The Asian wild ass typically has a sandy-colored coat with lighter-colored legs and belly, a short erect black mane, a black spinal stripe, and a black tail tuft. Its neigh is shrill. Different races of this species vary in size, but all are smaller than the African ass. They were once widely distributed across Asia, but they have been crowded out of their grazing lands by domestic livestock and have been hunted for their flesh and hides. Each race is now restricted to a very limited territory. Among them are the Persian ass, or onager, of central Asia; the Mongolian ass, or kulan, of NE Asia; the Tibetan ass, or kiang, presently the most numerous Asian wild ass; and the Indian ass, or ghorkhar. All are considered endangered, and the continued survival of the onager and the kulan is particularly threatened. The Syrian wild ass, of SW Asia, is probably already extinct.

The two wild races of the African species, called Nubian and Somali wild asses, are becoming rare. They are found in the mountains and deserts of NE Africa. The African ass averages about 4 1/2 ft (135 cm) in shoulder height; it is grayish in color, with longer ears and mane than the Asian ass, and with a characteristic loud, harsh bray. Its descendant, the donkey, is the oldest domestic beast of burden; it is believed to have been domesticated in Egypt by c.4000 b.c. A variety of the Asian ass was used in ancient Mesopotamia but did not survive as a domestic animal; all modern domestic donkeys are descended from the African species.

The donkey is still used as a pack and draft animal. Although not as swift or powerful as the horse, it is strong for its size and has great powers of endurance. Donkeys are more surefooted than horses in mountainous country and are cheaper to maintain, as they feed on dry scrub. They may live up to 47 years, about twice as long as a horse. In some regions the donkey is crossbred with the horse to produce a mule.

The donkey was once widely used in Mexico and the SW United States, where it was known by its Spanish name of burro. A large population of feral donkeys (wild descendants of domesticated animals) now exists in the deserts of that region. Feral donkeys are also found in the Old World, where they have given rise to some confusion about the number of true wild asses left in existence.

Asses are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Perissodactyla, family Equidae.