A mule is the offspring of a male donkey (jack) and a female horse (mare). Horses and donkeys are different species, with different numbers of chromosomes. Of the two first-generation hybrids between these two species, a mule is easier to obtain than a hinny, which is the offspring of a female donkey (jenny) and a male horse (stallion).
The size of a mule and work to which it is put depend largely on the breeding of the mule's mother (dam). Mules can be lightweight, medium weight, or when produced from draft mares, of moderately heavy weight.: 85–87 Mules are reputed to be more patient, hardy, and long-lived than horses, and are described as less obstinate and more intelligent than donkeys.: 5
The mule is valued because, while it has the size and ground-covering ability of its dam, it is stronger than a horse of similar size and inherits the endurance and disposition of the donkey sire, tending to require less feed than a horse of similar size. Mules also tend to be more independent than most domesticated equines other than its parental species, the donkey.
The median weight range for a mule is between about 370 and 460 kg (820 and 1,000 lb). While a few mules can carry live weight up to 160 kg (353 lb), the superiority of the mule becomes apparent in their additional endurance.
In general, a mule can be packed with dead weight up to 20% of its body weight, or around 90 kg (198 lb). Although it depends on the individual animal, mules trained by the Army of Pakistan are reported to be able to carry up to 72 kg (159 lb) and walk 26 km (16.2 mi) without resting. The average equine in general can carry up to roughly 30% of its body weight in live weight, such as a rider.
A female mule that has estrus cycles, and which could thus in theory carry a fetus, is called a "molly" or "Molly mule", though the term is sometimes used to refer to female mules in general. Pregnancy is rare, but can occasionally occur naturally, as well as through embryo transfer. A male mule is properly called a "horse mule", though often called a "john mule", which is the correct term for a gelded mule. A young male mule is called a "mule colt", and a young female is called a "mule filly".
With its short, thick head, long ears, thin limbs, small, narrow hooves, and short mane, the mule shares characteristics of a donkey. In height and body, shape of neck and rump, uniformity of coat, and teeth, it appears horse-like. The mule occurs in all sizes, shapes, and conformations. Some mules resemble huge draft horses, sturdy Quarter Horses, fine-boned racing horses, shaggy ponies, and more.
The mule is an example of hybrid vigor. Charles Darwin wrote: "The mule always appears to me a most surprising animal. That a hybrid should possess more reason, memory, obstinacy, social affection, powers of muscular endurance, and length of life, than either of its parents, seems to indicate that art has here outdone nature."
The mule inherits from its sire the traits of intelligence, sure-footedness, toughness, endurance, disposition, and natural cautiousness. From its dam it inherits speed, conformation, and agility.: 5–6, 8 Mules are reputed to exhibit a higher cognitive intelligence than their parent species, but robust scientific evidence to back up these claims is lacking. Preliminary data exist from at least two evidence-based studies, but they rely on a limited set of specialized cognitive tests and a small number of subjects. Mules are generally taller at the shoulder than donkeys and have better endurance than horses, although a lower top speed.
Handlers of working animals generally find mules preferable to horses; mules show more patience under the pressure of heavy weights, and their skin is harder and less sensitive than that of horses, rendering 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.
Mules occur in a variety of configurations, sizes, and colors. Minis weigh under 200 lb (91 kg), and other types range up to and over 1,000 lb (454 kg). The coats of mules have the same varieties as those of horses. Common colors are sorrel, bay, black, and grey. Less common are white, roan, palomino, dun, and buckskin. Least common are paint or tobiano patterns. Mules from Appaloosa mares produce wildly colored mules, much like their Appaloosa horse relatives, but with even more wildly skewed colors. The Appaloosa color is produced by a complex of genes known as the leopard complex. Mares homozygous for this gene complex bred to any color donkey will produce a spotted mule.
Mules historically were used by armies to transport supplies, occasionally as mobile firing platforms for smaller cannons, and to pull heavier field guns with wheels over mountainous trails such as in Afghanistan during the Second Anglo-Afghan War.
The Food and Agriculture Organization of the United Nations reports that China was the top market for mules in 2003, closely followed by Mexico and many Central and South American nations.
Mules and hinnies have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.
A few mare mules have produced offspring when mated with a purebred horse or donkey. Herodotus gives an account of such an event as an ill omen of Xerxes' invasion of Greece in 480 BC: "There happened also a portent of another kind while he was still at Sardis—a mule brought forth young and gave birth to a mule" (Herodotus The Histories 7:57), and a mule's giving birth was a frequently recorded portent in antiquity, although scientific writers also doubted whether it was really possible (see e.g. Aristotle, Historia animalium, 6.24; Varro, De re rustica, 2.1.28).
As of October 2002, only 60 cases of mules birthing foals had been documented since 1527. In China in 1981, a mare mule produced a filly. In Morocco in early 2002 and Colorado in 2007, mare mules produced colts. Blood and hair samples from the Colorado birth verified that the mother was indeed a mule and the foal was indeed her offspring.
