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The mule is a domestic equine hybrid between a donkey and a horse. It is the offspring of a male donkey (a jack) and a female horse (a mare).^[1] The horse and the donkey are different species, with different numbers of chromosomes;^[2] of the two possible first-generation hybrids between them, the mule is easier to obtain and more common than the hinny,^[3] which is the offspring of a male horse (a stallion) and a female donkey (a jenny).^[2]

A female mule is often called a "molly" or "Molly mule," though the correct term is "mare mule." A male mule is called a "john" or "John mule," though the correct term is "horse mule." A young male mule is called a "mule colt," and a young female is called a "mule filly." The donkey used to produce mules is called a "mule jack,"^[4] and a group of mules is often called a pack, though the words barren and span also apply.^[5]

The Hittites, a people of Asia Minor known for their horse-riding skills, held mules in higher esteem than their best horses. The price of a mule was three times that of a good horse for the Hittites. Similarly, mules were seven times as valuable as donkeys to the Sumerians.^[6]

Mules influenced the development of North America when Christopher Columbus introduced donkeys and horses to the continent during his expeditions in 1495. Bringing four male and two female donkeys in addition to horses allowed for the production of mules, which assisted the Spanish in their conquest of the continent. The defeat of the Aztecs, for example, opened up Mexico as a mule breeding ground, and the presence of mules in military operations and watch duties soon became commonplace.^[7]

George Washington bred mules at his Mount Vernon home. At the time, they were not common in the United States, but Washington understood their value, as they were "more docile than donkeys and cheap to maintain."^[8] By the nineteenth century, mules had become favored draught animals on farms and for pulling boats. Their numbers in the US reached 885,000, though they remained more popular in the south than the north.^[7] Most notably, the Twenty Mule Teams pulled, often in teams of 20 or more animals, wagonloads of borax out of Death Valley, California from 1883 to 1889. They pulled wagons carrying 10 short tons (9 metric tons) of borax ore during trips to the Borate mines before being replaced by railroads.^[9]

The use of mules for farming and for transportation of agricultural products largely gave way to diesel-powered tractors and transportation.^[10]

On 5 May 2003, Idaho Gem, a mule foal cloned by nuclear transfer of cells from foetal material, was born at the University of Idaho in Moscow, Idaho. Neither an equid nor a hybrid animal had been cloned before.^[11][12]

As of 2018, Mexico has the largest mule population in the world at 3,287,449.^[13]

Mules vary widely in size, from small miniature mules under 125 cm (50 in) to large and powerful draught mules standing up to 180 cm (70 in) at the withers.^[14][15] The median weight range is between about 370 and 460 kg (820 and 1000 lb).^[14]

The mule inherits from the donkey the traits of intelligence, sure-footedness, toughness, endurance, disposition, and natural cautiousness. From the horse it inherits speed, conformation, and agility.^[16] They grow faster and live longer, giving them a larger interval of productivity compared to the horse and donkey.^[17] They rarely become ill, unlike horses, who often need to be checked for worms, and like their donkey parent, mules can see their hind-legs and have hooves better suited to dry climates.^[18] Additionally, muleskin is tougher than that of a horse or donkey and tend to live longer on less resources than their horse counterpart.^[19] 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.^[20][21]

Similar to other equine animals, mules can sleep while standing (mostly to protect themselves against danger). To prevent predation, a group of mules may select members of the pack to "stand watch" while sleeping upright as the others sleep on the ground.^[22]

The speciation of horses and donkeys from their common ancestor happened sometime between 7.7 and 15.4 million years ago.^[17] Today, they are phenotypically and genetically different, with a horse having 64 chromosomes and a donkey having 62.^[23] A mule has 63 chromosomes, 32 from the horse and 31 from the donkey. Its odd number of chromosomes makes gamete formation difficult, often leaving mules infertile.^[23] This also disqualifies them as a species under the biological species model.^[24]

The conception of a mule is difficult due to the differences in behavior and mating patterns between donkeys and horses. When in close proximity, groups of horses and groups of donkeys do not interact with each other often, and donkeys remain at the bottom of the equine social hierarchy while horses rule the pasture and mules are left in a mid-tier social caste.^[25]

Mule pregnancy is rare, but can occasionally occur naturally, as well as through embryo transfer. A few mare mules have produced offspring when mated with a horse or a jack.^[26][27] 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). Between 1527 and 2002, approximately sixty such births were reported.^[27] In Morocco in early 2002 and Colorado in 2007, mare mules produced colts.^[27][28] Genetic testing from the Colorado birth verified that the mother was indeed a mule and the foal was indeed her offspring.^[29]

While a few mules can carry live weight up to 160 kg (353 lb), mules can generally be packed with dead weight up to around 90 kg (198 lb).^[30] With some variability, 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, showcasing mule strength and endurance.^[31]

Mules have uses beyond heavy lifting. In the Abruzzo region of Italy, for example, mules are used to defend herds of animals against predators like wolves and feral dogs. Horse breeders in Italy used to add a female mule to their flocks, finding that the mule tended to protect the foals with even more vigor than the actual mother.^[32]

Mule trains are also used to deliver food to remote areas of the world. In Nepal, for example, the World Food Programme provides food for children in the Sudurpaschim province with the help of mule drivers.^[33]

The most recent use for the mule is as a research specimen, especially in studies regarding the slicing, rearrangement, and compatibility of chromosomes.^[17]

A mule's diet is more similar to that of a donkey than a horse. They store water more efficiently, for example, and can consume 15 gallons of it daily, making them suitable for more desert-like climates. Their diet is also herbivorous, mainly consisting of grains, hay and greens, though they can also enjoy fruits and vegetables. And while they are not "picky-eaters" in the way humans view the term, mules do have similar tastebuds and therefore have dietary preferences based on taste and texture.^[22] Additionally, protein intake can be in issue with mules, so monitoring the amount of essential amino acids in their diet can be helpful.^[34]

A mule's hooves should be cleaned regularly to remove debris and should be shortened at least every two months to prevent overgrowth, pain, and discomfort. If necessary, mules may also wear muleshoes to protect their hooves.^[22] While similar to horseshoes, mule shoes are generally smaller due to their smaller hooves.^[35] Furthermore, a mule's coat and hair need to be groomed regularly, including clippings about twice monthly.^[22]

An adult mule's temperature should remain within the range of 37.5°C – 38.5°C, their pulse has a healthy range of 26-40 bpm, and they should take 8-16 breaths per minute as well.^[18]