Close relatives of the horse persist in the wild today in several parts of the Old World. Taxonomic interpretations vary, but most experts place them all in the genus Equus. The handsome striped equids of Africa fall into three distinct species: Grevy's zebra (Equus grevyi), the plains or Burchell's zebra (Equus burchelli =Equus quagga), and the mountain zebra (Equus zebra). Of these, the Grevy's zebra is most distinct. Taxonomically it is placed in the subgenus Dolichohippus, whereas the plains and mountain zebras are placed in the subgenus Hippotigris. The continued survival of most of these fascinating and spectacular species is threatened. Despite the establishment of sanctuaries, many populations are faced with diminishing ranges and decreasing numbers.
The digestive system of all equids is designed to extract energy and nutrients from coarse, low-quality forage by permitting passage of large quantities of plant matter through a long hindgut. The intestines of asses and Grevy's zebras are eleven to twelve times the body length; the intestines of mountain zebras and plains zebras are seventeen times the body length. The equid digestive system preadapts these animals to life in habitats dominated by grasses and shrubs. There are strong indications, however, that equids were not always restricted to such marginal habitats as they occupy today. During the Pleistocene (1.6 million to 10,000 years ago) some now-extinct North American species inhabited forested regions. Primarily grazers, most of these animals continue to exhibit considerable flexibility in diet and browse forbs, shurbs and small trees. The seasonal and geographic variations in forage quantity and quality and in water availability typical of arid and semiarid environments cause most wild equid populations to be migratory.
In body form all equids are quite similar. They are specialized for running and for grazing on siliceous grasses, which rapidly wear down the teeth. Species can be differentiated, however, on the basis of qualitative and quantitative characteristics of the skull, tooth morphology and length and robustness of the metapodials (foot bones). There are also variations in size, color, color patterning, hoof size and shape, mane length, presence or absence of a forelock, tail hair length and distribution, number and size of chestnuts and vocalizations.
One reason all equids are vulnerable in the face of hunting pressures and habitat loss is that they reproduce slowly. Gestation lasts eleven to thirteen months, depending upon the species, and almost always only one foal I dropped each season. Most researchers, however, report that mares foal only every other year. Sexual maturity occurs at age two in females and age three to five in males. Life span can extend twenty or more years.
The Grevy's zebra is the largest of the wild equids and is usually considered the most primitive morphologically. Adults attain shoulder heights of 140 to 150 centimeters (55-57 in.) and may weigh 400 kilograms (880 lbs.) or more. Its very narrow and closely spaced stripes make the Grevy the most strikingly beautiful of the zebras. The stripes extend all the way to the broad hooves, leaving only the belly white. A broad black dorsal stripe is set off by a narrow zone of white on either side. Grevys are long legged and rather slenderly built with a long head. The black-tipped mane is relatively long and erect; their ears are very large and rounded. Grevy's zebras bray in a manner similar to a donkey.
Grevy's zebras are essentially confined to the semi-desert of northern Kenya east of the Great Rift Valley and north of the Tana River. Their range extends into neighboring parts of Ethiopia and Somalia. During the rainy season mature stallions establish territories onto which mares come to foal and probably to breed. Gestation is thirteen months, longer than any other equid. Once the foals are born, the mares stay within two kilometers (1.2 mi.) of water and are almost always with the territorial stallion. Foals do not drink water until they are three months olds and -- unlike any other equid -- are left in "kindergartens" frequently guarded by the territorial male while their mothers go to water. Grevy foals begin to forage much earlier than do feral horse foals: a six-week-old Grevy's zebra will graze as often as a five-month old horse. This accelerated development of feeding capability allows the young Grevy foal to become independent of its mother at a relatively early age.
Grevy's zebras are under continuing pressure from human encroachment on their habitat. During severe East African droughts in recent years, pastoral peoples have blocked the zebras' access to vital water holes and mortality rates have been high.
African equids for the most part replace one another geographically. There is a zone of overlap, however, between the Grevy's zebra and the plains zebra on the floodplain of the Ewaso Nyiro in northern Kenya. Here the two species form mixed grazing herds, but there is no record of interbreeding.
The plains zebra is the most abundant and widespread of extant wild equids, occurring throughout the tropical grasslands of East and southern Africa. It is quite stout in comparison with the Grevy's zebra. Shoulder height varies from 120 to 140 centimeters (47-55 in.), and a mature male may weigh 300 kilograms (660 lbs.). Broad vertical stripes on the sides bend on the flanks to become horizontal across the rump. Stripes extend down the rather short legs to broad hooves. The stripes on the sides continue into the short, erect mane and meet under the belly. Stripes become less distinct on subspecies in the more southerly parts of its range. The plains zebra has a "bark" quite unlike the neigh of a horse or the bray of a donkey. Plains zebra have a harem-type social organization.
The pattern of stripes on all zebras is unique to each individual, with the variation greatest in the shoulder region. This has helped researchers identify and follow individuals over the course of long-term studies and may aid foals and adult zebras of a given harem in identifying each other in the large grazing herds.
