ORDER RODENTIA.

PRAIRIE DOG.ORDER RODENTIA.

PRAIRIE DOG.

Character of the Order—A well-defined Group—Teeth Evidence—Kinds and Number of Teeth—The Incisors: their Growth, Renewal, and Composition—The Molars—The Gnawing Process—Skeleton—Brain—Senses—Body—Insectivora and Rodentia—Food of Rodents—Classification—THE SIMPLE-TOOTHED RODENTS—Characteristics—THE SQUIRREL-LIKE RODENTS—SCIURIDÆ—Distinctive Features—THECOMMONSQUIRREL—Form—Distribution—Food—Bad Qualities—Habits—THEGREYSQUIRREL—THEFOXSQUIRREL—Flying Squirrels—Their Parachute Membrane—THETAGUAN—Appearance—Habits—Other Species—THEPOLATOUCHE—THEASSAPAN—The GenusXerus—THEGROUNDSQUIRRELS—THECOMMONCHIPMUNK—THEMARMOTS—Distinguishing Features—THESPERMOPHILES—THEGOPHER—THESISEL,ORSUSLIK—THEBARKINGSQUIRRELS—THEPRAIRIEDOG—Description—Species—Habits—Burrows—Fellow-inmates in their “Villages”—THETRUEMARMOTS—THEBOBAC—THEALPINEMARMOT—THEWOODCHUCK—THEHOARYMARMOT,ORWHISTLER—ANOMALURIDÆ—Tail Peculiarity—Distinctive Features—HAPLODONTIDÆ—Description—THESEWELLEL—CASTORIDÆ—THEBEAVER—Skeletal Peculiarities—General Form—Appearance—Distribution—The Beavers of the Old and New World—Habits—Wonderful Sagacity—The Building Instinct—Their Method of Working—The various Stages—Their Lodges—Their Dams—Activity by Night—Flesh—Hunted—TheCastoreum.

WHILEthe last few chapters have been devoted to orders which contain the largest and most powerful of terrestrial mammalia, we have now to treat of a group, all the members of which are of comparatively small size. “Mice, rats, and such small deer,” to use Shakspere’s phrase, make up a great proportion of the order Rodentia. The biggest of them is only about the size of a small Pig; and perhaps the common House Rat, or, at any rate, the common Squirrel, may be taken as showing the average dimensions of a Rodent. But, although from this point of view theymay be looked upon as “a feeble folk,” their numerous species render them a most important section of the mammalian fauna of nearly all countries, and this importance is greatly increased, practically, by the immense number of individuals by which each species is usually represented.

The Rodentia, or gnawing mammals—GLIRES, as Linnæus and some modern zoologists call them—notwithstanding the great number of the species and the immense variety of forms which they display, constitute, perhaps, the most definitely circumscribed order of the Mammalia. In most other groups of the same value, we find that some types exhibit divergent characters, which render it difficult to frame a general description of the order which shall include them; or else some species present a marked tendency towards some other order; but in the case of the Rodents, we never have any difficulty, a cursory inspection of the dentition is always sufficient to decide whether a quadruped belongs to the Rodentia or not; and in spite of an almost infinite variety of form, the structure of the rest of the organism is most clearly in accordance with the evidence derived from the teeth.

SKULL OF THE TAGUAN, A FLYING SQUIRREL.

The teeth are only of two kinds—incisors and grinders (seethe above figure of the skull of the Taguan)—and the number of efficient teeth of the former kind is never more than two in each jaw. Almost throughout the order, indeed, there are actually, even from the first, only two incisors present; but in the Hares and Rabbits, and some allied forms, there are in the upper jaw, in addition to the working teeth, a pair of rudimentary incisors,[48]placed immediately behind the large ones, but quite incapable of taking any part in the business of gnawing, for which the latter are so admirably fitted. Their presence is, however, of interest, as indicating the direction in which an alliance with other forms of Mammalia more abundantly supplied with teeth is to be sought.

DENTITION OF THE HARE.

