The donkey (Equus asinus) belongs to the family Equidae and genus Equus with horses and zebras. It’s classed as such because it has a long neck, a mane, and a single toe on each foot. Donkeys differ from their horse relatives in the shape of their faces (a donkey’s is much shorter), their ears (which are longer and thicker), and backs (flatter than a horse’s and containing one fewer vertebrae).
There are other types of donkey besides the domestic donkey, like the African wild ass (Equus africanus somaliensis), a wild subspecies that inhabits deserts in Eritrea, Ethiopia, and Somalia. Though not technically a donkey, the Asiatic wild ass (Equus hemionus) is closely related and named after its asinus cousin. It occurs in Iran, India, Mongolia, and parts of northern China.
Mules are bred from male donkeys (Equus asinus) and female horses (Equus caballus)—a female donkey that breeds with a male horse, rather, produces a hinny. Hinnies are not as common.
Mixing the chromosomes of horses and mules causes both mules and hinnies to be sterile, except on a few recorded occasions. And because “species” is technically defined as “a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding,” mules and hinnies are not considered species. They’re hybrids, rather. They do, however, have a scientific name, Equus mulus.
Mules, of course, belong to the same family and genus as donkeys and horses.
Characteristics of Donkeys vs. Mules
Donkeys and mules are tough to tell apart just by looking at them, but they do have some distinguishing physical features, one of the most prominent being a primitive marking belonging to only one.
Head
Both mules and donkeys are known to have short, thick heads, but the donkey’s is slightly shorter and thicker than the mule’s. Compared to a horse, a mule has long ears, but they resemble the shape of a horse’s whereas a donkey’s ears are thicker and more wideset. The openings are larger because they’re adapted for temperature regulation, a helpful trait to have in the hot desert from which donkeys originated. Another subtle difference: Donkeys’ ears, unlike mules’ ears, darken at the tips and around the edges.
Body Size and Shape
Whereas donkeys have flat backs, a mule’s is slightly curved, like a horse’s but less exaggerated. Mules are bigger than donkeys, taking their size after horses. Mules and horses both reach about 60 inches—or 15 hands—from hoof to withers (shoulders) whereas donkeys stand only about 45 inches at the withers.
Donkeys and mules both have thin limbs and narrow hooves.
Coat
Mules have short manes like donkeys, not like horses. Their coats, however, are more like horses’, meaning they have relatively fine hair that comes in an array of colors—including brown, reddish-brown (“bay”), black, gray, or even white, palomino, and dun.
Donkeys have coarser coats and are usually gray, although some are black or brown. They have a distinguishing dorsal line, a primitive and crosslike marking that begins at the base of the mane, trails down the spine, and typically intersects with a stripe that connects the shoulders.
Conservation Status
Most donkeys and mules are domestic. In the U.S., with the exception of a few domestic burro populations that have returned to the wild across the West and Southwest, they exist only in zoos and pastoral environments. Outside of the U.S., the equids also occur across Mexico, Central and South America, Asia, and Africa. They are some of the most common working animals in the world, used for tourism, transportation, and agriculture.
The African wild ass, a subspecies and suspected descendent of the donkey, still roams free but is rapidly disappearing across its native Horn of Africa range.
Its biggest threat is hunting—for both food and medicinal purposes, according to the IUCN.