In relationships, some say that opposites attract, but others believe that it’s better to be birds of a feather. The parents of mules clearly follow the “opposites attract” philosophy. Mules are unusual because they are hybrid animals, meaning that their parents are of different species.

A mule is the offspring of a female horse and a male donkey, and it is widely acknowledged that mules inherit the best qualities of each parent. A typical mule has the size and speed of a horse with the endurance and disposition of a donkey. They are typically longer lived with a longer working life than horses, significantly stronger, able to tolerate temperature extremes, sure-footed and very hearty—rarely becoming sick or lame. Charles Darwin said, “The mule always appears to me a most surprising animal. That a hybrid should possess more reason, memory, obstinacy, social affection, powers of muscular endurance, and length of life, than either of its parents, seems to indicate that art has here outdone nature.”

People who use their mules as work animals often prefer them to horses. Mules have long been valued as sturdy pack animals known for patience, strength and stamina. They make better plow animals when working clay soils, can handle sun and rain better, have harder hooves, are more resistant to problems with disease and insects, and they have an increased willingness to carry heavy loads. They require less food than horses of the same size.

Mules tend to have the body of a horse with the extremities of a donkey. They usually have long ears, a thin and short mane that may be upright and a tail with short hairs like a donkey as well as longer hairs typical of horses. The reverse cross of a male horse mated to a female donkey—a hinny—is far more variable in appearance than mules are. They may look essentially like a horse, like a donkey, or more mule-like in appearance, making it hard to distinguish mules from hinnies. One way to figure out if a horse-donkey cross is a mule or a hinny is to put the animal in a group of mixed equines and see who they choose for company. Equines tend to gravitate towards animals like their mother, so mules choose to be with horses and hinnies pick donkeys as their companions.

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Mules are almost always sterile due to having an odd number of chromosomes. Donkeys have 62 chromosomes, horses have 64 chromosomes and mules have 63. An even number of chromosomes is necessary to divide into pairs to reproduce. Mule offspring are so rare that only around 60 cases have been documented since the 1500s. The Romans used the expression “cum mula peperit” (“when a mule gives birth”) much as we use “when pigs fly” and “when hell freezes over”. Though pregnancy in mules can occasionally happen, no cases of fertile male mules are known.

When it comes to being fruitful and multiplying in the equine world, “birds of a feather” clearly triumphs over “opposites attract”.

Karen B. London, Ph.D. is a Certified Applied Animal Behaviorist, Certified Professional Dog Trainer, author, and an Adjunct Faculty in NAU’s Department of Biological Sciences.

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