See also: Horse sleep patterns and Sleep in non-humans

See also: Horse sleep patterns and Sleep in non-humans

Horses are able to sleep both standing up and lying down. In an adaptation from life in the wild, horses are able to enter light sleep by using a "stay apparatus" in their legs, allowing them to doze without collapsing.^[94] Horses sleep better when in groups because some animals will sleep while others stand guard to watch for predators. A horse kept alone will not sleep well because its instincts are to keep a constant eye out for danger.^[95]
Unlike humans, horses do not sleep in a solid, unbroken period of time, but take many short periods of rest. Horses spend four to fifteen hours a day in standing rest, and from a few minutes to several hours lying down. Total sleep time in a 24-hour period may range from several minutes to a couple of hours,^[95] mostly in short intervals of about 15 minutes each.^[96]
Horses must lie down to reach REM sleep. They only have to lie down for an hour or two every few days to meet their minimum REM sleep requirements.^[95] However, if a horse is never allowed to lie down, after several days it will become sleep-deprived, and in rare cases may suddenly collapse as it involuntarily slips into REM sleep while still standing.^[97] This condition differs from narcolepsy, although horses may also suffer from that disorder.^[98]

Taxonomy and evolution

From left to right: Size development, biometrical changes in the cranium, reduction of toes (left forefoot)

Main articles: Evolution of the horse, Equus (genus), and Equidae

The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not.^[99] Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), the tapir, and the rhinoceros—have survived to the present day.^[100] The earliest known member of the Equidae family was the Hyracotherium, which lived between 45 and 55 million years ago, during the Eocene period. It had 4 toes on each front foot, and 3 toes on each back foot.^[101] The extra toe on the front feet soon disappeared with the Mesohippus, which lived 32 to 37 million years ago.^[102] Over time, the extra side toes shrank in size until they vanished. All that remains of them in modern horses is a set of small vestigial bones on the leg below the knee,^[103] known informally as splint bones.^[104] Their legs also lengthened as their toes disappeared until they were a hooved animal capable of running at great speed.^[103] By about 5 million years ago, the modern Equus had evolved.^[105] Equid teeth also evolved from browsing on soft, tropical plants to adapt to browsing of drier plant material, then to grazing of tougher plains grasses. Thus proto-horses changed from leaf-eating forest-dwellers to grass-eating inhabitants of semi-arid regions worldwide, including the steppes of Eurasia and the Great Plains of North America.
By about 15,000 years ago, Equus ferus was a widespread holarctic species. Horse bones from this time period, the late Pleistocene, are found in Europe, Eurasia, Beringia, and North America.^[106] Yet between 10,000 and 7,600 years ago, the horse became extinct in North America and rare elsewhere.^{[107][108][109]} The reasons for this extinction are not fully known, but one theory notes that extinction in North America paralleled human arrival.^[110] Another theory points to climate change, noting that approximately 12,500 years ago, the grasses characteristic of a steppe ecosystem gave way to shrub tundra, which was covered with unpalatable plants.^[111]

Wild species surviving into modern times

A small herd of Przewalski's Horses

Main article: Wild horse

A truly wild horse is a species or subspecies with no ancestors that were ever domesticated. Therefore, most "wild" horses today are actually feral horses, animals that escaped or were turned loose from domestic herds and the descendants of those animals.^[112] Only one truly wild horse species (Equus ferus) with two subspecies, the Tarpan and the Przewalski's Horse, survived into recorded history.
The only true wild horse alive today is the Przewalski's Horse (Equus ferus przewalskii), named after the Russian explorer Nikolai Przhevalsky. It is a rare Asian animal, also known as the Mongolian Wild Horse; Mongolian people know it as the taki, and the Kyrgyz people call it a kirtag. The species was presumed extinct in the wild between 1969 and 1992, while a small breeding population survived in zoos around the world. In 1992, it was reestablished in the wild due to the conservation efforts of numerous zoos.^[113] Today, a small wild breeding population exists in Mongolia.^[114][115] There are additional animals still maintained at zoos throughout the world.
The Tarpan or European Wild Horse (Equus ferus ferus) was found in Europe and much of Asia. It survived into the historical era, but became extinct in 1909, when the last captive died in a Russian zoo.^[116] Thus, the genetic line was lost. Attempts have been made to recreate the Tarpan,^{[116][117][118]} which resulted in horses with outward physical similarities, but nonetheless descended from domesticated ancestors and not true wild horses.
Periodically, populations of horses in isolated areas are speculated to be relic populations of wild horses, but generally have been proven to be feral or domestic. For example, the Riwoche horse of Tibet was proposed as such,^[115] but testing did not reveal genetic differences with domesticated horses,^[119] Similarly, the Sorraia of Spain was proposed as a direct descendant of the Tarpan based on shared characteristics,^[120][121] but genetic studies have shown that the Sorraia is more closely related to other horse breeds and that the outward similarity is an unreliable measure of relatedness.