Horse

• 1. Introduction

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  The horse, Equus ferus caballus, travels on the same bones a human would use to walk on tiptoe. It carries an average weight of 500 kg on a single toe per foot, capped in keratin no different from a fingernail. This one-toed, hoofed mammal evolved over the past 45 to 55 million years from a small multi-toed creature called Eohippus, an animal that first appeared in North America. Somewhere around 4000 BCE in Central Asia, people began to tame it. By 3000 BCE that taming had spread widely. How does an animal built entirely to flee end up pulling plows, charging into battle, and calming people in prison? What separates a horse from a pony, a true wild horse from a feral one, a hot blood from a cold blood? And why can this creature fall asleep on its feet, yet collapse if it is never allowed to lie down? The answers live in its body, its mind, and its long entanglement with us.

• 2. Built To Flee

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  More than 350 degrees of vision wraps around a horse's head, because its eyes sit on the sides rather than the front. Of that range, roughly 65 degrees is binocular and the remaining 285 degrees is monocular. The equine eye is one of the largest of any land mammal, with excellent day and night sight but dichromatic, two-color vision, so red and related colors appear as a shade of green. This is the architecture of a prey animal that must watch its surroundings at all times.

  Each ear can rotate up to 180 degrees, giving a horse the potential for 360-degree hearing without moving its head. Sound shapes behavior in measurable ways. A 2013 study in the UK found that stabled horses stayed calmest in quiet settings or while listening to country or classical music, but grew nervous with jazz or rock, and the study recommended keeping music under 21 decibels. An Australian study found that stabled racehorses exposed to talk radio had a higher rate of gastric ulcers than those listening to music.

  The first reaction to a threat is to startle and flee, though a horse will stand and defend itself when flight is impossible or its young are threatened. Curiosity tempers the instinct. When startled, a horse often hesitates an instant to find the cause and may not flee at all from something it judges harmless. Balance underpins the escape, drawn partly from the ability to feel footing and partly from highly developed proprioception, the unconscious sense of where the body and limbs are at all times. So sensitive is the skin that a horse can feel an insect landing anywhere on its body.

• 3. The One Toe

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  Splint bones, a set of small vestigial bones below the knee, are the only trace left of the toes a horse once had. The earliest known member of the family Equidae, Hyracotherium, lived between 45 and 55 million years ago during the Eocene and carried four toes on each front foot and three on each back foot. The extra front toe vanished with Mesohippus, which lived 32 to 37 million years ago, and the side toes shrank away over time as the legs lengthened. By about 5 million years ago the modern Equus had evolved.

  The skeleton that resulted averages 205 bones, and it differs from a human's in a telling way. A horse has no collarbone; its forelimbs attach to the spinal column through a powerful set of muscles, tendons, and ligaments that bind the shoulder blade to the torso. The part called the knee is really the carpal bones of a human wrist, and the hock holds bones equivalent to the human ankle and heel. Below the knees and hocks a horse has no muscles at all, only skin, hair, bone, tendons, ligaments, cartilage, and the specialized tissues of the hoof.

  That hoof grows continuously, and in most domesticated horses it must be trimmed every five to eight weeks. The traditional adage sums up the stakes, "no foot, no horse". Horses in the wild wear down and regrow the hoof at a rate suited to their terrain, while domestic horses may have horseshoes set by a professional farrier. The same single-toed limb that carries this weight also lets the animal reach a galloping speed averaging 40 to 48 km/h, with a recorded sprint record of 70.76 km/h.

• 4. A Gut For Grass

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  Twelve incisors sit at the front of an adult horse's mouth, shaped for biting off grass, with 24 premolars and molars at the back for chewing. Between them lies an empty interdental space, the bars, where a bit rests directly on the gums when the horse is bridled. Stallions and geldings carry four extra canine teeth called tushes, and some horses develop tiny vestigial wolf teeth that are usually removed because they interfere with the bit. Because the teeth keep erupting through life and wear down by grazing, the incisors offer a very rough estimate of a horse's age.

  A forage diet eaten steadily through the day drives the whole digestive design. A horse has a relatively small stomach but very long intestines, and a 450 kg horse will eat 7 to 11 kg of food and drink 38 to 45 L of water in a day. Horses are not ruminants, having only one stomach, yet they digest cellulose through hindgut fermentation, where symbiotic bacteria break it down in the cecum and large intestine. They have no gallbladder but tolerate high amounts of fat.

  One anatomical fact turns digestion dangerous. Horses cannot vomit, so digestive trouble can quickly become colic, a leading cause of death. Their advanced sense of taste lets them sort fodder, and prehensile lips can pick out even small grains. Generally they avoid poisonous plants, but there are exceptions, and a horse will occasionally eat toxic amounts even when healthy food is available.

• 5. Standing Sleep

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  A stay apparatus in the legs lets a horse doze without collapsing, an adaptation carried over from life in the wild. Horses sleep better in groups, because some animals rest while others stand guard against predators, and a horse kept alone sleeps poorly because instinct keeps it watching for danger. Rather than one unbroken stretch, sleep comes in many short periods, mostly intervals of about 15 minutes. The average sleep time of a domestic horse is said to be 2.9 hours per day.