The plains zebra has differentiated into several subspecies, two of which are now extinct. The Grant's zebra (Equus burchelli boehmi) is the most common of the plains zebra subspecies. The Grant's zebra is the best studied of the plains zebras, and much of what we know of the behavior and biology of the species comes from work done with this subspecies in the wild and in zoos. With broad black stripes on a white background (Africans, reportedly, see white strips on a black background), this subspecies is the zebra most frequently seen in zoos and circuses around the world. In the wild its distribution extends from southern Sudan through East Africa south to the Zambesi River. There may be some 300,000 left in the wild; on the Serengeti-Mara Plains alone there are an estimated 150,000 plains zebras. During the rainy season in Serengeti, aggregate herds of up to 10,000 individuals may form, part of one of the last great wildlife spectacles in the world.
The Chapman's zebra or the Damara zebra (Equus burchelli antiquorum) is a subspecies of plains zebra occurring from Angola and Namibia across northern South Africa to Transvaal. It is characterized by a pattern of broad, dark stripes alternating with thin, light shadow-stripes. The stripes fade into the brownish color of the body on the hindquarters and are absent altogether on the legs.
Another southern subspecies of the plains zebra, the Burchell's zebra (Equus burchelli burchelli), now extinct, lacked stripes on the hindquarters. Its basic body color was reddish-yellow. Burchell's zebra existed from southern Botswana into the Orange Free State of South Africa. As European settlement spread northward from the Cape to colonial Southern Rhodesia, this subspecies was hunted to extinction. The wild herds had disappeared by 1910, and the last known individual died in the Berlin Zoo in 1918.
The southernmost subspecies, the quagga (Equus burchelli quagga) of South Africa, is also extinct. It occurred in large numbers south of the Orange River at the beginning of the nineteenth century, but Boer settlers decimated the population for meat and hides. The quagga disappeared from the wild by 1878, and the last zoo specimen died in 1883. All that remains today are nineteen pelts, a few skulls, three photographs and a few paintings. The quagga was yellowish-brown with stripes that were confined to the head, neck and forebody. DNA from one of the pelts has been retrieved and analyzed, establishing that the quagga was, indeed, a variant of the plains zebra and not a separate species as previously believed. There is currently an experimental breeding program in progress in South Africa to try to reconstruct the quagga from the Chapman's subspecies.
The third zebra species is the mountain zebra. The most diagnostic feature of both mountain zebra subspecies is a square flap of skin or dewlap on the throat, best developed on males. Mountain zebras never form the large herds characteristic of plains zebras, but do exhibit a harem-type social system. During the winter they move up to twenty kilometers (12 mi.) from a water source. Where they are hunted, they water at night; where they are unmolested, they water at any time.
Two subspecies are recognized. Hartmann's zebra (Equus zebra hartmanni) occupies the rugged, broken terrain at the edge of the African Plateau east of the Namib Desert. Its habitat grades from an open woodland with a divers, grassy understory in southern Angola and Namibia to the succulent steppe of the Karroo in South Africa. In the 1950s, mountain zebras numbered between 50,000 and 75,000 and were regarded as vermin by an expanding livestock industry. Especially in drought years zebras competed with cattle for forage and water, and stampeding zebras occasionally tore down fences. By 1960 only 10,000 were left; and in 1973 Hartmann's zebra was considered and endangered species, with approximately 7,000 head remaining.
Hartmann's zebras have broad black stripes on an off-white body. The stripes extend down the legs to narrow hooves, but do not meet on the belly. These animals stand from 118 to 132 centimeters (46-52 in.) high. This subspecies seeks shade and rests during the hottest parts of the day and has been demonstrated to orient its body with respect to the sun. At midday zebras present the least amount and lightest parts of their bodies to the direct rays of the sun, there by decreasing potential heat load. The vocalizations of the Hartmann's zebra are similar to the neigh of a horse.
The Cape mountain zebra (Equus zebra zebra) is the smallest of the extant zebras -- with a shoulder height of about 120 centimeters (47 in.) -- and the most restricted geographically. Its broad black stripes are closely spaced on a pure white body. Overall it is stockier than the Hartmann's zebra, has longer ears, and has a larger dewlap. The Cape mountain zebra formerly inhabited all the mountain ranges of the southern Cape Province of South Africa. By 1922, however, only 400 were believed to survive. To counteract the continued decline, Mountain Zebra National Park was established in 1937 on acacia veld near Cradock, South Africa, but its small population of Cape mountain zebra became extinct in 1950. That same year reintroductions from nearby remnant populations began. Eleven animals were donated from a nearby farm in 1950, and in 1964 another small herd was added. By the late 1960s, the total Cape mountain population was only 140 but grew to 200 by 1979, with 75 percent of the animals in Mountain Zebra National Park. In 1984, the population was back to 400 head. Since then a few zebras have been reintroduced to the Cape Point Nature Reserve.