A hybrid animal that is the offspring of a mare (female horse) and a jack (male donkey). The offspring of a stallion (male horse) and a jenny (female donkey) is a hinny, which is slightly smaller than a mule and more difficult to produce by cross-breeding. Mules vary greatly in size and appearance, but are valued because they are stronger and require less food than a horse of similar size and have the endurance and temperament of a donkey. A mule is also more intelligent than either of its parents. Most mules are sterile, because its parents have different numbers of chromosomes. Its superior qualities result from hybrid vigour (heterosis).