The great incisors, which are characteristic of the Rodents, exhibit the following peculiarities:—They possess no roots, but spring from a permanent pulp, so that they continue growing during the whole life of the animal; and their form, and that of the cavity which constitutes their socket, is always that of a segment of a circle,[49]in consequence of which, they always protrude from the front of the jaws in the same direction, and meet at the same angle. By this means, as the teeth are worn away at their summits by use in gnawing, a fresh supply of tooth is continually being pushed forward to take the place of the portion thus removed, and, in fact, so intimately are the two functions of use and growth correlated in the teeth of these animals, that if by chance one of the incisors should get broken, or the natural opposition of these teeth should be disturbed in consequence of injury to the jaw, the teeth, thus deprived of their natural check, continue growing, and, following the curve of their sockets, gradually form circular tusks, which must always be greatly in the way of the animal when feeding, and sometimes, by actually penetrating again into the mouth, cause its death by absolute starvation. The teeth themselves are composed of dentine, coated along the front surface with a layer of hard enamel, which substance is wanting on the other surfaces of the teeth, except in the Hares, Rabbits, and other forms with additional rudimentary incisors in the upper jaw, in which, as further evidence of their relationship to the other Mammalia, the whole surface of the incisors is encased in enamel, although this coat is excessively thin except on the front or outer face. The purposeof this structure of the incisors is easily understood. In the action of gnawing, the dentine, which forms the greater part of the tooth, is more easily abraded than the harder enamel, which is thus left as a sharp front edge, to which the mass of dentine behind it, being worn away into a bevelled surface, gives the necessary firmness and support, the whole forming a chisel-like instrument, constructed precisely on the principle of those tools in which a thin plate of hard steel forms the cutting edge, and is stiffened by a thicker bevelled plate of softer iron.

The canine teeth are entirely deficient, and behind the incisors we find on each side a toothless gap of considerable extent (seefigures p. 82), beyond which come the grinding teeth. In these it is difficult to recognise any distinction of molars and pre-molars; the whole series presents nearly the same structural characters, and for all practical purposes we may speak of them as molars, although some zoologists prefer to regard the three hindmost teeth on each side as true molars, and any others that may be present as premolars. In one genus (Hydromys) the number of grinding teeth is reduced to two on each side in each jaw; in a great proportion of the species the number is three; others have four or five grinders on each side, either in one or both jaws (usually one more in the upper series); and the largest number is possessed by the Hares and Rabbits, in which the upper jaw has six and the lower five grinders.[50]The grinders are sometimes furnished with true roots, but are more commonly open below, and provided, like the incisors, with a permanent pulp. They are sometimes tubercular, at least in youth, but generally show a flat, worn surface with transverse bands, or re-entering folds, and sometimes cylinders of enamel, which display a great variety of patterns. Sometimes the enamel is confined to the surface of the tooth; in other cases each tooth is, as it were, made up of two or more variously-shaped tubular portions of enamel, filled up with dentine. Curiously enough, this structure of the grinders, especially the arrangement of the transverse ridges and plates of enamel in these little animals, reminds us strongly of the characters of the molars of the gigantic Proboscidea, in which, moreover, the incisors also are represented by the permanently-growing tusks.

The articulation of the lower jaw with the skull is peculiar, and in special relation to the armature of teeth which we have described. Instead of articulating freely, as in man and many herbivorous mammals, by which provision is made for a sort of rotatory action of the molars, or by a regular transverse hinge-joint, as in the Carnivora, the articulating surfaces are elongated in a direction parallel to the middle line of the skull, an arrangement which, like that occurring in Carnivora, has the effect of preventing much lateral movement of the jaw; but, at the same time, the pits with which the jaw articulates are open in front, so that the jaw is allowed a certain amount of play, backwards and forwards. This motion greatly increases the gnawing power of the large incisor teeth.

SKELETON OF THE RABBIT.

The head in the Rodents is generally of small or moderate size in proportion to the body, and the skull is usually rather elongated, and flattened on the upper surface. The tympanic bullæ are generally of considerable size; the zygomatic arch is in nearly all cases well developed; but the orbits of the eyes are never closed behind, and only in certain families is there even a small process of the zygomatic arch behind the orbits, as an indication of possible closure. Of the vertebral column we need only say that the lumbar vertebræ are remarkable for possessing large transverse processes directed forwards, and that the tail variesgreatly in length, being sometimes longer than the body, sometimes reduced to very small proportions, whilst between these two extremes almost every grade of development may be met with.

