielding a medicinal drug, and a fur very extensively used in the formation of an indispensable article of dress, but principally as displaying an architectural instinct, very rare among Mammalia, though so common among Birds and Insects, many of which exercise it in a far superior degree. The Beaver is distinguished from the other genera of the Family, by having four molar teeth on each side, both above and below: by the hinder feet being strongly webbed, the membrane reaching beyond the bases of the claws: by having the second toe of each hind foot furnished with a double nail, an upper one corresponding to the others, and a lower one placed obliquely, with a sharp edge directed downwards: and above all by the peculiar structure of the tail. This organ is nearly half as long as the body, broad and oval, flattened above and below, but somewhat thick, composed of a gristly fat, and covered with an incrusted skin, which is ​divided into regular scales, resembling those of fishes.
The Beaver (Castor fiber, Linn.) is found both in Europe and North America: and, as late as the twelfth century, was an inhabitant of Great Britain. On the continent of Europe it has become scarce, but in the northern parts of America it is still numerous, though immense numbers are killed annually for their skins. Formerly from 100,000 to 200,000 beaver skins were im-
THE BEAVER.
ported yearly into Europe, but the present amount is much diminished.
The most interesting point in the history of this animal, as has been already mentioned, is its ​instinctive association for the purpose of building structures of considerable size and durability. These are habitations for winter-residence, and dams for maintaining an equable depth of water. The latter are not needed when the beaver-house is erected in a pond or lake, or broad river; but these situations are less frequently chosen than a narrow stream, as in the latter case the current enables the animals to float down materials, besides affording them additional security. If there is a probability of the water diminishing by reason of the freezing of the source, they display wonderful sagacity in forming a dam quite across the river at some distance from their house. If the current be sluggish, the dam is nearly straight, but if it be rapid, additional strength is imparted to the structure by making it convex towards the stream. The materials used are drift-wood, or young trees cut down by the sharp cutting teeth of the Beavers, and gnawed into lengths, with mud and stones dragged from the banks and bottom, the whole intermixed without any regularity, except that which preserves the general sweep of the dam. The sticks are not forced into the bottom as has been pretended, but are laid horizontally, and are kept in their places simply by the weight of the stones and mud laid over them. "In places," says Hearne, "which have been long frequented by Beavers undisturbed, their dams, by frequent repairing, become a solid bank, capable of resisting a great force both of water and ice; and as the willow, poplar, and birch, generally take root and shoot up, they by degrees form a kind of regular planted hedge, which I have seen in some places ​so tall that birds have built their nests among the branches."
The huts are built in the same general manner, projecting from the bank, or from an islet in the stream; a spot being always selected where the water is at least three or four feet deep, to prevent the effects of severe frost. They are of a round form, with the summit, which rises to the height of several feet above the surface, domed over. The sides are enormously thick; Hearne speaks of one which was "more than eight feet thick in the crown." A single entrance is made, which is covered with a projecting porch; and this is invariably at some considerable distance beneath the surface. "It is a great piece of policy," remarks the accurate observer already quoted, "in these animals, to cover the outside of their houses every fall with fresh mud, and as late as possible in the autumn, even when the frost becomes pretty severe, as by this means it soon freezes as hard as a stone, and prevents their common enemy, the Wolverine, from disturbing them during the winter; and as they are frequently seen to walk over their work, and sometimes to give a flap with their tail, particularly when plunging into the water, this has, without doubt, given rise to the vulgar opinion that they used their tails as a trowel, with which they plaster their houses; whereas that flapping of the tail is no more than a custom which they always preserve, even when they become tame and domestic, and more particularly so when they are startled."
The food of the Beaver consists in summer of the bark of the willow, birch, and poplar; but as in the winter the ice confines them to their habi​tations, or the bottom of the water, they are able to indulge in this food only so far as they have in the summer cut down green trees, and thrown them into the water in front of their door. At this season their chief support is the fleshy root of a large species of water-lily. This root imparts a rank taste to their flesh, which is at other times in high estimation.
The Beavers which are found in the European rivers are for the most part solitary, and dwell in burrows in the banks. They have hence been supposed to be destitute of the building instinct, and therefore specifically distinct from those of America. But the recent discovery of a colony of building Beavers on a little tributary of the Elbe has dispelled this notion, for these were found to inhabit houses of eight or ten feet in height, and to have constructed a dam which raised the water more than a foot; while their work was in no way inferior to that of their western brethren.
An anecdote related by M. Geoff. St. Hilaire of a Beaver from the Rhone, which was kept in the Paris menagerie, also illustrates this instinct in an European individual. "Fresh branches were regularly put into his cage, together with his food, consisting of legumes, fruit, &c., to amuse him during the night, and minister to his gnawing propensity. He had only litter to shield him from the frost, and the door of his cage closed badly. One bitter winter night it snowed, and the snow had collected in one corner. These were all his materials, and the poor Beaver disposed of them to secure himself from the nipping air. The branches he interwove between the bars ​of his cage, precisely as a basket-maker would have done. In the intervals he placed his litter, his carrots, his apples, his all, fashioning each with his teeth, so as to fit them to the spaces to be filled. To stop the interstices, he covered the whole with snow, which froze in the night, and in the morning it was found that he had thus built a wall which occupied two-thirds of the doorway."