In its common modern meaning, a mule is the offspring of a male donkey and a female horse, which is classified as a kind of F1 hybrid. The much rarer offspring of a male horse and a female donkey is called a hinny. The term "mule" (Latin mulus) was formerly applied to the infertile offspring of any two creatures of different species. The mule, easier to breed and usually larger in size than a hinny, has monopolized 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 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.
The median weight range for a mule is between 800 pounds to 1000 pounds.
With its short thick head, long ears, thin limbs, small narrow hooves, short mane, absence of chestnuts (horny growths) inside the hocks, the mule looks like a donkey; in height and body, shape of neck and croup, uniformity of coat, and teeth, it appears horse-like; the mule comes in all sizes, shapes and conformities. There are mules that resemble quarter horses, huge draft mules, fine-boned racing mules, shaggy pony mules and many more types.
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".
Mules are generally less tolerant towards dogs than horses are. They are also capable of striking out with any of their hooves in any direction, even sideways if needed.
Mules exhibit a higher cognitive intelligence than their parent species - horses and donkeys. This is believed to be the result of hybrid vigour, similar to how mules acquire greater height and endurance than either parents.
Several female mules have produced offspring when mated with a purebred horse or donkey. Since 1527 there have been more than 60 documented cases of foals born to female mules around the world. There are no recorded cases of fertile mule stallions. 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. 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. Some examples of recorded fertile mules include:
After World War II, mules fell on hard times. The use of mules for farming and transportation of agricultural products gave way to modern tractors and trucks. A dedicated number of mule breeders, however, continued the tradition as hobby and continued breeding the great lines of mammoth jacks started in the United States by George Washington with the gift from the King of Spain of two Catalan jacks. These hobby breeders began to utilize better mares for mule production until today's modern saddle mule emerged. Exhibition shows where mules pulled heavy loads have now been joined with mules competing in Western and English pleasure riding, as well as dressage and show jumping competition. There is now a cable TV show dedicated to the training of donkeys and mules. Mules, once snubbed at traditional horse shows, have been accepted for competition at the most exclusive horse shows in the world in all disciplines.
Amish farmers, who reject tractors and most other modern technology, commonly use teams of six or eight mules to pull plows, diskers, and other farm equipment, though they use horses for pulling buggies on the road.
In 2003, researchers at University of Idaho and Utah State University found a way to reproduce mules -- by cloning the first mule as part of Project Idaho The research team includes Gordon Woods, UI professor of animal and veterinary science, Kenneth L. White, USU professor of animal science, and Dirk Vanderwall, UI assistant professor of animal and veterinary science. The baby mule, Idaho Gem, was born May 4. It is 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.
Mules today come in a variety of shapes, sizes and colors, from minis under 50 pounds to maxis over 1000 pounds, and in many different colors. Mules from Appaloosa mares produce wildly colored mules, much like their Appaloosa horse relatives, but with even wilder skewed colors. The Appaloosa color is produced by a complex of genes known as the Leopard Complex (Lp). Mares homozygous for the Lp gene bred to any color donkey will produce an Appaloosa colored mule.