Horses can doze while standing up, and the figures behind that habit are striking. 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. In a herd, some horses lie down while at least one stays standing to watch for danger, and horses are prey animals that depend on running away to stay safe.
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. Horses see color, but fewer colors than people do.
A horse stands on a single toe on each foot, and the toe is covered by a hoof. Over millions of years the ancestors of horses lost toes, and losing the side toes made the end of a horse's foot lighter. 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, 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. Their cheek teeth are tall-crowned and specialized for grazing, and they erupt continuously during a horse's 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.
A mare gives birth to a baby horse called a foal. A mare carries her foal for about 11 months, which is usually 335 to 342 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.
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. The modern horse, Equus caballus, evolved on the North American continent.
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, which came 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. 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. Wild horses live in herds of one or two stallions, a group of mares, and their foals. Przewalski's horses became extinct in the wild, and the last were seen in the late 1960s, but they were reintroduced to Mongolia. In August 2022, 433 Przewalski's horses were living in Hustain National Park in Mongolia. Przewalski's horses have 66 chromosomes, while domestic horses have 64. A 2018 study found that they have domestic ancestors, and it suggested that they descend from early domestic Botai herds that escaped. Whether they are truly wild is therefore debated.
Words to know
- premature: born too early
- dentine: tooth bone
- chromosomes: DNA bundles
- domesticated: tamed for people
- reintroduced: brought back