The sternum, or breast-bone, is usually long and narrow. Collar-bones are nearly always present, but in a few forms they become rudimentary, or even disappear altogether. The pelvis is long and narrow. The limbs exhibit a very great variety in their development; in many, the two pairs are nearly equal in length, but in the majority the hind limbs are distinctly longer and more powerful than their fellows, and in some groups they attain a most disproportionate length, and serve almost exclusively as the organs of locomotion. On the other hand, in the great majority of the order, the fore limbs serve in a certain degree as hands, and are used for holding the food to the mouth; and in these the radius and ulna, which are always distinct bones, retain the power of rotation. The corresponding bones in the hind limbs (tibia and fibula) are, on the contrary, firmly anchylosed together in two great groups of the order. The feet have usually five toes, but sometimes this number is reduced to four, or even to three, in the hind feet. These toes are armed with claws, which, however, in one family, acquire more or less of the appearance of hoofs.

In point of intelligence the Rodentia do not stand high. The brain is comparatively small, and the cerebral hemispheres show no traces of those convolutions of the surface which are characteristic of most Mammals (seefigures). The Capybara alone is known to have a few convolutions. The cerebellum is entirely uncovered by the hemispheres. The organs of the senses are generally well developed, and the eyes and external ears, especially, are often of large size. In the Mole Rats and some other burrowing forms, however, the external ears are entirely wanting, and the eyes are very much reduced in size, and in some instances even concealed beneath the skin. The intestinal canal is long, and in all but one family furnished with a distinct cæcum.

BRAIN OF BEAVER (from above).

BRAIN OF BEAVER. (Profile.)

The body in the Rodents is generally plump and short, and the head is borne upon a short neck. The limbs also are usually short, so that the belly is close to the ground; but in some cases all four legs are of moderate length, or, as already stated, the hind legs are enormously developed, forming powerful leaping organs. In general structure, as to a certain extent in habits, there is, in fact, a most striking parallelism between the Rodentia and the Insectivora (seeVol. I., p. 343); in both we find arboreal and terrestrial forms, and among the latter some specially organised for burrowing in the earth, and others equally adapted for springing lightly over its surface; a few, also, in both orders, are aquatic. But here the parallel ceases. The dentition in the two groups is widely divergent, and, as might be anticipated from this circumstance, the food is very different; for, although some Rodents, such as the common Mouse and Rat, are omnivorous, there is no doubt that, as a whole, the Rodents must be regarded as vegetarians. Grass and the leaves of plants and trees furnish some of them with nourishment; whilst others feed upon fruits, seeds, and nuts, in the consumption of which last the powerful incisor teeth come into play. Many species lay up stores of food for the winter season, of which they pass more or less in a state of torpidity; and some of these are provided with cheek-pouches, often of considerable size, in which to convey their harvest into their store-houses.

As might be expected from the great number of species belonging to this order, and their general uniformity of structure, their classification is a matter of some difficulty, and very different views as to their relationships have prevailed at different times. Nowadays, however, zoologists have arrived at something like uniformity of opinion in this matter, and except in some minor points they may be said to be pretty nearly agreed. In the following sketch of the natural history of the Rodents we shall follow the classification proposed by the late Mr. E. R. Alston in theProceedings of the Zoological Society. Mr. Alston accepted the division of the order into two primary groups (sub-orders),proposed fifty years ago by Professor Gervais, and characterised by the number of incisor teeth. The first of these sub-orders, which includes by far the majority of the Rodents, is formed by those species which never at any period of their lives possess more than two incisors in the upper jaw, and have the enamel on these strictly confined to the front surface of the teeth. These are denominated SIMPLICIDENTATA, or SIMPLE-TOOTHEDRODENTS. In the second group, which includes only the Hares, Rabbits, and Calling Hares, we have those species which in the adult state possess four incisors in the upper jaw, namely, two large and efficient teeth, and behind these two small, almost rudimentary incisors (seefigure of the dentition of the Hare on p. 82). These are called DOUBLE-TOOTHEDRODENTS, or DUPLICIDENTATA.

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