This family, made up of the horses, asses and zebras, contains one genus with nine species. Domestic equids range worldwide; in the wild equids occur mainly in East Africa and the Near East to Mongolia. They inhabit a variety of habitats from lush grasslands and savanna to sandy and stony deserts.
Equids are generally thick-skulled animals with stocky bodies. They are heavily haired, but the length of hair is variable. Most species have a mane on the neck and a lock of hair on the forepart of the head known as a forelock. Some are swift runners: these have long thin limbs with only one functional digit (mesaxonic). Equids walk on the tips of their toes (unguligrade). In the equid foreleg, radius and ulna are united, and the ulna is greatly reduced so that all weight is born on the radius. In the hind leg, the enlarged tibia supports the weight and the fibula is reduced and fused to the tibia. Wild equids are large animals, ranging in body size from around 200 to 500 kg. Their domestic descendants are more variable, varying from less than 140 kg to over 1000 kg.
Equids have 40-42 teeth with a dental formula 3/3, 1/1, 3-4/3, 3/3. The canines are vestigial or absent in females. Their cheek teeth have a complex structure; they are hypsodont with four main columns and various infoldings with much cement. Age of an equid is often estimated by the degree to which surface pattern of cheek teeth is worn, but the abrasive character of food plays too large a role in tooth wear to make this entirely accurate.
Equid skulls are long with the nasal bones long, narrow and freely projecting anteriorly to points. The orbit is far back in the skull, behind the teeth, and the postorbital processes are broad. Tympanic bullae are small.
Females come into heat several times a year or until they become pregnant. Most species give birth every 2 years to a single offspring after a gestation period of 11-13 months. Weaning occurs after about 6-8 months and offspring become sexually mature at about 2 years. Potential lifespan is 25-35 years.
All equids are relatively swift, alert runners and generally flee from danger rather than fight. However, among their own kind or in attempting defense, they kick with the hind feet, strike with the forefeet and sometimes bite. Equids are active both day and night but are mainly crepuscular. They are entirely herbivorous, feeding largely on grass and some browse. Most drink water daily, although they can go without water for long periods of time.
Equids are polygynous herd animals that generally live in extended family groups occupying large territories in open country (grasslands, semi-arid areas, deserts, and mountains). Communication of moods and other information takes place with changes in ear, mouth, and tail positions. Also, some vocal communication through nickering takes place in horses and zebras.
The fossil record of horses is especially rich. It has provided classic examples that supposedly document gradual change in teeth and limbs. The first horse, Hyracotherium (=Eohippus), is known from the early Eocene and appears to have been derived from a condylarth. It was a small animal with relatively simple quadrate teeth, a modestly enlarged third metacarpal, and digitigrade stance. Through the Oligocene, Miocene, and Pliocene horses increased in size, their lateral digits shrank and lost contact with the ground, their brains enlarged, and their molariform teeth became much more complex. The first true grazer, Merychippus, lived in Miocene times. Its cheek teeth were hypsodont and had strongly developed lophs on their occlusal surfaces. Three of its toes contacted the ground. The first one-toed horse, Pliohippus, lived in the late Miocene. The genus Equus first appeared during the Pliocene. Horses were once widespread, inhabiting temporal grasslands, savannahs, and steppe habitats through North America, Asia, Africa, and Europe. North America has been the center of equid evolution. Equids disappeared completely from that continent around 8000 years ago, not to return until Europeans brought them in their ships a few hundred years ago.
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