The size of a mule and work to which it is put depend largely on the breeding of the mule's female parent (dam). Mules can be lightweight, medium weight or when produced from draft horse mares, of moderately heavy weight. Mules are reputed to be more patient, hardy and long-lived than horses and are described as less obstinate and more intelligent than donkeys.
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In general, a mule can be packed with dead weight of up to 20% of its body weight, or approximately 90 kg (198 lb).
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. A male mule is properly called a horse mule, though often called a john mule, which is the correct term for a gelded mule. A young male mule is called a mule colt, and a young female is called a mule filly.
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A hinny is a domestic equine hybrid that is the offspring of a male horse, a stallion, and a female donkey, a jenny. It is the reciprocal cross to the more common mule, which is the product of a male donkey, a jack, and a female horse, a mare. The hinny is distinctive from the mule both in physiology and temperament as a consequence of genomic imprinting.