Horse

The horse (Equus ferus caballus)^[2][3] is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved (ungulate) mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today. Humans began to domesticate horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Horses in the subspecies caballus are domesticated, although some domesticated populations live in the wild as feral horses. These feral populations are not true wild horses, as this term is used to describe horses that have never been domesticated, such as the endangered Przewalski's Horse, a separate subspecies, and the only remaining true wild horse. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, size, colors, markings, breeds, locomotion, and behavior.
Horses' anatomy enables them to make use of speed to escape predators and they have a well-developed sense of balance and a strong fight-or-flight instinct. Related to this need to flee from predators in the wild is an unusual trait: horses are able to sleep both standing up and lying down. Female horses, called mares, carry their young for approximately 11 months, and a young horse, called a foal, can stand and run shortly following birth. Most domesticated horses begin training under saddle or in harness between the ages of two and four. They reach full adult development by age five, and have an average lifespan of between 25 and 30 years.
Horse breeds are loosely divided into three categories based on general temperament: spirited "hot bloods" with speed and endurance; "cold bloods", such as draft horses and some ponies, suitable for slow, heavy work; and "warmbloods", developed from crosses between hot bloods and cold bloods, often focusing on creating breeds for specific riding purposes, particularly in Europe. There are over 300 breeds of horses in the world today, developed for many different uses.
Horses and humans interact in a wide variety of sport competitions and non-competitive recreational pursuits, as well as in working activities such as police work, agriculture, entertainment, and therapy. Horses were historically used in warfare, from which a wide variety of riding and driving techniques developed, using many different styles of equipment and methods of control. Many products are derived from horses, including meat, milk, hide, hair, bone, and pharmaceuticals extracted from the urine of pregnant mares. Humans provide domesticated horses with food, water and shelter, as well as attention from specialists such as veterinarians and farriers.

Biology

Main article: Equine anatomy

Points of a horse^[4][5]

Specific terms and specialized language is used to describe Horse anatomy, different life stages, colors and breeds.

Lifespan and life stages

Depending on breed, management and environment, the modern domestic horse has a life expectancy of 25 to 30 years.^[6] It is uncommon, but a few animals live into their 40s and, occasionally, beyond.^[7] The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62.^[6] In modern times, Sugar Puff, who had been listed in the Guinness Book of World Records as the world's oldest living pony, died in 2007 at age 56.^[8]
Regardless of a horse's actual birth date, for most competition purposes an animal is considered a year older on January 1 of each year in the northern hemisphere^[6][9] and August 1 in the southern hemisphere.^[10] The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.^[11]
The following terminology is used to describe horses of various ages:

• Foal: a horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling.^[12] Most domesticated foals are weaned at 5 to 7 months of age, although foals can be weaned at 4 months with no adverse physical effects.^[13]
• Yearling: a horse of either sex that is between one and two years old.^[14]
• Colt: a male horse under the age of four.^[15] A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses.^[16]
• Filly: a female horse under the age of four.^[12]
• Mare: a female horse four years old and older.^[17]
• Stallion: a non-castrated male horse four years old and older.^[18] Some people, particularly in the UK, refer to a stallion as a "horse".^[19]
• Gelding: a castrated male horse of any age.^[12]

In horse racing, these definitions may differ: For example, in the British Isles, Thoroughbred horse racing defines colts and fillies as less than five years old.^[20] However, Australian Thoroughbred racing defines colts and fillies as less than four years old.^[21]

Size and measurement

See also: Hand (length)

The height of horses is measured at the highest point of the withers, where the neck meets the back. This point was chosen because it is a stable point of the anatomy, unlike the head or neck, which move up and down.
The English-speaking world measures the height of horses in hands and inches. One hand is equal to 4 inches (101.6 mm). The height is expressed as the number of full hands, followed by a point, then the number of additional inches, then the abbreviation "h" or "hh" (for "hands high"). Thus, a horse described as "15.2 h" is 15 hands (60 inches, 152 cm) plus 2 inches (5.1 cm), for a total of 62 inches (157.5 cm) in height.^[22]

Size varies greatly among horse breeds, as with this full-sized horse and a miniature horse.

The size of horses varies by breed, but also is influenced by nutrition. Light riding horses usually range in height from 14 to 16 hands (56 to 64 inches, 142 to 163 cm) and can weigh from 380 to 550 kilograms (840 to 1,200 lb).^[23] Larger riding horses usually start at about 15.2 hands (62 inches, 157 cm) and often are as tall as 17 hands (68 inches, 173 cm), weighing from 500 to 600 kilograms (1,100 to 1,300 lb).^[24] Heavy or draft horses are usually at least 16 to 18 hands (64 to 72 inches, 163 to 183 cm) high and can weigh from about 700 to 1,000 kilograms (1,500 to 2,200 lb).^[25]
The largest horse in recorded history was probably a Shire horse named Mammoth, who was born in 1848. He stood 21.2½ hands high (86.5 in/220 cm), and his peak weight was estimated at 1,500 kilograms (3,300 lb).^[26] The current record holder for the world's smallest horse is Thumbelina, a fully mature miniature horse affected by dwarfism. She is 17 inches (43 cm) tall and weighs 57 pounds (26 kg).^[27]

