| Zebras | |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Perissodactyla |
| Family: | Equidae |
| Genus: | Equus |
| Subgenus: | Hippotigris and Dolichohippus |
| Species | |
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Equus zebra |
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Zebras are several species of African equids (horse family) united by their distinctive black and white stripes. Their stripes come in different patterns unique to each individual. They are generally social animals that live in small harems to large herds. Unlike their closest relatives, horses and asses, zebras have never been truly domesticated.
There are three species of zebras: the plains zebra, the Grévy's zebra and the mountain zebra. The plains zebra and the mountain zebra belong to the subgenus Hippotigris, but Grevy's zebra is the sole species of subgenus Dolichohippus. The latter resembles an ass, to which it is closely related, while the former two are more horse-like. All three belong to the genus Equus, along with other living equids.
The unique stripes of zebras make these among the animals most familiar to people. They occur in a variety of habitats, such as grasslands, savannas, woodlands, thorny scrublands, mountains, and coastal hills. However, various anthropogenic factors have had a severe impact on zebra populations, in particular hunting for skins and habitat destruction. Grevy's zebra and the mountain zebra are endangered. While plains zebras are much more plentiful, one subspecies, the quagga, went extinct in the late 19th century, though they have now been rebred from zebra DNA.
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Zebra in English dates back to c.1600, from Italian Zebra, perhaps from Portuguese, which in turn is said to be Congolese (as stated in the Oxford English Dictionary). The Encarta Dictionary says its ultimate origin is uncertain, but perhaps it may come from Latin Equiferus meaning "Wild horse," from equus "horse" and ferus "wild, untamed". The pronunciation is /ˈzɛbrə/ zeb-rə or /ˈziːbrə/ zee-brə.[1]
Zebras evolved among the Old World horses within the last 4 million years. Grevy's zebras (and perhaps also Mountain Zebras) are with asses and donkeys in a separate lineage from the other zebra lineages.[2] This means either that striped equids evolved more than once, or that common ancestors of zebras and asses were striped and only zebras retained the stripes. Extensive stripes are posited to have been of little use to equids that live in low densities in deserts (like asses and some horses) or ones that live in colder climates with shaggy coats and annual shading (like some horses).[3]
Fossils of an ancient equid were discovered in the Hagerman Fossil Beds National Monument in Hagerman, Idaho. It was named the Hagerman horse with a scientific name of Equus simplicidens. It is believed to have been similar to the Grevy's zebra.[4] The animals had stocky zebra-like bodies and short, narrow, donkey-like skulls.[5] Grevy's zebra also has a donkey-like skull. The Hagerman horse is also called the American zebra or Hagerman zebra.
There are three extant species. Collectively, two of the species have eight subspecies (seven extant). Zebra populations are diverse, and the relationships between and the taxonomic status of several of the subspecies are not well known.
The plains zebra (Equus quagga, formerly Equus burchelli) is the most common, and has or had about twelve subspecies distributed across much of southern and eastern Africa. It, or particular subspecies of it, have also been known as the common zebra, the dauw, Burchell's Zebra (actually the subspecies Equus quagga burchellii), Chapman's zebra, Wahlberg's zebra, Selous' zebra, Grant's zebra, Boehm's zebra and the quagga (another extinct subspecies, Equus quagga quagga).
The mountain zebra (Equus zebra) of southwest Africa tends to have a sleek coat with a white belly and narrower stripes than the plains Zebra. It has two subspecies and is classified as vulnerable.
Grévy's Zebra (Equus grevyi) is the largest type, with a long, narrow head, making it appear rather mule-like. It is an inhabitant of the semiarid grasslands of Ethiopia and northern Kenya. Grévy's zebra is the rarest species, and is classified as endangered.
