Meet the Relatives
Part of the Horse exhibition.
The horse family (Equidae) today is quite small. All horse breeds, from slim thoroughbred racehorses to stocky plow horses to tiny ponies, belong to a single species, Equus caballus. What's more, all surviving branches of the horse family tree are also members of this same genus Equus, which now consists of only seven living species. Other equids include donkeys, asses, and zebras.
The horse (Equus caballus) includes all domesticated horse breeds. Some scientists also consider the Asiatic wild horse, or Przewalski horse, to be a variety of Equus caballus, though it is often called a separate species, Equus przewalskii. Domestic horses are thought to have been bred from the European wild horse, or tarpan, extinct since 1919.
The donkey (Equus asinus) is a domesticated African ass native to eastern Africa.
The onager and kulan are varieties of the Asiatic ass (Equus hemionus), which has five subspecies in the Middle East and Asia.
The kiang (Equus kiang) is an Asian ass with three subspecies ranging from China to India.
The largest zebra, Grevy's zebra (Equus grevyi) of eastern Africa, has the narrowest stripes.
Known for the "gridiron" stripes on its rump, the mountain zebra (Equus zebra) of southern Africa is endangered due to poaching and habitat loss.
Burchell's zebra (Equus burchelli) has wide stripes. It has several subspecies with distinctive patterns.
The quagga, a form of Burchell's zebra that is sometimes considered its own species, disappeared in the mid-1800s. It formerly lived in southeastern Africa.
About Scientific Names
Scientists use scientific names to catalog life--ideally, each true species should have a name different from every other. With very closely related organisms, however, it may be difficult to draw a sharp line between species. While many scientists think that all living horses can be grouped in one species (Equus caballus), agreement is not universal.
Next of Kin
The only surviving branch of the horse family is the genus Equus, which includes zebras, asses, and donkeys along with the horse. But which living animals outside the horse family are the horse's closest relatives? Hint: You won't find them on a farm.
Here's another hint: Follow the feet. Horses belong to a group of mammals with an odd number of toes. That rules out mammals with two toes, or "cloven hooves," like goats, pigs, cows, deer, and camels.
So who are the other odd-toed, plant-eating animals? Most members of this group, known as perissodactyls, are extinct. But several species survive at present. They include rhinoceroses and tapirs, the horse's closest living relatives.
Extinct Relatives
Horses are more closely related to extinct perissodactyls like this brontothere than they are to cows, pigs, and goats.