Equus (genus)

The Equus genus includes animals like horses, asses, and zebras. These are special kinds of mammals that belong to a group called perissodactyls, which means they have an odd number of toes on each foot. There are seven living species in this genus today, and many more that lived long ago and are known only from fossils.

These animals have slender legs, long heads, and necks, and most have manes and tails. They eat plants and prefer to graze, which means they mostly eat grass and other soft vegetation. Unlike some other plant-eating animals, they have simpler stomachs but can still survive on foods that are not very nutritious.

Today, domestic horses and donkeys can be found all around the world, but wild horses and their relatives live mainly in Africa and Asia. In the wild, these animals live in groups led by a male, or they live in areas where males protect resources that attract females. Females usually take care of the babies, but males sometimes help too. These animals talk to each other using sounds and body language. Unfortunately, human activities have made it harder for wild equine populations to survive.

Etymology

The word equus comes from Latin and means "horse". It is related to the Greek word hippos, which also means "horse". Even older forms of the Greek word, like i-qo written in a very ancient script, show how people have long used similar words to describe these animals.

Taxonomic and evolutionary history

Further information: Evolution of the horse

The genus Equus was first described by Carl Linnaeus in (/wiki/10th_edition_of_Systema_Naturae). It is the only living group in the family Equidae. The earliest equids were small, about the size of a dog, and lived around 54 million years ago during the Eocene. They had three toes on their back legs and four on their front legs, with small hooves instead of claws.

Over millions of years, these animals grew larger and their side toes got smaller. By the time the genus Equus appeared, they had just one main toe on each foot. Equus includes all living equines today. The oldest known species, Equus simplicidens, looked a bit like a zebra but had a head shape similar to a donkey. Fossils of early Equus have been found in places like Idaho, the USA, and also in Europe and Russia.

Today, Equus includes seven living species. Scientists believe the common ancestor of all modern equines lived around 5.6 million years ago. Studies of DNA from very old horse bones suggest this ancestor may have lived as recently as 4 million years ago. The species in Equus split into two main groups: the noncaballoids, which include zebras and asses, and the caballoids, or “true horses,” which include the common horse E. caballus and the wild horse E. ferus przewalskii. Zebras evolved in Africa, while asses and true horses spread into other parts of the world.

Extant species

Prehistoric species

Many extinct species of Equus have been found in fossils. Some of these may actually be the same species, just with different names. For example, Equus niobrarensis might be the same as Equus scotti, and Equus alaskae could be the same as Equus lambei. DNA studies of old horse remains from North America and Europe suggest that many of these “different” species were actually one highly-variable widespread species.

• American species

  • Equus alaskae – Alaskan horse
  • Equus conversidens – Mexican horse
  • Equus fraternus
  • Equus francisi – now placed in a separate genus, Haringtonhippus
  • Equus giganteus – Giant horse
  • Equus lambei – Yukon horse
  • Equus neogeus – often placed in a separate genus, Amerhippus
  • Equus niobrarensis – Niobrara horse
  • Equus occidentalis – Western horse
  • Equus scotti – Scott's horse
  • Equus semiplicatus
  • Equus simplicidens – Hagerman horse

• Eurasian species

  • Equus altidens^{[citation needed]}
  • Equus major
  • Equus suessenbornensis
  • Equus livenzovensis
  • Equus senezensis
  • Equus dalianensis^{[citation needed]}
  • Equus lenensis – Siberian horse
  • Equus latipes^{[citation needed]}
  • Equus hydruntinus – European wild ass
  • Equus namadicus
  • Equus ovodovi
  • Equus yunnanensis
  • Equus stenonis – Stenon zebra
  • Equus sivalensis

• African species

  • Equus algericus^{[citation needed]}
  • Equus capensis – Giant zebra
  • Equus mauritanicus – Saharan zebra

Domestic species

• Equus caballus – Domestic horse
• Equus asinus – Domestic donkey

Hybrids

Main article: Equid hybrid

Equine species can crossbreed with each other. The most common hybrid is the mule, a cross between a male donkey and a female horse. These hybrids usually cannot have babies of their own. Another hybrid is a hinny, made from a male horse and a female donkey. Other mixes include the zorse, a mix of a zebra and a horse, and a zonkey or zedonk, a mix of a zebra and a donkey. In places where Grévy's zebras live near plains zebras, they can have babies together. Ancient DNA shows that the Bronze Age kunga was a mix between the Syrian wild ass and the donkey.

Biology

Further information: Equine anatomy and Horse behavior

Equines, which include horses, asses, and zebras, come in many sizes but all have long heads and necks with slender legs that end in a single toe. Grévy's zebra is the largest wild type, standing up to 13.2 hands tall and weighing up to 405 kg. Domesticated horses can be much bigger, with some draft horses reaching 18 hands tall and weighing over 1,000 kg, while miniature horses may be as small as 30 inches tall.

These animals are built for running long distances. Their teeth are designed for grazing, with strong molars for grinding grass. Males often have special canine teeth they can use in fights. Equines have good eyesight and can move their ears to locate sounds. Their coats may change with the seasons, and zebras have distinctive black-and-white stripes that may help protect them from flies.

Human relations

The earliest proof that people began to tame horses comes from places in Ukraine and Kazakhstan around 4000–3500 BCE. By 3000 BCE, horses were fully tamed, and by 2000 BCE, many more horse bones appeared in places across northwestern Europe, showing that tamed horses were spreading. The best proof of taming comes from graves where horse remains were buried with chariots from the Sintashta and Petrovka cultures around 2100 BCE. Studies of genes show that only a few wild male horses, maybe all from one group, helped create the tamed horses by mating with many female horses.

Przewalski's horse is not an ancestor of tamed horses, but the two can have babies together. The split between Przewalski's horse and the common horse happened between 120,000 to 240,000 years ago, long before people tamed horses. The European wild horse, called the tarpan, shares family ties with today’s tamed horses. Donkeys were first tamed by people moving with animals in North Africa more than 5,000 years ago to help with the dry lands of the Sahara and the Horn of Africa. Scientists think donkeys were tamed twice from one wild ancestor, the Nubian wild ass. Trying to tame zebras did not work well, although Walter Rothschild taught some to pull a carriage in England.