A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny (the offspring of a male horse and a female donkey). While there is no known instance of a male mule siring offspring, female mules have on very rare occasion given birth to viable offspring. The size of a mule and work to which it is put depends largely on the breeding of the mule's dam. Mules can be lightweight, medium weight, or even, when produced from draught horse mares, of moderately heavy weight.
It has been claimed that mules are "more patient, sure-footed, hardy and long-lived than horses, and they are considered less obstinate, faster, and more intelligent than donkeys."
A female mule that has estrus cycles and thus, in theory, could carry a fetus, is called a "molly" or "Molly mule," though the term is sometimes used to refer to female mules in general. Pregnancy is rare, but can occasionally occur naturally as well as through embryo transfer. One of several terms for a gelded mule is a "John mule."
A pack animal or beast of burden is a working animal used by humans as means of transporting materials by attaching them so their weight bears on the animal's back; the term may be applied to either an individual animal or a species so employed. The term pack animal is sometimes used in contrast to draft animal, which is a working animal that typically pulls a load behind itself (such as a plow or a wheeled cart) rather than carrying cargo directly on its back.
The term is not routinely applied to humans carrying loads on their backs except to make a pejorative point about the injustice of so employing them, or about the privation that usually occasions accepting such work without explicit coercion. (The 1978 Rolling Stones song "Beast of Burden" refers to a sense of abuse, accepted within a romantic relationship.) Nevertheless, from a physical point of view, certainly many considerations apply equally to human and other pack animals, without considering the range of social conditions ranging through slaves, abused women and children, Himalayan and African natives employed as expedition porters, vacationing students whose duties as staff of mountaineering huts include packing heavy loads of supplies up steep slopes, and purely recreational hikers and backpackers including both short-trip ones, and long-trip backpackers who court injury and emaciation in carrying their heavy loads.