Definition of a mule
A mule is a cross between two species of equine: the horse or pony (Equus caballus) and the domestic donkey (Equus asinus). The word 'mule' is used for either the cross of male donkey on female horse or female donkey on male horse, although the latter cross is more correctly known as a 'hinny'.
Despite the fact that both mules and hinnies each have one horse and one donkey parent, the two crosses generally differ from each other in appearance and stature and to some extent in temperament - a fact which has been recognised since they were first bred.

What do mules look like?
The mule proper is said to have the body of a horse with the extremities of a donkey. The most noticeable points are its long ears, short thin mane, which may stand upright like a donkey's or be a little longer and flop over, and a tail which has shortish hairs on the dock a little longer than the donkey's but also has long hairs like the horse's, and is much fuller than the donkey's. The withers are low or non-existent, the back flat with a goose rump, the body flat-sided with weaker quarters than the horse, and also narrower and less deep-shouldered. The legs are, like the donkey's, straight, with small, hard, dense, upright, straight-sided hooves. The head is a little narrower than the donkey's, but otherwise very similar, except for the eyes, which are specifically mule - but difficult to describe.
The hinny generally has the body of a donkey with the extremities of the horse. The ears are shorter than a mule's, the mane and tail longer and thicker, the legs stronger with hooves which are rounder and less upright, the body rounder and deeper, and the head shorter and narrower. The most common coat colour is blue-grey roan. However, it is noticeable that hinnies differ far more from each other than do mules, with every imaginable variation, from being almost indistinguishable from a horse, through being mule-like, to being almost indistinguishable from a donkey.

Advantages of mules
Mules, having hybrid vigour, can grow taller than both parents. Weight for weight they are stronger than horses, and are much longer-lived with much longer working lives, although maturing slightly later. They rarely become ill or lame or suffer wounds, can withstand extremes of temperature, can live on frugal rations, have tremendous stamina and resilience and are exceptionally sure-footed.
Hinnies are said to lack hybrid vigour, and it has always been recognised that they are smaller than mules (although this may partly be due to their being carried in a smaller womb), less strong and with less stamina and hardiness.

Mules' temperament
Mules have a reputation for being obstinate and bad-tempered, but as with donkeys, the mule's legendary stubbornness is in fact a manifestation of its talent for self-preservation. There are times when a human finds this 'talent' annoying, when he is disobeyed by a mule, but there are many other times when it can be a great advantage: if a mule takes care of itself, then it follows that it is also taking great care of its cargo, human or otherwise. It is not for nothing that mules are chosen rather than horses to take tourists down the Grand Canyon! By intelligent handling, it is quite possible to foresee occasions on which a mule is likely to be 'stubborn' and to avoid them.
The undeserved reputation for bad temper is, I believe, due to the mule's unexpectedly sensitive and untrusting nature. Until he has learnt to trust a person, he is worried that the person may do him harm, and will take defensive action (never offensive) by kicking them, should he feel the occasion merits it. And mules are splendid kickers - they kick fast and accurately, and if a mule misses, it is because he intended to. Unfortunately many of the people who have worked with mules over the centuries have not appreciated this sensitivity, and have not understood another characteristic of the mule: that you cannot force him to do anything, but must persuade him, or organise his work so that he is only asked to do those things which he will want to do. Failure to appreciate this has led to many a battle between man and mule, and to the mule's bad reputation.

Mules are highly intelligent - mule devotees would say more intelligent than horses - and are very quick to learn, with a grasp of a situation which often seems little short of miraculous. This means that their handlers need to be quick-witted to stay one jump ahead of them. A well-trained and handled mule is obliging, kind, patient, persevering, calm, tolerant, sensible, loyal, affectionate, playful - and also proud, jealous and calculating. Being so intelligent, a badly trained and handled mule can be a problem.

Hinnies tend to be more donkey-like in temperament, which may be partly due to the fact that they were reared by donkeys, although this is unlikely to be the whole of the story. They are generally quieter, more compliant, less curious, less adventurous and less independent than mules. Being less sensitive and untrusting, they are less likely to kick, preferring to avoid trouble rather than confront it. Despite their reputation for being less useful than mules, there are many examples of their being much appreciated.

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