How can an animal weighing from 227 to 900 kg sleep on its feet? Horses can doze while standing up, and the figures are striking, and adult horses sleep only 2.5 to 5 hours a day, and 80% of that sleep is in a standing position. Horses spend about 80% of their time standing, even during light sleep, and they sleep in a polyphasic pattern, spread over 5 to 7 episodes, with most sleep between midnight and 4:00 am. A horse stands on a single toe on each foot, and the toe is covered by a hoof. Even though horses stand on their middle digits, they still have parts of all five ancestral toes. Over millions of years the ancestors of horses lost toes, and losing the side toes made the end of a horse's foot lighter. The hoof wall does not contain blood vessels or nerves, and it keeps growing and must be worn or trimmed off, at about 1/4 to 3/8 of an inch a month. The frog of the hoof is a highly elastic wedge-shaped mass that normally touches the ground first.
Horses are grazers that mostly eat grass, and they naturally graze for 12 to 16 hours a day, and a mare gives birth to a baby horse called a foal. Average gestation for horses is 335 days, but ranges from 287 to 419 days, and a foal delivered before day 320 of pregnancy is considered premature. A foal can stand up within about an hour of being born, and a mare's first milk, called colostrum, has antibodies that help protect the foal from infection. A horse's cheek teeth are tall-crowned and specialized for grazing, and they erupt continuously during its life to make up for wear of 3 to 4 mm per year. In a horse's cheek teeth, dentine and cementum wear out faster than enamel, forming dentine cups and enamel ridges, and with increasing age, the dentine content of a horse's chewing surface increases while the enamel content decreases.
The oldest known horse, Hyracotherium, is better known as eohippus, which means the dawn horse. It was a small, dog-sized animal that lived in forests and ate leaves, from around 55 million years ago to around 45 million years ago. It had 4 toes on each front foot and 3 toes on each hind foot, and it had low-crowned teeth, but with the beginnings of the characteristic horse-like ridges on the molars. Horse ancestors shown in textbooks are not necessarily in the direct line that led to modern horses, and the modern horse, Equus caballus, evolved on the North American continent. The Spanish brought horses back to North America in the 1500s, and stray horses became known as mustangs, from the Spanish word mesteno. The Bureau of Land Management estimated 85,466 wild horses and burros on its lands nationwide as of March 1, 2026.
Scientists use ancient DNA to trace where horses came from. A 2021 study of ancient DNA found that today's domestic horses were first domesticated in the Pontic-Caspian steppes, northern Caucasus, around 2200 years BCE. The study identified a domestication center in the Western Eurasian steppes, especially the lower Volga-Don region, and an international team of 162 scientists collected, sequenced and compared 273 horse genomes, from horses that lived between 50,000 and 200 years BC. DNA showed that the 5,500-year-old horses from Botai, Central Asia, were not the ancestors of modern domestic horses. The study also found two genetic changes in modern domestic horses: one linked to more docile behaviour and one to a stronger backbone. The new domestic horse replaced all previous populations across Eurasia within around 500 years of the initial domestication, and horseback riding and the use of spoke-wheeled war chariots supported its spread. People ride horses, and a rider should wear a helmet every time and have a grown-up instructor nearby.
Przewalski's horses live in groups with several mares, a dominant stallion, and their young. They eat mostly grass and some leaves, and they became extinct in the wild, and the last were seen in the late 1960s. They were reintroduced to Mongolia, where hunting has been prohibited since 1930 and the horses are legally protected. In August 2022, 433 Przewalski's horses were living in Hustain National Park in Mongolia, and all living Przewalski's horses are descended from 14 founders. Przewalski's horses have 66 chromosomes, while domestic horses have 64. A 2018 study sequenced the genomes of 20 horses from Botai and 22 horses from across Eurasia, and it suggested that Przewalski's horses descend from early domestic Botai herds that escaped, rather than from horses that were never tamed. Whether they are truly wild is therefore debated, and wild horses live in herds of one or two stallions, a group of mares, and their foals.
Horses also have a mane of coarse hair along the neck. Horses are prey animals that depend on running away to stay safe, and a horse uses its ears to hear, and ears pointed stiffly forward mean the horse is alert. Horses can hear sounds from 14 Hz to 25 kHz, while the human range is 20 Hz to 20 kHz. A horse sees mostly with each eye separately, which gives it a panoramic view, and it has blind spots directly in front of it and directly behind it. In a herd, some horses lie down while at least one stays standing to watch for danger, and special tendons and ligaments let a standing horse stay upright without using much muscle effort. Horses see color, but fewer colors than people do.
Words to know
- polyphasic: in many periods
- gestation: pregnancy length
- genomes: full DNA sets
- chromosomes: DNA bundles
- docile: calm and gentle
