How can an animal weighing from 227 to 900 kg sleep on its feet? Horses can doze while standing up. Horses spend about 80% of their time standing, even during light sleep, and adult horses sleep only 2.5 to 5 hours a day, with 80% of that sleep in a standing position. Special tendons and ligaments let a standing horse stay upright without using much muscle effort. Horses are also grazers that mostly eat grass, and they naturally graze for 12 to 16 hours a day. A horse stands on a single toe on each foot, and the toe is covered by a hoof, and even though horses stand on their middle digits, they still have parts of all five ancestral toes. The hoof wall does not contain blood vessels or nerves, and it keeps growing and must be worn or trimmed off.
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, while in a horse's cheek teeth, dentine and cementum wear out faster than enamel, forming dentine cups and enamel ridges. 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, and it had low-crowned teeth, but with the beginnings of the characteristic horse-like ridges on the molars. Over millions of years the ancestors of horses lost toes. Horse ancestors shown in textbooks are not necessarily in the direct line that led to modern horses.
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. 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. One study of ancient DNA suggested there were two independent events of horse domestication. The 2021 study found two genetic changes in modern domestic horses: one linked to more docile behaviour and one to a stronger backbone. After domestication, the new domestic horses replaced all the wild horse populations from the Atlantic to Mongolia within a few centuries. 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, and they eat mostly grass and some leaves. 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. All living Przewalski's horses are descended from 14 founders, and they have 66 chromosomes, while domestic horses have 64. A 2018 DNA study suggested that they descend from early domestic Botai herds that escaped, so whether they are truly wild is debated. Wild horses live in herds of one or two stallions, a group of mares, and their foals.
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. 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. 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. It 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.
Words to know
- gestation: pregnancy length
- dentine: tooth bone
- docile: calm and gentle
- chromosomes: DNA bundles
- alert: ready to act
