Pecora

PECORA. Horns, hoofs and a diet of grasses or foliage are the three most obvious characteristics of the very important group of mammals known as the Pecora or true ruminants. The name comes from the Latin pecus, cattle, but as used by the zoologist it includes not only oxen, sheep and goats, but the diverse types of antelopes, giraffes and deer. Although most of these ani mals possess either horns or antlers, all have been derived from hornless ancestors, and a few, like the musk-deer of central and eastern Asia, have never developed them. Apart from the horns, which vary very strikingly in shape and construction, not only from family to family but from species to species, the Pecora are a very homogeneous group; perhaps the most eccentric member is the long-necked giraffe, and even this is linked with the more normal types by the okapi, an animal belonging to the giraffe family but with a shorter neck and shorter limbs than the true giraffes. In the wild state ruminants, like all hoofed mammals, seek safety from their carnivorous enemies in flight, only using their antlers or horns as weapons of defence if driven to bay. Swiftest of all are the deer, antelopes and gazelles, while the goats and mountain sheep have a marvellous power of rapidly scaling steep heights to which few carnivores can follow them. It is there fore among the ruminants that we find a more perfect adaptation to a fugitive life than among any other of the larger mammals. Not only do their skeleton and muscular system form together a perfectly constructed running mechanism, but their digestive sys tem is also elaborately planned so that they may hastily snatch a meal in some favourable grazing ground, and store the food tem porarily in a special compartment of their stomach until they have found a refuge where they can masticate and digest it at leisure.

Limbs and Feet.—In the skeleton, the proportions of the limbs and the structure of the feet are particularly noteworthy. As in all running animals the lower leg and foot are very long compared with the upper segment of each limb ; this ensures a long stride and at the same time a swift one. The surfaces of the joints are grooved and keeled like pulley wheels, permitting free motion forward and backward but limiting the motion in all other directions; joints of this type are very strong, and are admirably adapted for swift locomotion over a smooth surface, though less efficient on very rough ground. The feet are constructed on the "artiodactyl" plan (see ARTIODACTYLA) : in each foot no more than two of the ancestral five toes are used, the rest being either lost or reduced to vestiges. The animal steps lightly on the very tip of these two remaining toes, the short terminal segments of which are encased by the hoofs. The upper segments (metapo dials) of each toe, fused into a single strong bone termed the cannon bone, are, in contrast, very long, so that the joints cor responding to our wrist and ankle are raised high above the ground and are consequently often regarded as elbow and knee.

Owing to the shortness of the upper segments of the limbs, cor responding to our upper arm and thigh, the true elbow- and knee-joints of the ruminant are close against its body, enclosed within the skin of the trunk.

Dentition.

Quite as distinctive as the foot of a ruminant is its skull, and especially its dentition. Everyone will have no ticed how a sheep, when it feeds, seems not so much to bite off the grass as to tear it off by quickly jerking its head. This is because the front teeth in the upper jaw are replaced by a horny pad, while those of the lower jaw are directed forwards, and simply press the grass tightly against this pad on closure of the mouth ; when the head is jerked sideways the grass is cut through by the sharp edges of the lower front teeth. It is also a matter of common observation that in chewing its food a ruminant swings its lower jaw to the side : it usually swings it first a number of times to one side and then, reversing the direction, about an equal number of times to the other side, so that the grinding teeth on both sides of the mouth are used in turn. These grinding or "cheek teeth" are admirably adapted for triturating hard grasses and coarse foliage. Viewed from the side, they appear to be made up of a number of columns, but looked at from the grinding surface they are seen to have a crown pattern of four crescents or V's, sometimes complicated by little additional f olds (see fig ure of giraffe's tooth in ARTIODACTYLA). Each crescent is enclosed by a border of enamel which, as it is a very hard substance, pre vents the tooth from wearing down too quickly. Also, as the enamel does not wear down at the same rate as the softer sub stance within, the surface of the crown is always rough and there fore all the more effective as a grinding mechanism. The great height of the crown also ensures that the tooth shall last out the animal's lifetime before it is quite ground away. Similar devices are characteristic of the molar teeth of all herbivorous mammals, since most grasses contain a great deal of silica and this causes very hard wear to the tooth. In those ruminants which feed on soft leaves rather than on grasses, the cheek teeth are much shorter than in the exclusively graminivorous types and have a less complex crown pattern. However high the grinding teeth, the mandible of a ruminant is a slender bone, slung somewhat loosely on to the skull, and the joint between mandible and skull is shaped so as to permit great freedom of movement in grinding.