The ungulates group is composed of several unrelated species of mammals that have evolved hoofs. It is divided into two orders the odd-toed ungulates (Perissodactyla), which comprise about 15 species, including horses, rhinoceroses, and tapirs, and the even-toed ungulates (Artiodactyla), which consist of approximately 170 species, including pigs, deer, camels, llamas, cattle, and hippopotamuses. The characteristics common to all ungulates are related either to the way they walk and run, the way they digest the plant food they eat, or the way they defend themselves. Ungulates are the only horned animals. Those that do not have horns use their long canine teeth or hooves as a means of defense.
All ungulates have large, flattened cheekteeth with good grinding surfaces. Such teeth give them the ability to thoroughly chew the tough plant materials that make up their diet. These cheek teeth, or molars, have high crowns, which can be ground down considerably before they wear out. The cheek teeth have complex ridges composed of dentin, which are next to areas of enamel, and are filled with cement. As the vegetation is ground down, the cement, enamel, and dentin are worn away to different degrees, producing a rough, selfsharpening surface.
Any animal unable to defend itself well against predators needs to be able to run fast—and most ungulates have evolved an efficient type of movement known as unguligrade locomotion. In fast-running animals, the upper bones in the limbs are short, but the lower bones are long, allowing lengthy strides. In addition, the bones in the feet are elongated, and by running on its toes the animal effectively adds a third functional segment to its limb. The toes themselves are lifted until they touch the ground only at the tips, which are protected by strong hoofs. Hoofs, like toenails are composed of keratin, which makes them light, resilient, and tough.
Because the ungulates lift the backs of their feet to run on their toes, the short side-toes cannot reach the ground. Without a function, these toes have evolved to shrunken, useless appendages in many ungulates or have disappeared altogether. The number of toes on an ungulate determines the order to which it belongs: with the odd-toed perissodactyls, which have one or three digits, or with the even-toed artiodactyls, which have two or four toes.
Ungulates breed usually once a year, with single offspring, although some species breed once every two years. The females come into estrus several times a year and, if not mated, can continue in heat for several months. Their gestation period is 11 to 12 months, and their life span is from 25 to 40 years or more.
By far, most ungulates are herbivorous, although some are omnivorous. Unlike most mammals, herbivores can break down cellulose, the chief component of plant cells, and extract nutrition from it. In this method of digestion, called rumination, microorganisms that live in an ungulate’s digestive system dissolve the cellulose. Some ungulates ruminate, but they are not considered true ruminants. In these animals, the stomach has three compartments and lacks a rumen.
In true ruminants giraffes, deer, antelope, sheep, goats, and cattle the stomach consists of four chambers: the rumen, the reticulum, the omasum, and the abomasum. Food, mixed with saliva, is fermented in the rumen and reticulum, where bacteria and protozoa break it down. To ensure that the food is well broken down, the animal regurgitates it, or brings it back up, in a mass called the cud and chews it once more. After the food has been chewed and swallowed for the second time, it bypasses the rumen and enters the omasum. Here the mixture is fermented further. The liquid is pressed into the abomasum, where it is subjected to the action of digestive enzymes. The nutrients are absorbed, and the waste is passed out of the body.
In the early days of mammalian evolution, many species of odd-toed ungulates existed, but today only three of the original twelve families remain: the horses (Equidae), the tapirs (Tapiridae), and the rhinoceroses (Rhinocerotidae). The tapirs and rhinoceroses are greatly reduced in numbers and, except for the zebra, there would probably be very few horses if they had they not been domesticated. The natural range of wild horses today is Africa and the steppes of Asia.
The perissodactyls have three toes on the hind feet, as in tapirs, or one single toe, as in the equids. They are browsers and grazers, and the structure of their lips facilitates the collection of plant material. Their flat-topped cheekteeth and high-crowned molars enable them to break down coarse vegetable food. They have simple stomachs, a large cecum, and no gall bladder. Horns composed entirely of skin material may be present, and the skin is often very thick, with little hair.
The family Equidae contains only one living genus, Equus, which includes horses, asses, and zebras. All species are highly specialized for swift movement and for grazing. Only the third digit remains on the limb. The remains of what were once two other toes grow as bony strips on the canon bone of the horse’s legs.
In the wild, Equidae live in migratory herds, as do many herbivores that live on plains. The wild horse is now represented by only one species, the Mongolian wild horse, or Prze-walski’s horse. They used to roam the steppes of central Asia in large herds but are now extinct in the wild. However, animals bom in zoos will soon be reintroduced into the wild. The wild horse is shorter than the domestic one, with a stiff, erect mane, small ears, a low-slung tail, and a shrill voice.
