A mule is the offspring of a jackass (yes, that is really what a male donkey is called, the females are known as jennies) and a female horse. When this is reversed, and a female donkey and a male horse breed, the offspring is known as a hinny. Because a mule is a hybrid, the chances of it reproducing are extremely slim, although it has been known to happen.
Mules have larger heads than horses, and huge ears. Their ears can measure as much as 33 inches from tip to base. Their ears are sensitive to being touched, and much can be learned about a mule's mood by watching the ears. When a mule's ears are perked forward, the animal is intently watching or listening for something. This trait has alerted many mule skinners to danger long before they would have become aware of it using human senses. A mule with ears folded back is angry, and likely to bite or kick.
Mules vary in size, depending on their specific breed. Today there are five mule breeds. The smallest is the pack mule, standing about 4 feet at the shoulders and weighing about 600 pounds. Cotton mules are the next largest, and are the result of a burro sire and a pony mother. They were bred to carry cotton sacks on southern plantations. Their size was such that slaves picking the cotton didn't have to stand up to put cotton into the bags carried by the mule. Farm mules are slightly larger than cotton mules, but smaller than the sugar mule, who weighs in at 1,150 pounds. The largest and heaviest mule breed is the draft mule, who weighs over 3,000 pounds and is the mule used in the famous twenty mule trains that hauled borax from Death Valley in the 19th century.
The first mules in the United States were bred by George Washington. He imported the first jackasses to America and began a mule breeding program in 1785 to furnish mules to the US Army. Mules were the backbone of the military until the invention of the internal combustion engine.
Mules are stronger than horses, having inherited the heavier, thicker bones from their donkey father. This also makes them slower than most horses as well, however. Mules are also credited with being very intelligent and quite stubborn at times. Mules also have a very long memory and have been known to remember a person who mistreated them for many years. Mules live over 30 years, so that's a pretty long memory.