Mule

• 1. Introduction

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  The mule, a hybrid born from a male donkey and a female horse, once pulled twenty-mule teams of borax wagons out of California's Death Valley, hauling ten short tons of ore per trip between 1883 and 1889. That partnership of eighteen mules and two horses was only one late chapter in a story that stretches back thousands of years, to the point where the range of the domestic horse first overlapped with that of the domestic ass in Western Asia. Hittites once valued a mule at three times the price of a good horse. The Sumerians rated one worth seven donkeys. What is it about crossing a horse and a donkey that made the result more prized, in some times and places, than either parent, and why does breeding one still come down to luck rather than certainty?

• 2. Naming The Hybrid

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  A female mule carries the name molly, or more formally a mare mule, while a male is called a john, or a horse mule. A young male is a mule colt and a young female a mule filly. The donkey used to sire mules is known as a mule jack, and a group of mules is usually called a pack, though the terms barren and span are also used. The hybrid is easier to produce than its reverse pairing, a hinny, which comes from a male horse and a female donkey. Breeders across history have needed this precise vocabulary because the animal itself sits between two species, sharing traits from both parents in ways that are visible from the moment it is born.

• 3. An Ancient Cross Between Species

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  Breeding mules only became possible once the range of the domestic horse, which originated in Central Asia around 3500 BC, extended into the territory of the domestic ass from north-eastern Africa. That overlap likely happened in Anatolia and Mesopotamia, and mules were being bred there before 1000 BC. A painting in the Tomb of Nebamun at Thebes, dating from around 1350 BC, shows a chariot pulled by a pair of animals that have been identified variously as onagers, mules, or hinnies. Mules also appear in Israel and Judah during the time of King David, and Mesopotamian art from the first millennium BC depicts them repeatedly, including bas-reliefs from the Lion Hunt of Ashurbanipal at the North Palace of Nineveh, which show two mules loaded with hunting nets. Homer recorded their arrival in Asia Minor in the Iliad, set around 800 BC.

• 4. George Washington's Mules

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  After the American Revolutionary War, George Washington began breeding mules at his Virginia estate, Mount Vernon, sending his donkeys out to farms across the country to sire them with local horses. He found that mules worked harder and longer than horses on the same tasks while needing less food and water, and he came to be called the father of the American mule. In 1785 Mount Vernon held 132 horses; by 1799 the count had shifted to 27 horses, 20 donkeys, and 63 mules. Mules had been uncommon in the United States before Washington, who judged the animals more docile than donkeys and cheaper to maintain. His breeding program helped set the animal on a path toward becoming a fixture of American farm labor in the century that followed.

• 5. Working Animals At War And In The Field

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  By the nineteenth century, mules were the preferred draught animal on many American farms and for pulling boats, and their national numbers reached 885,000, concentrated more in the south than the north. Twenty-mule teams of eighteen mules and two horses hauled borax ore out of Death Valley to the Borate mines before railroads took over the route. Armies also relied on mules to move supplies, sometimes as mobile platforms for small cannons, and to haul heavier field guns over mountain trails, including during the Second Anglo-Afghan War in Afghanistan. That military and agricultural demand faded in the second half of the twentieth century, as diesel tractors and motorized transport replaced mules across industrialised countries. Even as their working numbers fell, the mule found a new kind of significance: the first cloned equine, a foal named Idaho Gem, was produced by nuclear transfer of cells from foetal material and born at the University of Idaho in Moscow, Idaho, on the 5th of May 2003, the first hybrid animal of any kind ever cloned.

• 6. Hybrid Vigor

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  Charles Darwin wrote that the mule always struck him as a most surprising animal, one whose reason, memory, obstinacy, social affection, muscular endurance, and length of life seemed to exceed either parent, as though art had here outdone nature. Mules take intelligence, sure-footedness, toughness, endurance, and natural cautiousness from their donkey fathers, while inheriting speed, conformation, and agility from horses. They grow faster and live longer than either parent species, and they rarely fall ill the way horses often do with worms. Like donkeys, mules can see their own hind legs and have hooves suited to dry climates, and their skin is tougher than that of either parent. In size they range from miniature mules under 125 centimetres to draught mules standing up to 180 centimetres at the withers, with a median weight between about 370 and 460 kilograms. Their coats can be any color seen in a horse or donkey, often marked with the pale light points typical of donkeys around the muzzle and eyes, along with primitive dorsal, shoulder, or zebra-like leg stripes. Mules are also reputed to be more cognitively intelligent than either parent, though the evidence is thin, resting on preliminary results from only a couple of studies using a limited set of cognitive tests and few subjects. Like other equines they can sleep standing up, and a group will sometimes post members to stand watch while the rest sleep on the ground.

• 7. An Odd Number Of Chromosomes

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  A horse carries 64 chromosomes and a donkey 62, the product of a split between the two lineages that occurred somewhere between 7.7 and 15.4 million years ago. A mule inherits 63, 32 from its horse mother and 31 from its donkey father, and that odd number makes gamete formation difficult, leaving most mules infertile and disqualifying the mule as a species in its own right under the biological species concept. Conception is complicated further by behavior: groups of horses and groups of donkeys rarely interact, with donkeys sitting at the bottom of the equine social hierarchy, horses ruling the pasture, and mules occupying a mid-tier caste between them. Herodotus recorded a mule giving birth as an ill omen before Xerxes' invasion of Greece in 480 BC, writing that at Sardis a mule brought forth young and gave birth to a mule. Between 1527 and 2002, roughly sixty such births were documented, including confirmed cases in Morocco in 2002 and Colorado in 2007, where blood and hair samples verified the mother was genuinely a mule. A 1939 article in the Journal of Heredity described a fertile mare mule named Old Bec, owned by Texas A&M University in the late 1920s, whose foals by a jack and by a Saddlebred stallion showed sterility and pure horse traits respectively. In 1995 researchers at the Federal University of Minas Gerais documented a mule pregnant for a seventh time, having already produced foals with the standard mule count of 63 chromosomes as well as horse-like foals carrying 64 chromosomes; the elder two of those horse-like foals had already proved fertile, with typical horse progeny of their own.

• 8. Uses And Daily Care

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  As of 2018, Mexico held the largest population of mules in the world, at 3,287,449 animals. Mules still serve as pack animals, generally carrying dead weight up to around 90 kilograms, and specimens trained by the Army of Pakistan have carried 72 kilograms over 26 kilometres without resting. In Nepal, the World Food Programme delivers food to children in Sudurpashchim Province with the help of mule drivers navigating remote terrain. In the Abruzzo region of Italy, mules are used to guard herds against wolves and feral dogs, and horse breeders in Italy have long added a female mule to their flocks after finding it protects foals more fiercely than the mares themselves; mules are also used as research specimens in studies of chromosome slicing, rearrangement, and compatibility. A mule's diet leans closer to a donkey's than a horse's, and it can drink 15 gallons of water a day on a mix of grains, hay, and greens, with fruits and vegetables as occasional additions and its protein intake watched for essential amino acids. Its hooves need shortening at least every two months to prevent overgrowth and pain, and it typically wears a smaller horseshoe, or a muleshoe, than a horse of comparable size. A healthy adult mule runs a temperature between 37.5 and 38.5 °C, with a pulse of 26 to 40 beats per minute and 8 to 16 breaths per minute.