Ponies

Main article: Pony

The general rule for height distinguishing between a horse and a pony at maturity is 14.2 hands (58 inches, 147 cm). An animal 14.2 h or over is usually considered to be a horse and one less than 14.2 h a pony.^[28] However, there are many exceptions to the general rule. In Australia, ponies measure under 14 hands (56 inches, 142 cm).^[29] The International Federation for Equestrian Sports, which uses metric measurements, defines the cutoff between horses and ponies at 148 centimetres (58.27 in) (just over 14.2 h) without shoes and 149 centimetres (58.66 in) (just over 14.2½ h) with shoes.^[30] Some breeds which typically produce individuals both under and over 14.2 h consider all animals of that breed to be horses regardless of their height.^[31] Conversely, some pony breeds may have features in common with horses, and individual animals may occasionally mature at over 14.2 h, but are still considered to be ponies.^[32]
The distinction between a horse and pony is not simply a difference in height, but other aspects of phenotype or appearance, such as conformation and temperament. Ponies often exhibit thicker manes, tails, and overall coat. They also have proportionally shorter legs, wider barrels, heavier bone, shorter and thicker necks, and short heads with broad foreheads. They may have calmer temperaments than horses and also a high level of equine intelligence that may or may not be used to cooperate with human handlers.^[28] In fact, small size, by itself, is sometimes not a factor at all. While the Shetland pony stands on average 10 hands (40 inches, 102 cm),^[33] the Falabella and other miniature horses, which can be no taller than 30 inches (76 cm), the size of a medium-sized dog, are classified by their respective registries as very small horses rather than as ponies.^[34]

Colors and markings

Bay and chestnut (sometimes called "sorrel") are two of the most common coat colors, seen in almost all breeds.

Main articles: Equine coat color, Equine coat color genetics, and Horse markings

Horses exhibit a diverse array of coat colors and distinctive markings, described with a specialized vocabulary. Often, a horse is classified first by its coat color, before breed or sex.^[35] Horses of the same color may be distinguished from one another by white markings,^[36] which, along with various spotting patterns, are inherited separately from coat color.^[37]
Many genes that create horse coat colors have been identified, although research continues to further identify factors that result in specific traits. One of the first genetic relationships to be understood was that between recessive "red" (chestnut) and dominant "black" allele that is controlled by the Melanocortin 1 receptor, also known as the "extension gene" or "red factor." Additional genes control suppression of base red and black color to point coloration as seen in bay, spotting patterns such as pinto or leopard, dilutions such as palomino or dun, as well as graying, and all the other factors that create the dozens of possible coat colors found in horses.^[38] These colors can be modified by at least ten other genes to create all other colors.^[38]
Horses which have a white coat color are often mislabeled; a horse that looks "white" is usually a middle-aged or older gray. Grays are born a darker shade, get lighter as they age, and usually have black skin underneath their white hair coat (with the exception of pink skin under white markings). The only horses properly called white are born with a white hair coat and have predominantly pink skin, a fairly rare occurrence.^[39] Different and unrelated genetic factors can produce white coat colors in horses, including several different alleles of dominant white and the sabino-1 gene.^[40] However, there are no "albino" horses, defined as having both pink skin and red eyes.^[41]

Reproduction and development

Main article: Horse breeding

Gestation lasts for approximately 335–340 days^[42] and usually results in one foal. Twins are rare.^[43] Horses are a precocial species, and foals are capable of standing and running within a short time following birth.^[44]
Horses, particularly colts, sometimes are physically capable of reproduction at about 18 months, but domesticated horses are rarely allowed to breed before the age of three, especially females.^[42] Horses four years old are considered mature, although the skeleton normally continues to develop until the age of six; maturation also depends on the horse's size, breed, sex, and quality of care. Also, if the horse is larger, its bones are larger; therefore, not only do the bones take longer to actually form bone tissue, but the epiphyseal plates are also larger and take longer to convert from cartilage to bone. These plates convert after the other parts of the bones, and are crucial to development.^[45]
Depending on maturity, breed, and work expected, horses are usually put under saddle and trained to be ridden between the ages of two and four.^[46] Although Thoroughbred race horses are put on the track as young age two in some countries,^[47] horses specifically bred for sports such as dressage are generally not put under saddle until they are three or four years old, because their bones and muscles are not solidly developed.^[48] For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (5 years) old.^[11]

Anatomy

Main articles: Equine anatomy, Muscular system of the horse, Respiratory system of the horse, and Circulatory system of the horse