Although zebra species may have overlapping ranges, they do not interbreed. This held true even when the quagga and Burchell's race of plains zebra shared the same area. In captivity, plains zebras have been crossed with mountain zebras. The hybrid foals lacked a dewlap and resembled the plains zebra apart from their larger ears and their hindquarters pattern. Attempts to breed a Grévy's zebra stallion to mountain zebra mares resulted in a high rate of miscarriage. In captivity, crosses between zebras and other (non-zebra) equines have produced several distinct hybrids, including the zebroid, zeedonk, zony, and zorse. In certain regions of Kenya, plains zebras and Grévy's Zebra coexist, and fertile hybrids occur.[6]
The common plains zebra is about 50–52 inches (12.2-13 hands, 1.3 m) at the shoulder with a body ranging from 6–8.5 feet (2–2.6 m) long with an 18-inch (0.5 m) tail. It can weigh up to 770 pounds (350 kg), males being slightly bigger than females. Grévy's Zebra is considerably larger, while the mountain zebra is somewhat smaller.[7]
It was previously believed that zebras were white animals with black stripes, since some zebras have white underbellies. Embryological evidence, however, shows that the animal's background color is black and the white stripes and bellies are additions.[3]
The stripes are typically vertical on the head, neck, forequarters, and main body, with horizontal stripes at the rear and on the legs of the animal. The "zebra crossing" is named after the zebra's black and white stripes.
A wide variety of hypotheses have been proposed to account for the evolution of the striking stripes of zebras. The more traditional of these (1 & 2, below) relate to camouflage.
1. The vertical striping may help the zebra hide in grass. While seeming absurd at first glance, considering that grass is neither white nor black, it is supposed to be effective against the zebra's main predator, the lion, which is color blind. In addition, even at moderate distances, the striking striping merges to an apparent grey.
2. Another hypothesis is that since zebras are herd animals, the stripes may help to confuse predators—a number of zebras standing or moving close together may appear as one large animal, making it more difficult for the lion to pick out any single zebra to attack.[8]
3. It has been suggested that the stripes serve as visual cues and identification.[3] Although each striping pattern is unique to each individual, it is not known whether zebras can recognize one another by their stripes.
4. One theory suggested by an innovative experiment posits that the disruptive colouration is an effective means of confusing the visual system of the blood-sucking tsetse fly.[9]
5. Alternative theories include that the stripes coincide with fat patterning beneath the skin, serving as a thermo-regulatory mechanism for the zebra, or that wounds sustained disrupt the striping pattern to clearly indicate the fitness of the animal to potential mates.
Like horses, zebras walk, trot, canter and gallop. They are generally slower than horses, but their great stamina helps them outpace predators. When chased, a zebra will zig-zag from side to side, making it more difficult for the predator. When cornered, the zebra will rear up and kick or bite its attacker.
Zebras have excellent eyesight. It is believed that they can see in color. Like most ungulates, the zebra has its eyes on the sides of its head, giving it a wide field of view. Zebras also have night vision, although not as advanced as that of most of their predators.
Zebras have excellent hearing, and tend to have larger, rounder ears than horses. Like horses and other ungulates, zebra can turn their ears in almost any direction. In addition to eyesight and hearing, zebras have an acute sense of smell and taste.
Like most members of the horse family, zebras are highly social. Their social structure, however, depends on the species. Mountain zebras and plains zebras live in groups, known as 'harems', consisting of one stallion with up to six mares and their foals. Bachelor males either live alone or with groups of other bachelors until they are old enough to challenge a breeding stallion. When attacked by packs of hyenas or wild dogs a zebra group will huddle together with the foals in the middle while the stallion tries to ward them off.
Unlike the other zebra species, Grevy's zebras do not have permanent social bonds. A group of these zebras rarely stays together for more than a few months. The foals stay with their mothers, while adult males live alone. Like the other two zebra species, bachelor male zebras will organize in groups.
Like horses, zebras sleep standing up, and only sleep when neighbors are around to warn them of predators.
