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
Mule and Ass by Hendrik Goltzius or Hieronymus Wierix, 1578
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.
Color and size variety
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.
Distribution and use
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 the thing 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 2001, 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 chromsomes 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 sires known lineages, and one had ears noticably 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