A 1939 article in the Journal of Heredity describes two offspring of a fertile mare mule named "Old Bec", which was owned at the time by Texas A&M University in the late 1920s. One of the foals was a female, sired by a jack. Unlike her mother, she was sterile. The other, sired by a five-gaited Saddlebred stallion, exhibited no characteristics of any donkey. That horse, a stallion, was bred to several mares, which gave birth to live foals that showed no characteristics of the donkey. In a more recent instance, a group from the Federal University of Minas Gerais in 1995 described a female mule that was pregnant for a seventh time, having previously produced two donkey sires, two foals with the typical 63 chromosomes of mules, and several horse stallions that had produced four foals. The three of the latter available for testing each bore 64 horse-like chromosomes. These foals phenotypically resembled horses, though they bore markings absent from the sire’s known lineages, and one had ears noticeably longer than those typical of her sire's breed. The elder two horse-like foals had proved fertile at the time of publication, with their progeny being typical of horses.
The mule is "the most common and oldest known manmade hybrid." It was likely invented in ancient times in what is now Turkey. They were common in Egypt by 3000 BCE. Homer noted their arrival in Asia Minor in the Iliad in 800 BCE. Mules are mentioned in the Bible (Samuel 2:18:9, Kings 1:18:5, Zacharia 14:15, Psalms 32:9). Christopher Columbus brought mules to the New World. George Washington is known as the father of the American mule due to his success in producing 57 mules at his home at Mount Vernon. At the time, mules were not common in the United States, but Washington understood their value, as they were "more docile than donkeys and cheap to maintain." In the 19th century, they were used in various capacities as draft animals - on farms, especially where clay made the soil slippery and sticky; pulling canal boats; and famously for pulling, often in teams of 20 or more animals, wagonloads of borax out of Death Valley, California from 1883 to 1889. The wagons were among the largest ever pulled by draft animals, designed to carry 10 short tons (9 metric tons) of borax ore at a time.
In the second half of the 20th century, widespread usage of mules declined in industrialized countries. The use of mules for farming and transportation of agricultural products largely gave way to steam-, then gasoline-powered, tractors and trucks.
Mules are still used extensively to transport cargo in rugged, roadless regions, such as the large wilderness areas of California's Sierra Nevada mountains or the Pasayten Wilderness of northern Washington. Commercial pack mules are used recreationally, such as to supply mountaineering base camps, and also to supply trail-building and maintenance crews, and backcountry footbridge-building crews. As of July 2014, at least 16 commercial mule pack stations are in business in the Sierra Nevada. The Angeles chapter of the Sierra Club has a mule pack section that organizes hiking trips with supplies carried by mules.
During the Soviet–Afghan War, mules were used to carry weapons and supplies over Afghanistan's rugged terrain to the mujahideen.
About 3.5 million donkeys and mules are slaughtered each year for meat worldwide.
Mule trains have been part of working portions of transportation links as recently as 2005 by the World Food Programme.
A "mule train" is a connected or unconnected line of pack mules, usually carrying cargo. Because of the mule's ability to carry at least as much as a horse, its trait of being sure-footed along with tolerance of poorer, coarser foods and abilities to tolerate arid terrains, mule trains were common caravan organized means of animal-powered bulk transport back into preclassical times. In many climate and circumstantial instances, an equivalent string of pack horses would have to carry more fodder and sacks of high-energy grains such as oats, so could carry less cargo. In modern times, strings of sure-footed mules have been used to carry riders in dangerous but scenic backcountry terrain such as excursions into canyons.
Pack trains were instrumental in opening up the American West, as they could carry up to 250 lb (110 kg), survive on rough forage,[a] did not require feed, and could operate in the arid, higher elevations of the Rockies, serving as the main cargo means to the west from Missouri during the heyday of the North American fur trade. Their use antedated the move west into the Rockies as colonial Americans sent out the first fur trappers and explorers past the Appalachians, who were then followed west by high-risk-taking settlers by the 1750s (such as Daniel Boone), who led an increasing flood of emigrants who began pushing west over into southern New York, and through the gaps of the Allegheny into the Ohio Country (the lands of western Province of Virginia and the Province of Pennsylvania), into Tennessee and Kentucky before and especially after the American Revolution.
In 2003, researchers at University of Idaho and Utah State University produced the first mule clone as part of Project Idaho. The research team included Gordon Woods, professor of animal and veterinary science at the University of Idaho; Kenneth L. White, Utah State University professor of animal science; and Dirk Vanderwall, University of Idaho assistant professor of animal and veterinary science. The baby mule, Idaho Gem, was born May 4. It was the first clone of a hybrid animal. Veterinary examinations of the foal and its surrogate mother showed them to be in good health soon after birth. The foal's DNA comes from a fetal cell culture first established in 1998 at the University of Idaho.