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 Grévy'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 them one of 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. Grévy's zebra and the mountain zebra are endangered. While plains zebras are much more plentiful, one subspecies, the quagga, became extinct in the late 19th century – though there is currently a plan, called the Quagga Project, that aims to breed zebras that are phenotypically similar to the quagga in a process called breeding back.
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. It is likely that the stripes are caused by a combination of factors. 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.
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 and 2, below) relate to camouflage.
1. The vertical striping may help the zebra hide in the grass by disrupting its outline. In addition, even at moderate distances, the striking striping merges to an apparent grey. However, the camouflage has been contested with arguments that most of a zebra's predators (such as lions and hyenas) cannot see well at a distance, and are more likely to have smelled or heard a zebra before seeing it from a distance, especially at night.
2. The stripes may help to confuse predators by motion dazzle—a group of zebras standing or moving close together may appear as one large mass of flickering stripes, making it more difficult for the lion to pick out a target. It has been suggested that when moving, the stripes may confuse observers, such as mammalian predators and biting insects, by two visual illusions: the wagon-wheel effect, where the perceived motion is inverted, and the barberpole illusion, where the perceived motion is in a wrong direction.
3. The stripes may serve as visual cues and identification. Although the striping pattern is unique to each individual, it is not known whether zebras can recognize one another by their stripes.
4. Experiments by different researchers indicate that the stripes are effective in attracting fewer flies, including blood-sucking tsetse flies and tabanid horseflies. A 2012 experiment in Hungaryshowed that zebra-striped models were nearly minimally attractive to tabanid horseflies. These flies are attracted to linearly polarized light, and the study showed that black and white stripes disrupt the attractive pattern. Further, attractiveness increases with stripe width, so the relatively narrow stripes of the three living species of zebras should be unattractive to horseflies.
5. Stripes may be used to cool the zebra. Air may move more quickly over black light-absorbing stripes while moving more slowly over white stripes. This would create convection currents around the zebra that would cool it. One study analyzes that zebras have more stripes in hotter habitats.
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, Grévy'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. Grévy's zebras make mulelike 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.