Skeletal system

Main article: Skeletal system of the horse

The skeletal system of a modern horse

Horses have a skeleton that averages 205 bones.^[49] A significant difference between the horse skeleton and that of a human, is the lack of a collarbone—the horse's forelimbs are attached to the spinal column by a powerful set of muscles, tendons, and ligaments that attach the shoulder blade to the torso. The horse's legs and hooves are also unique structures. Their leg bones are proportioned differently from those of a human. For example, the body part that is called a horse's "knee" is actually made up of the carpal bones that correspond to the human wrist. Similarly, the hock contains bones equivalent to those in the human ankle and heel. The lower leg bones of a horse correspond to the bones of the human hand or foot, and the fetlock (incorrectly called the "ankle") is actually the proximal sesamoid bones between the cannon bones (a single equivalent to the human metacarpal or metatarsal bones) and the proximal phalanges, located where one finds the "knuckles" of a human. A horse also has no muscles in its legs below the knees and hocks, only skin, hair, bone, tendons, ligaments, cartilage, and the assorted specialized tissues that make up the hoof.^[50]

Hooves

Main articles: Horse hoof, Horseshoe, and Farrier

The critical importance of the feet and legs is summed up by the traditional adage, "no foot, no horse".^[51] The horse hoof begins with the distal phalanges, the equivalent of the human fingertip or tip of the toe, surrounded by cartilage and other specialized, blood-rich soft tissues such as the laminae. The exterior hoof wall and horn of the sole is made of essentially the same material as a human fingernail.^[52] The end result is that a horse, weighing on average 500 kilograms (1,100 lb),^[53] travels on the same bones as would a human on tiptoe.^[54] For the protection of the hoof under certain conditions, some horses have horseshoes placed on their feet by a professional farrier. The hoof continually grows, and in most domesticated horses needs to be trimmed (and horseshoes reset, if used) every five to eight weeks,^[55] though the hooves of horses in the wild wear down and regrow at a rate suitable for their terrain.

Teeth

Main article: Horse teeth

Horses are adapted to grazing. In an adult horse, there are 12 incisors at the front of the mouth, adapted to biting off the grass or other vegetation. There are 24 teeth adapted for chewing, the premolars and molars, at the back of the mouth. Stallions and geldings have four additional teeth just behind the incisors, a type of canine teeth called "tushes". Some horses, both male and female, will also develop one to four very small vestigial teeth in front of the molars, known as "wolf" teeth, which are generally removed because they can interfere with the bit. There is an empty interdental space between the incisors and the molars where the bit rests directly on the gums, or "bars" of the horse's mouth when the horse is bridled.^[56]
An estimate of a horse's age can be made from looking at its teeth. The teeth continue to erupt throughout life and are worn down by grazing. Therefore, the incisors show changes as the horse ages, but a distinct wear and growth pattern, and changes in the angle at which the chewing surfaces meet. This allows a very rough estimate of a horse's age, although diet and veterinary care can also affect the rate of tooth wear.^[6]

Digestion

Main articles: Equine digestive system and Equine nutrition

Horses are herbivores with a digestive system adapted to a forage diet of grasses and other plant material, consumed steadily throughout the day. Therefore, compared to humans, they have a relatively small stomach but very long intestines to facilitate a steady flow of nutrients. A 450-kilogram (990 lb) horse will eat 7 to 11 kilograms (15 to 24 lb) of food per day and, under normal use, drink 38 litres (8.4 imp gal; 10 US gal) to 45 litres (9.9 imp gal; 12 US gal) of water. Horses are not ruminants, so they have only one stomach, like humans, but unlike humans, they can digest cellulose, a major component of grass. Cellulose digestion occurs in the cecum, or "water gut", which food goes through before reaching the large intestine. Unlike humans, horses cannot vomit, so digestion problems can quickly cause colic, a leading cause of death.^[57]

Senses

A horse's eye

See also: Equine vision

The horse's senses are generally superior to those of a human. As prey animals, they must be aware of their surroundings at all times.^[58] They have the largest eyes of any land mammal,^[59] and are lateral-eyed, meaning that their eyes are positioned on the sides of their heads.^[60] This means that horses have a range of vision of more than 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision.^[59] Horses have excellent day and night vision, but they have two-color, or dichromatic vision; their color vision is somewhat like red-green color blindness in humans, where certain colors, especially red and related colors, appear as a shade of green.^[61]
A horse's hearing is good,^[58] and the pinna of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head.^[62] Their sense of smell, while much better than that of humans, is not their strongest asset; they rely to a greater extent on vision.^[58]
Horses have a great sense of balance, due partly to their ability to feel their footing and partly to highly developed proprioception—the unconscious sense of where the body and limbs are at all times.^[63] A horse's sense of touch is well developed. The most sensitive areas are around the eyes, ears, and nose.^[64] Horses sense contact as subtle as an insect landing anywhere on the body.^[65]
Horses have an advanced sense of taste that allows them to sort through fodder to choose what they would most like to eat,^[66] and their prehensile lips can easily sort even the smallest grains. Horses generally will not eat poisonous plants. However, there are exceptions and horses will occasionally eat toxic amounts of poisonous plants even when there is adequate healthy food.^[67]