  REM sleep, though, demands that a horse lie down. It only needs to lie down for an hour or two every few days to meet its minimum requirement. A horse never allowed to lie down will grow sleep-deprived after several days, and in rare cases may suddenly collapse as it slips involuntarily into REM sleep while still standing. This differs from narcolepsy, though horses can suffer from that disorder too.

  Younger horses tend to sleep significantly more than adults, and the precocial newborn arrives ready to move. A foal can stand and run within a short time of birth, after a gestation lasting roughly 340 days that almost always produces a single foal, with twins rare. Most foals are weaned between four and six months of age.

• 6. Hot, Cold, And Warm

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  "Hot bloods" such as the Akhal-Teke, the Arabian, the Barb, and the now-extinct Turkoman horse are spirited, bold, and quick to learn. Bred for agility and speed, they tend to be thin-skinned, slim, and long-legged. Over the centuries European breeders imported these oriental horses from the Middle East and Northern Africa to sharpen their best racing and light cavalry stock. The Thoroughbred itself was developed in England from those older oriental breeds.

  "Cold bloods" carry the opposite temperament, bred for strength and the calm patience needed to pull a plow or a heavy carriage. Nicknamed "gentle giants", they include the Belgian and the Clydesdale. The Percheron is lighter and livelier, developed to pull carriages or plow drier fields, while the Shire is slower and more powerful, bred for heavy clay soils. The largest horse in recorded history was a Shire named Mammoth, born in 1848, whose peak weight was estimated at 1524 kg.

  "Warmblood" breeds such as the Trakehner and Hanoverian arose when European carriage and war horses were crossed with Arabians or Thoroughbreds, producing a riding horse with more refinement than a draft horse but greater size and milder temperament than a light breed. Strictly, the term describes any cross between cold-blooded and hot-blooded breeds, like the Irish Draught or the Cleveland Bay. These categories of temperament now sit alongside more than 300 distinct breeds, each shaped by a need for form to function.

• 7. Wild, Feral, And Lost

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  Only two truly wild subspecies survived into recorded history, the tarpan and Przewalski's horse, and only the latter survives today. The Przewalski's horse, named after the Russian explorer Nikolai Przhevalsky, is known in Mongolia as the taki and to the Kyrgyz as the kirtag. It was presumed extinct in the wild between 1969 and 1992, kept alive by a small breeding population in zoos, then reestablished in the wild in 1992 through the efforts of numerous zoos.

  The tarpan met a darker end. The European wild horse ranged across Europe and much of Asia, surviving into the historical era before becoming extinct in 1909, when the last captive died in a Russian zoo, and its genetic line was lost. Attempts to recreate it produced horses that look similar but descend from domesticated ancestors. The same disappointment met other candidates. The Riwoche horse of Tibet and the Sorraia of Portugal were each proposed as relict wild populations, but genetic testing tied both to domesticated horses.

  Most so-called wild horses are in fact feral, descended from domesticated animals that escaped or were turned loose. Studies of feral herds reveal a clear social order led by a dominant individual, usually a mare. Even Przewalski's claim to wildness has been questioned, after domestic horses of the 5,000-year-old Botai culture of Central Asia were found more closely related to Przewalski's horses than to E. f. caballus. That Botai population appears to have been an independent domestication attempt that left no genetic mark on modern domesticated horses.

• 8. The Long Partnership

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  Horse remains buried with chariots in the graves of the Indo-European Sintashta and Petrovka cultures, dated to around 2100 BCE, offer the most irrefutable evidence of domestication. By 2000 BCE horse bones had multiplied sharply in human settlements across northwestern Europe. The genetics tell a lopsided story of how it happened. Very few wild stallions contributed to the domestic horse, shown by very low Y-chromosome variability, while a great deal of mitochondrial variation reveals that many mares joined early herds. The DOM2 gene that all modern domesticated horses carry spread rapidly around 2200 BCE, alongside the Sintashta culture and the spoke-wheeled chariot.

  The scale of that partnership today is enormous. The Food and Agriculture Organization estimated almost 59,000,000 horses worldwide in 2008, with around 33,500,000 in the Americas. An estimated 100 million horses, donkeys, and mules still work in agriculture and transport in less developed areas, including about 27 million working animals in Africa alone. In the United States, the American Horse Council put the direct economic impact of horse-related activity at over 39 billion dollars, supporting 1.4 million full-time jobs.

  The relationship reaches into culture and even slaughter. Horses appear in the mythologies of Greco-Roman, Hindu, Islamic, and Germanic traditions, and in the 12-year cycle of the Chinese zodiac. The Mongols fermented mare's milk into kumis and drank their own horses' blood to ride for extended periods without stopping to eat. Approximately 5 million horses are slaughtered each year for meat worldwide. From the same animal comes a stranger product still, the Italian spinto, a probe sharpened from the horse tibia and used to test whether a curing pig ham is ready.