Mules are hybrid animals of the equine family that result from the mating of a donkey stallion (called a jack) with a horse mare. In genetic terms, this is classified as an F1 hybrid between.Equus caballus X Equus asinus. Male mules are called mules or Johns, and are unable to reproduce. However, although they are genetically sterile, they are still affected by the same male hormones as any other stallion and they can become dangerous if not castrated. An uncastrated male mule cannot participate in shows and it is useless to keep them as a stallion. Since no male mule has ever sired a foal, you cannot mate two mules and expect offspring. In fact, there appears to be no authenticated instances of mules breeding among themselves even when the stallion has not been castrated. Interestingly, jack donkeys appear to be reluctant to mate with horse mares and many have to be trained to do so, though in these more modern times, artificial insemination can be attempted if all else fails.
In contrast, a fertile female mule (called a molly) can sometimes occur from this breeding combination. But this is so rare that the Romans had a saying, "cum mula peperit," meaning "when a mule foals" which is similar to saying "when hell freezes over." In some regions, a mule giving birth can trigger fear and superstition. When a mule produced a foal in Albania in 1994, it was thought to have unleashed the spawn of the devil on the small village; and when a mule in Morocco produced a foal in 2002, the locals feared it signaled the end of the world.
But it is true that a molly can be mated to either a horse or a donkey stallion; sometimes with very interesting results. It has been reported that a mare mule in Brazil has produced two 100% horse sons sired by a horse stallion. In the 1920s, when a molly at Texas A&M named "Old Beck" was bred to a horse stallion, she produced a horse son and when bred to a donkey, she produced mule offspring, including a mule daughter. And according to the American Donkey and Mule Society, there is an unverified case of a mare mule that produced a mule daughter. The daughter was also a fertile mule and produced a horse-like foal with some mule traits which was called a "hule". However, there are no reports as to whether the hule was fertile since it may have been castrated. It has also been reported that female hules may be fertile, but their offspring are usually feeble and die at birth. In another case, a molly gave birth to 2 foals in Nebraska in the mid-1980 and this was the first genetic testing of a mule's offspring. Tests showed no evidence that the mother had passed along any genetic markers from her donkey father, who was also the father of the foals. This is called "hemiclonal transmission". It is now known that in most fertile mule mares, they only pass on their maternal horse DNA.
However, even if sterile, mollies still have the female hormones coursing through their veins, which give them a strong maternal drive. They have been known to kidnap the foals of other equines sharing the same paddock.
The hinny is also a hybrid equine, but it is the result of the opposite mating: a horse stallion crossed with a donkey mare (called a jenny) but it is much less common. They are harder to produce because a stallion/jenny mating is less likely to result in pregnancy. This was once believed to be due to the donkey mare having a smaller womb, but the difficulty in impregnation suggests that it is largely a genetic issue. Donkeys (Equus asinus) have 62 chromosomes while horses (Equus caballus) have 64 chromosomes; infertile mules and hinnies have 63 chromosomes that are a mixture of one from each parent. The chromosomes have different structures and with the difference in number, too, this usually prevents the chromosomes from pairing up properly and creating successful embryos. In attempts among other species to create hybrids, the hybrid is much less likely to occur when the male has more chromosomes than the female, than when it is the other way around. It is almost like the female egg does not recognize sperm that has too many chromosomes, but does not seem to mind if the sperm has too few. Hinnies are also sterile, but a fertile female hinny in China is believed to be a unique case.
Hinnies and mules look slightly different from each other, yet for all practical purposes, hinnies, mollies and mules are classified and shown together under the general term of Mule.
The rarer hinnies are often said to be more horse-like than the mule, but more often it is difficult to tell them apart. The mule has a large head that is more donkey-like than horse-like, while the hinny has a smaller head that is more horse-like. The tail of the hinny is much more like that of the horse than is the tail of the mule. Generally, this is accounted for by the males of both species transmitting with greater power these parts of their structure. Hinnies are smaller and finer boned than mules, simply because of the fact that most donkeys are smaller than horses.
Mules come in every size and shape imaginable and all colors except overo (again due to genetics). Miniature mules under 36" can be seen all the way up to 17 hand Percheron draft mules (by Mammoth Jacks). The Poitou donkey was used exclusively for breeding huge draft mules from a breed of draft horse called the Mullasier - the Mule producer.
Mules' ears are usually somewhat smaller than a donkey's, but longer than the ears of the horse parent, and are roughly the same shape. Every mule will have a unique bray that is a combination of the horse's whinny and the grunting of the donkey's bray. And they show a natural resistance to disease and insects.
Mules are used in the same sports as horses; under saddle, in harness, for cutting, roping or dressage. They have more stamina and can carry more weight than a horse of equal size. This is due to what geneticists call "hybrid vigor". There is one aspect where the mule outshines the horse: high-jumping. Mules only 50 inches tall at the withers have been known to clear jumps of up to 72 inches. Amazingly, these jumps are from a standing start inside a marked area, not from a galloping approach.
Mules are not really stubborn, they simply will not put themselves in danger. A horse can be worked until it drops, but not a mule. That stubborn streak is the mule's way of telling humans that something is not right. It is not a good idea to abuse a mule, for they are clever and seem intelligent enough to plot revenge. However, treated kindly, they will do their best for their owner and with the utmost patience.