Zebras communicate with each other with high pitched barks and whinnying. Grevy's zebras make mule-like brays. A zebra's ears signify its mood. When a zebra is in a calm, tense or friendly mood, its ears stand erect. When it is frightened, its ears are pushed forward. When angry, the ears are pulled backward. When surveying an area for predators, zebras will stand in an alert posture; with ears erect, head held high, and staring. When tense they will also snort. When a predator is spotted or sensed, a zebra will bark (or bray) loudly.
Zebras feed almost entirely on grasses, but may occasionally eat shrubs, herbs, twigs, leaves and bark. Their digestive systems allow them to subsist on diets of lower nutritional quality than that necessary for other herbivores.
Female zebras mature earlier than the males, and a mare may have her first foal by the age of three. Males are not able to breed until the age of five or six. Mares may give birth to one foal every twelve months. She nurses the foal for up to a year. Like horses, zebras are able to stand, walk and suckle shortly after they are born. A zebra foal is brown and white instead of black and white at birth.
Plains and mountain zebra foals are protected by their mothers, as well as the head stallion and the other mares in their group. Grevy's zebra foals have only their mother as a regular protector, since, as noted above, Grevy's zebra groups often disband after a few months.
Attempts have been made to train zebras for riding, since they have better resistance than horses to African diseases. Most of these attempts failed, though, due to the zebra's more unpredictable nature and tendency to panic under stress. For this reason, zebra-mules or zebroids (crosses between any species of zebra and a horse, pony, donkey or ass) are preferred over purebred zebras.
In England, the zoological collector Lord Rothschild frequently used zebras to draw a carriage. In 1907, Rosendo Ribeiro, the first doctor in Nairobi, Kenya, used a riding zebra for house calls. In the mid-19th century, Governor George Grey imported zebras to New Zealand from his previous posting in South Africa, and used them to pull his carriage on his privately owned Kawau Island.
Captain Horace Hayes, in "Points of the Horse" (circa 1893), compared the usefulness of different zebra species. In 1891, Hayes broke a mature, intact mountain zebra stallion to ride in two days time, and the animal was quiet enough for his wife to ride and be photographed upon. He found the Burchell's zebra easy to break, and considered it ideal for domestication, as it was immune to the bite of the tsetse fly. He considered the quagga (now extinct) well-suited to domestication due to being easy to train to saddle and harness.[10]
Modern man has had great impact on the zebra population. Zebras were, and still are, hunted for their skins, and for meat. They also compete with livestock for forage,[11] and sometimes culled.
The Cape mountain zebra was hunted to near extinction, with less than 100 individuals by the 1930s. The population has increased to about 700 due to conservation efforts, though. Both mountain zebra subspecies are currently protected in national parks, but are still endangered.
The Grevy's zebra is also endangered. Hunting and competition from livestock have greatly decreased their population. Because of the population's small size, environmental hazards, such as drought, are capable of affecting the entire species. Plains zebras are much more numerous and have a healthy population. Nevertheless, they too have been reduced by hunting and loss of habitat to farming. One subspecies, the quagga, is now extinct.
Zebras have been the subject of African folk tales which tell how they got their stripes. According to a Bushmen folk tale of Namibia, the zebra was once all white, but acquired its black stripes after a fight with a baboon over a waterhole. After kicking the baboon so hard, the zebra lost his balance and tripped over a fire, and the fire sticks left scorch marks all over his white coat.[12] In the film Fantasia, two centaurs are depicted being half human and half zebra, instead of the typical half human and half horse.[13]
Zebra are a popular subject in art.[14] The fourth Mughal emperor Jahangir (r.1605-24), commissioned a painting of the zebra, which was completed by Ustad Mansur.[15] Zebra stripes are also a popular style for furniture, carpets and fashion.
When depicted in movies and cartoons, zebras are most often miscellaneous characters, but have had some starring roles, notably in Madagascar and Racing Stripes. Zebras also serve as mascots and symbols for products and corporations, notably Zebra Technologies and Fruit Stripe gum. Zebras are featured on the coat of arms of Botswana.
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