A mule is the offspring of a male donkey and a female horse. Historically, and today in technical jargon, "mule" refers to the infertile offspring of any two animals of different species. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny (the offspring of a male horse and a female donkey). All male mules and most female mules are infertile.

The size of a mule and work to which it is put depends largely on the breeding of the mule's dam. Mules can be lightweight, medium weight, or even, when produced from draught horse mares, of moderate heavy weight. pp. 85–87.

The mule, easier to breed and usually larger in size than a hinny, has monopolized the attention of breeders Fact: date=September 2007. Reproductive success occurs more often when a donkey is the sire and the horse is the dam. Sometimes people let a stallion (male horse) run with a jenny (female donkey) for as long as six years before she becomes pregnant. Mules and hinnies are almost always sterile (see fertile mules below for rare cases). The sterility is attributed to the differing number of chromosomes of the two species: donkeys have 62 chromosomes, whereas horses have 64.

A female mule that has estrus cycles and can carry a fetus is called a "molly" and can occasionally occur naturally as well as through embryo transfer.

Size and Performance

The median weight range for a mule is between .

Although it depends on the individual animal, an army mule can "carry up to 172 kg and walk 26 km without resting."

The virtue of the mule is that a mule is pound-for-pound stronger than a horse of similar size and inherits the strong will and higher intelligence of the donkey father. Mules also tend to require less food than a horse of similar size.

Characteristics

A mule does not sound exactly like a donkey or a horse. Instead, a mule makes a sound that is similar to a donkey's but also has the whinnying characteristics of a horse (often starts with a whinny, ends in a hee-haw). Sometimes, mules whimper. The coats of mules come in the same varieties as those of horses. Common colors are sorrel, bay, black, and grey. Less common are white, roans (both blue and red), palomino, dun, and buckskin. Least common are paint mules or tobianos.

The mule possesses the sobriety, patience, endurance and sure-footedness of the donkey, and the vigour, strength and courage of the horse. Operators of working animals generally find mules preferable to horses: mules show less impatience under the pressure of heavy weights, and their skin, harder and less sensitive than that of horses, renders them more capable of resisting sun and rain. Their hooves are harder than horses', and they show a natural resistance to disease and insects. Many North American farmers with clay soil found mules superior as plow animals, especially in the U.S. state of Missouri, hence the expression "stubborn as a Missouri mule".