Mule

The mule is the offspring of a male donkey (jackass) and a female horse (mare). The opposite cross-a female donkey (jenny) bred to a male horse (stallion)-produces a hinny. A mule has a donkey's large head and long ears, the body of a horse, and an asslike tail. Its voice is a bray that is somewhat different than that of the ass. Some mules are quite large: as much as sixteen hands (sixty-four inches) at the withers. Mules are strong and durable. They are more surefooted and can bear heavier loads than either the horse or the donkey. This is a good example of hybrid vigor or heterosis: the tendency of hybrids to be larger and more durable than their parent species. It is often said that a mule is smarter than a horse because a horse may overeat and "lounder" (become ill fromovereating) if given unlimited access to grain, but a mule will not. Mules are almost always infertile, apparently due to differing numbers of chromosomes in the gametes of the horse and the donkey.

Mules in History
Mules were essential in transport for at least three thousand years, from ancient times until the invention of the steam engine and the building of railroads. Mules can be ridden but were used mostly in mule teams or mule trains to pull wheeled vehicles in both peace and war. For example, the twenty-mule teams of Death Valley, California, hauled two wagons loaded with borax plus a water wagon with a combined weight of over twenty-seven tons. Wild populations of equids have been greatly reduced by humans through hunting and habitat destruction, some to the point of extinction. Domestic horses, donkeys, and mules, however, have thrived for thousands of years in association with humankind. Despite being largely replaced by mechanical power, domestic equids are still common.