Equids are herbivorous mammals belonging to the family Equidae (also called horse family), which includes horses, asses, and zebras. They have long limbs with a single toe and, hence, are part of the order Perissodactyla (odd-toed ungulates), just like tapirs and rhinoceroses. These animals have a large head and a sturdy neck covered with a long strip of coarse hair called the mane.
This group is believed to have evolved around 50 million years ago from tiny, multi-toed ungulates and currently comprises 7 extant species under a single genus, Equus. They largely depend on a herbivorous diet and are hindgut fermenters equipped with high-crowned (hypsodont) teeth for shearing tough plant parts.
Horses typically measure about 8 ft (2.4 m) in length from nose to tail. Plains zebras are slightly smaller, ranging from 85 to 97 in (217 to 246 cm), while mountain zebras exhibit more variation, measuring between 83 and 102 in (210 to 260 cm). African wild asses reach a length of approximately 6.6 ft (2 m), whereas Asiatic wild asses range from 6 ft 11 in to 8 ft 2 in (2.1 to 2.5 m).
Wild equids typically range between 440 and 1,100 lb (200 to 500 kg) in weight, but their domestic counterparts show wider variation, measuring around 308 to 2,204 lb (140 to 1,000 kg).
These mammals have stocky bodies with large heads, sturdy necks, and long, slender limbs equipped with a single functional toe. Most equids feature a distinctive mane, a strip of long, bristly hair running along their neck, and a forelock, a section of hair on the front of their head. Their tails are tipped with long tufts of hair, often referred to as tassels.
In their forelimbs, the radius and ulna are fused, with the ulna being significantly reduced, leaving the radius to support the animal’s weight. Similarly, in their hind limbs, the tibia and fibula are fused, with the fibula being greatly reduced.
Their skulls are characterized by long, narrow nasal bones, small tympanic bullae, and broad post-orbital processes (thick-skulled). The orbit lies far back in the skull and is separated from the temporal fossa by a post-orbital plate.
They are hypsodonts, having teeth with high crowns and short roots. The canines are either vestigial or absent altogether in females.
The dental formula of equids is 3.1.3-4.3/3.1.3.3 = 40-42.
Currently, the 7 extant species of equids are all grouped under a single genus, Equus, which falls under the subfamily Equinae and tribe Equini.
The earliest equid fossils have been unearthed from North America, dating back to the Early Eocene Epoch, about 54 million years ago. These early equids, including members of the genus Hyracotherium, were similar to the size of foxes, with three toes on the hindlimbs and four on the front limbs. They browsed and fed on soft plants and were already adapted for running with a digitigrade stance. These ancestors also had modestly enlarged third metacarpals, quadrate teeth, and enlarged brains, indicating their high intelligence. With time, the number of toes in their feet reduced, and they gradually developed teeth adapted to feed on tough plant matter.
The diversification of this group began during the Miocene Epoch. Around this time, they developed distinct horse-like traits, with Merychippus, the first true grazer, having hypsodont teeth adapted for grazing. The first one-toed horse, Pliohippus, also appeared during this time.
Members of the genus Equus evolved during the Early Pleistocene Epoch and continued to diversify over the years.
Although horses were once abundant in North America, they completely disappeared from the continent around 8,000 years ago.
Wild equids typically range from East Africa and the Near East to Mongolia in East Asia, whereas domesticated equids are almost found worldwide. Some isolated populations of wild horses, in particular, are found in Bosnia, Croatia, New Zealand, Portugal, Scotland, and islands along the Atlantic coast of North America.
Although these animals live in a variety of habitats, they are mostly found in grasslands and savannas. The African wild ass, however, lives in the stony deserts of North Africa, while the Asian wild ass species inhabit the dry deserts of the Near East and Central Asia.
The plains zebra thrives in the lush grasslands of Eastern and Southern Africa, whereas the mountain zebra is restricted to the mountainous regions of southwest Africa.
They are herbivorous and primarily consume grasses and sedges. However, some equids, like asses, also consume buds, fruits, roots, and bark, if their preferred resources are unavailable.
These animals are most active during twilight and are thus crepuscular. They are unguligrade and walk on the tips of their toes.
They typically exist in two basic social structures. The first, observed in horses and zebras, includes closed family groups called harems. These groups comprise a single adult male, several females, and their offspring. Hence, harems maintain a polygynous mating system, where a single male mates with multiple females. The members of harems (typically nomadic) maintain overlapping home ranges, which they actively defend from predators.
In the second social structure, often observed in Grévy’s zebras and wild asses, the adult males establish large territories near standing water, where they are more likely to find sexually receptive females. These females enter the territories and are monopolized by the males.
In both these systems, males who are not yet capable of forming a harem or territory live in separate bachelor groups.
Equids are swift runners, relying on flight rather than confrontation when faced with danger. However, they can defend themselves by kicking with their hindlimbs, striking with their forelimbs, or biting their attackers.
These mammals have an average lifespan of 25 to 35 years.
Male equids mate with females during estrus or heat, a 10 to 15-day phase when females are sexually receptive. During this phase, the females stand with their hindlimbs spread and raise their tails to assert their willingness to mate. The males sense female receptivity by raising their upper lip and flashing their teeth (Flehmen response).
After mating, most equids give birth to a single young (foal) after a gestation period of 11 to 13 months. The foal is capable of running within an hour of its birth and is typically weaned in 6 to 8 months, though in some species, it may be nursed till 13 months after birth. The young become sexually mature at about 2 years.
Equids are preyed upon by carnivorous mammals, like lions, leopards, cheetahs, wild dogs, hyenas, coyotes, and wolves. They are also attacked by aquatic reptiles, like Nile crocodiles.
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