The zebra (family Equidae) is a large, single-hoofed animal of the genus Equus, which includes the horse and the ass. Zebras generally grow to around 4 to 5 feet (1.2 to 1.5 m) tall and 7.5 feet (2.3 m) long, and weigh close to 800 pounds (365 kg). The animal is known for its colorful and distinctive stripes, which differ in thickness and pattern depending on the species. Zebras can run at speeds approaching 40 mph (64 kph), especially when chased by predators.
The ancestors of the Equus family first appear in the fossil record rougly 60 million years ago. The Hyracotherium was a forest dweller which resembled a smaller version of today's horse. As grasses spread during the Oligocene and Miocene (5 to 38 million years ago), the animal's descendants moved onto the plains. There, they evolved the physical adaptations seen in today's Equus - from longer limbs to flatter teeth suitable for grazing.
Until about 2 million years ago, almost 20 genera roamed the earth. Today, there are 6 surviving species of Equus including the horse (E. przewalskii) and the Asian wild ass (E. hemionus). The most rare species is the Somali Ass (E. africanus), only a handful of which are believed to survive in Somalia and Ethiopia.
There are three species of zebra alive today in Africa. The Mountain Zebra (E. zebra) and Grevy's Zebra are rare and restricted in their distribution to arid zones on the continent. The Plains or Burchell's Zebra (E. burchelli) is the most abundant member of the family, and can be found in eastern, central, and southern Africa.
Zebras either form harems (herds of males and females) or socialize in bachelor herds (all-male herds from 2-15 zebras). To create a new harem, a stallion must abduct a filly from her herd. The stallion must then fight other males that approach the filly every time she is in heat, which occurs monthly. The filly will finally stay with the stallion that impregnates her.
Bachelor herds are usually led by young stallions whose rank depends on age. Bachelors will spend much of their time preparing for their later roles as herd stallions by play fighting and engaging in challenge rituals. By age 5, bachelors are ready to start harems, leaving the bachelor herd in search of a filly.
Plain zebras spend most their day grazing, although they will also engage in dust bathing, drinking, and brief periods of resting. Zebras will move little at night, and sleep laying down. At least one member of the herd will stand guard to protect against lions or hyenas. Zebras have a powerful back kick that can easily break the jaw of an attacking predator. If a harem with young foals is approached by a lion or hyena, the females will surround the young while the stallion attacks the predator.