A mule is a sterile cross between a male donkey, or jack, and a female horse. When a stallion is mated with a female donkey, or jenny, the result is a hinny. Mules have been bred for centuries as riding, work, and driving animals, and were widely used throughout Europe and the New World until the automobile began to replace equines as a transportation method. Breeders continue to raise mules as pleasure animals, and many ranches continue to have a working force of mules.
The mule is prized as a work animal because it is intelligent, surefooted, and rugged. Mules can endure hot weather and difficult working conditions which could injure horses, but they are not as difficult to manage as donkeys. Especially in the American West, the durability of the mule was a very valuable trait, as weather and trail conditions could sometimes be extreme. Mules are also known for being stubborn, but they are unusually patient and gentle with beginning riders.
In appearance, a mule superficially resembles a donkey, with a few horse-like traits. Mules have long ears, short manes, and partially hairless tails. They come in a range of sizes, depending on breeding, and are also available in an assortment of colors. Because of their versatility, mules are used in many of the same sports horses are, and sometimes outperform horses in competition. In addition, mules are often chosen as work animals by trail riding vacation companies, as they can carry a great deal of weight and tolerate less skilled riders.
Although a few rare instances of fertile mules have been recorded, the majority of mules are sterile. This is because donkeys have 62 chromosomes, while horses have 64, resulting in 63 chromosomes in a mule. As the chromosomes cannot be split, the mule will be unable to produce offspring. As a general rule, male mules, also called johns, are gelded to reduce behavioral problems. Female mules are known as mollies.
Because mules themselves are sterile, mule breeders maintain stocks of horses and donkeys to breed with, taking note of particularly good crosses and replicating them. Most breeders prefer to breed mules, as they are larger than hinnies, and also easier to breed in general, as female donkeys do not impregnate easily. Mule breeders select for attractive, strong mules with amenable dispositions, and are pleased to match their mules with prospective owners.