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In its common modern meaning, a mule is the offspring of a male donkey and a female horse. Compare hinny – the offspring of a male horse and a female donkey. The term "mule" (Latin mulus) was formerly applied to the offspring of any two creatures of different species – in modern usage, a "hybrid".
The mule, easier to breed and usually larger in size than a hinny, has monopolised the attention of breeders. The chromosome match-up more often occurs when the jack (male donkey) is the sire and the mare (female horse) is the dam. Sometimes people let a stallion (male horse) run with a jenny (female donkey) for as long as six years before getting her pregnant. Mules and hinnies are almost always sterile (see fertile mules below for rare cases) (see External links). The sterility is attributed to the different number of chromosomes the two species have: donkeys have 62 chromosomes, while horses have 64. Their offspring thus have 63 chromosomes which cannot evenly divide.
A female mule, called a "molly", that has estrus cycles and can carry a fetus, as has occasionally happened naturally but also through embryo transfer. The difficulty is in getting the molly pregnant in the first place.
From experience, mules are generally considered to be more intelligent than either horses or donkeys. 
In its short thick head, long ears, thin limbs, small narrow hooves, short mane, absence of chestnuts (horny growths) inside the hocks, and tail hairless at the root, the mule appears asinine. In height and body, shape of neck and croup, uniformity of coat, and teeth, it appears equine. It has the voice neither of the donkey nor of the horse, but emits a distinctive sound that is similar to a donkey's but also has the whinnying characteristics of horse, sometimes mules whimper. The coat of mules comes in the same variety as that of horses. Many times however, mules are Bay or Sorrel due to the type of Jack(Sire) used. 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 ass, 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, while 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".
Mules are generally less tolerant towards dogs than horses, and are capable of defending their rider against, even killing, a mountain lion. They are also capable of striking out with any of their hooves in any direction.
Humans have used mules from early times; the inhabitants of Mysia and Paphlagonia allegedly bred the first mules. The ancient Greeks and especially Romans valued mules for transport, employing them to draw carriages and carry loads. In the 19th century, mules hauled barges on the Erie Canal and other North American and European canals, and mule teams were often used by teamsters on the U.S. Western frontier. In the early 20th century use of mules survived mainly in military transport, being used to haul caissons and artillery through nearly impassable terrain, the bravery and focused intelligence of the animal serving it well in the midst of the noise and confusion of warfare. Mules have become far less common since the rise of the automobile, the motorized tractor, and other internal combustion-powered vehicles. They still find employment in less-developed countries, and in certain specialized roles for which they are still aptly suited. Mules can negotiate well on narrow, steep trails – such as the route from the South Rim down into the Grand Canyon. (The mule used for tourist transport on the Bright Angel Trail in the Grand Canyon is usually bred out of Quarter Horse Mares to get the compact size needed for that job.) Mules (and burros) can handle extremely rugged terrain and tracks that are too steep and twisted for either the less sure-footed horse or for a motor vehicle. Mules are also shown at equine shows.
Several female mules have produced offspring when mated to a purebred horse or ass. Since 1527 there have been more than 60 documented cases of foals born to female mules around the world. Mules and hinnies have 63 chromosomes that are a mixture of one from each parent. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos.
- Cornevin and Lesbre stated that in 1873 an Arab mule in Africa was bred to a stallion and produced female offspring. The parents and the offspring were sent to the Jardin d'Acclimatation in Paris. The mule produced a second female offspring sired by the same stallion and then two male offspring, one sired by an ass and the other by a stallion. The female progeny were fertile, but their offspring were feeble and died at birth. Cossar Ewart recorded an Indian case in which a female mule gave birth to a male colt. The best documented fertile mule mare was "Krause" who produced two male offspring when bred back to her own sire.
- In most fertile mule mares, the mare passes on a complete set of her maternal genes (i.e. from her horse/pony mother) to the foal; a female mule bred to a horse will therefore produce a 100% horse foal. In the 1920s, "Old Beck" (Texas A&M) produced a mule daughter called "Kit". When Old Beck was bred to a horse stallion she produced a horse son (he sired horse foals). When bred to a donkey, she produced mule offspring. Likewise, a mare mule in Brazil has produced two 100% horse sons sired by a horse stallion.
- In Morocco, a mare mule produced a male foal that was 75% donkey and 25% horse i.e. she passed on a mixture of genes instead of passing on her maternal chromosomes in the expected way. There are no recorded cases of fertile mule stallions. There is an unverified case of a mare mule that produced a mule daughter. The daughter was also fertile and produced a horse-like foal with some mule traits; this was dubbed a "hule". There are no reports as to whether the mule was fertile.
- A comparable case is that of a fertile hinny (ass mother, horse sire – the reverse of a mule) in China. Her offspring, "Dragon Foal", was sired by a donkey. Scientists expected a donkey foal if the mother had passed on her maternal chromosomes in the same way as a mule. However, Dragon Foal resembles a strange donkey with mule-like features. Her chromosomes and DNA tests confirm that she is a previously undocumented combination.
- African Wild Ass
- British Mule Society
- A case of a mule giving birth
- LongearsMall.com – Online Donkey & Mule Community and Resource Directory
- Supply Mules in Kashmir
This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.