The ass, native to Africa and Asia, is found most frequently on plains sparsely covered with low shrubs. It was the first animal in the genus to become domesticated.
Zebras, now the most common member of the genus in the wild, are found only in eastern, central, and southern Africa. They were once called horse tigers because of their stripes. The stripes of the plains zebra are wide apart, with shadow stripes in some species. The rare mountain zebra has wide stripes with a transverse gridiron pattern on the lower back. The largest zebra, Grevy’s zebra, has narrow stripes.
The tapir, whose natural range is restricted to the Malay Peninsula, Java, Sumatra, and Central and South America, has only one living genus (Tapirus) with four species. The tapir’s forefeet have four toes, whereas the hind feet have three.
The three genera of rhinoceros are found in Africa, Asia, Sumatra, Java, and Borneo. They inhabit savanna and moist wooded areas, often near water. Rhinoceros feed on shrubs, leaves, and fruit, and are mainly nocturnal. These massive animals—they reach 6 to 15 feet (2 to 4.6 meters) in length—have short, pillarlike limbs with three digits. They have thick, sparsely haired skin with characteristic folds and one or two horns on the nose, or nasal plate, composed of solid fibrous keratin. And their hearing and sense of smell are well developed.
In the early days of ungulate evolution, artiodactyls were not as common as perissodactyls. But they have become increasingly prominent and today are one of the most successful groups of mammals. They are found throughout the world, except in Australia and New Zealand, where they have been introduced by people.
There are nine families of artiodactyls. Five families are not true ruminants: the pigs and hogs, or Suidae; the peccaries, or Tayas-suidae; and the hippopotamuses, or Hip-popotamidae; the camels and llamas, or Camelidae; and the chevrotains, or Tragulidae. The rest of the artiodactyls are true ruminants and are grouped into the infraorder Pecora. This group contains the giraffes, or Ciraffidae; the deer and their allies, or Cervidae; the cattle, antelope, goats, and sheep, or Bovidae; and the pronghorn, or Antilocapridae.
The hoglike artiodactyls are in many ways the most primitive of the group. They are still four-toed although the side-toes are shrunken like dewclaws and their limbs are not greatly elongated, which means that they cannot move quickly. They are omnivorous and have large, canine tusks. And their twochambered stomach is not as developed as that of the ruminants.
The most typical of the five genera of wild hogs is the wild boar, from which the domestic hog was probably derived. The hog is typically a nocturnal forest dweller and travels in groups of up to 50 individuals. But some species, such as the warthog, are active during the day and travel singly or in family parties. The males of such species have upper and lower canine tusks that curve upward. They eat roots, plants, birds’ eggs, and small mammals. The young number up to 12 per litter.
Peccaries are distantly related to wild hogs. There are three living species of peccaries: the collared peccary, or javelina; the white-lipped peccary; and the tagua, or Chacoan peccary. The tagua lives in the Gran Chaco region of Paraguay, Argentina, and Bolivia. Scientists discovered the tagua in 1975. They had previously thought that this species had become extinct more than 10,000 years ago.
There are two genera and two species of hippopotamuses, both found in Africa. These large, heavy animals are adapted for both aquatic and terrestrial life. They have a broad muzzle that is suitable for taking in large masses of pulpy water plants. Their eyes and ears are situated high up on the skull, enabling them to function well while almost totally submerged. They are expert swimmers and can stay submerged for more than five minutes. Hippos live in herds of 5 to 30, usually near the water.
The camelids can be split into two groups: camels, which live in the deserts of Africa and Asia, and llamas, which are found in South America. Camels are able to conserve water by reducing evaporation and concentrating their urine, and can lose up to 25 per cent of their body weight by water loss. They also carry a food supply in the form of a hump, which is a large lump of fat that provides energy for the animal if food is scarce.
There are two chief kinds of camels, the Bactrian camel, which has two humps, and the dromedary, which has one. Nearly all camels that exist today are domesticated. Only a few hundred wild Bactrian camels may still wander in remote areas of Mongolia.
The South American camelids, or llamas, live in a variety of habitats, from cool plains to mountains of permanent snow. The llamas are smaller than camels, have no humps, and are covered with a thick coat of wool. Two wild species of South American camelid are the vicuna and the guanaco.
The hoofs have disappeared on all camelids and have been replaced by a nail and a large pad, which enables them to walk on soft or sandy ground. They have two digits on each limb. The camelids are ruminants but have no separation of the omasum and abomasum.
The chevrotains, or mouse deer, are timid forest browsers, not much bigger than a rabbit, found in the tropics of the Eastern Hemisphere. They have no horns and use their large, tusklike upper canines as weapons. They resemble deer in the white patterning of their red-brown coat. With a three-chambered stomach, the chevrotains like the camels represent a halfway point between the nonruminants and the true ruminants.
The rest of the artiodactyls, all of which are true ruminants, contain the most successful and numerous of the ungulates. Almost all of them have horns or antlers and a rumen. The side-toes of their limbs have disappeared, leaving two functional digits, although lateral ones may be represented on these animals by imperfect dew-claws. The incisors have been lost from the upper jaw, and the lower teeth bite against the upper gum.
Among these artiodactyls are the giraffes and okapis, browsing animals now found only in tropical Africa. Their long necks contain the usual number of vertebrae for mammals— seven—but each vertebra is greatly elongated. The neck of the okapi is shorter than that of the giraffes. Horns, which are bony knobs that grow continually but slowly, are found in both sexes. Okapis are solitary, but giraffes usually move in loose knit groups or family herds.
The deer, or cervids, which are mainly forest dwellers, are essentially temperate-zone animals, although some are found in southern Asia. The males have antlers, which are bony growths from the skull. They are shed each year and form progressively more branches as the animals mature. The older the cervid, the larger are the antlers.
Cervids are divided into four subfamilies: Moschinae, Muntiacinae, Cervinae, and Odocoileini. Moschinae contains only one genus, the musk deer. Restricted to Asia, this genus is thought to be the most primitive of the cervids and, with the water deer, is the only type with no antlers. The male has an abdominal gland that exudes musk. Because of the demand for musk by the perfume trade, the musk deer has been hunted out of most of its former range.
Muntiacinae, or muntjacs, contains two genera that are also restricted to Asia. They are small deer, and although they have antlers they also have well-developed upper canines. They are solitary animals that are most active at dusk.
The subfamily Cervinae includes the red deer and wapitis, the axis deer, and the fallow deer. They are found in Asia, Europe, and North America. The Odocoileini is the most widely distributed subfamily and includes such species as white-tailed deer; moose and elk; and caribou and reindeer, whose range is the northernmost of all species of cervid.
The bovids are the largest group of ungulates found throughout the world. They include such animals as antelopes, gazelles, and most of the animals that human beings depend on for food, such as cattle, sheep, and goats. There are nine subfamilies: the cowlike Bovinae; the duikers, or Cephalophinae; the reedbuck and their relatives, or Hippotragi-nae; antelopes and gazelles, or Antilopinae; and the sheep and goats, or Caprinae.
All bovids have bony horns which, unlike antlers, are never shed. The horns, which are firmly attached to the skull, can occur in many different shapes, such as spirals or curves, but they are not branched. Bovids are usually found in herds, and most inhabit grassland, scrubland, or deserts, but goats and sheep are generally found in rocky mountainous or desert areas. They feed by twisting vegetation around the tongue and cutting it with the lower incisors.
Domestic cattle, sheep, and goats have been bred from wild European and Asian species. There are six species of wild sheep found in North Africa, Canada, and mountainous areas of Asia and the Mediterranean. The largest is the argali of the Altai Mountains in Siberia and Mongolia. Males stand 4 feet (1.2 meters) high at the shoulders and have large spiral horns. None of the species of wild sheep has as woolly a coat as the domestic sheep.
Most wild goats live in Asia. One species, the ibex, lives in the mountainous areas of Sudan and Siberia, and in the Alps and Caucasus Mountains. The male ibex has spectacular curved horns that may grow more than 30 inches (76 centimeters) long. Despite its name, the Rocky Mountain goat is not a true goat, but a type of animal called a goat antelope.
Together with rats and hogs, feral goats— goats that have escaped domestic captivity-are among the most destructive of animals because they eat anything in sight. In destroying vegetation, they have been instrumental in the extinction of other animal species, such as some birds.
The pronghorn, native only to North America, is the only remaining species of its family. Herds of about a thousand animals used to be common, but their numbers are greatly reduced, and today they live in herds of about 50 to 100. Like cattle, their horns consist of a bony core that is never shed. Pronghorns do, however, shed the outer covering of their horns. The horns are branched, as are those of the cervids.