# Horses

Horses

The Grevy's zebra ( Equus grevyi ), an East African Horse style

Systematics
 Class : Mammals (mammalia) Subclass : Higher mammals (Eutheria) Superordinate : Laurasiatheria Order : Unpaired ungulate (Perissodactyla) Family : Horses (Equidae) Genre : Horses
Scientific name
Equus
Linnaeus , 1758

The horse ( Equus ) are the only extant species of the family of Equidae . The genus includes the wild horses (the Przewalski horse and the now extinct wild horse ), the various types of donkey (the African and Asian donkey or the Kiang ) and at least three zebra species (the steppe , mountain and Grevy's zebra ) . It also excludes those domesticated from the wildHouse shapes a. The number of species and their delimitation from one another are still controversial today. A total of seven or eight species are often distinguished, the majority of which are endangered. The animals live today in Africa south of the Sahara and in southern, central and eastern Asia . The inhabited habitats consist of open, often grassy landscape areas, which can sometimes be very dry to desert-like . Horses are adapted to these regions due to their strong physique and long, slender limbs. The characteristic feature of the genus can also be found on the legs, as both the front and rear feet each have only one toe, which is covered by a broad hoof . The decrease in the number of toes, which also earned the horses the higher-ranking designation "equine", enables fast and low-friction locomotion in the steppe and savannah areas .

In general, horses are sociable animals. Two types of groups can be distinguished in the social community: one with mother-young animal groups and stallions living individually and a second with larger groups of mares and young animals, which also include one or more male animals, the so-called “harems”. The training of one or the other basic type is usually dependent on external factors. This includes above all the food supply, which can be constant over the year or - due to the stronger influence of the seasons - also changing. The main diet of the animals consists of grass, occasionally they also eat leaves and twigs. To chew the hard grass food, molars with extremely high crowns formed in the horses, which is used as another typical characteristic of the species. The gastrointestinal tract , which is less efficient than other ungulates , means that the horses spend most of their active time eating. The stallions mate with several mares, while the mares, depending on the social group type in which they live, have either one or more stallions as mating partners. Usually a single foal is born which is independent after a maximum of six years. The male and female offspring then leave the parental group.

The horses are of great importance in the history of human development. In prehistoric times they were used as an important source of raw materials and food. In the course of settling down , two species were domesticated. The house horse emerged from the wild horse, the house donkey from the African donkey. Both forms of domestic animals play an important role as riding animals and pack animals and achieved worldwide distribution in the wake of humans. The systematic study of the genus began in 1758 with the establishment of the generic name Equus . In the period that followed, various proposals for subdivision were made, but most of them were unsustainable. From a phylogenetic point of view, horses are the youngest member of a 56 million year development process in the family. The earliest representatives of the horse genus appeared in North America in the Pliocene around three and a half million years ago . Only a little later, these early horses had settled in Eurasia and Africa. The American horse branch died out around 10,000 years ago.

## features

### Habitus

A short standing mane is characteristic of Przewalski horses .

Horses are generally stocky animals with a barrel-shaped body and long neck, a comparatively large head and long limbs. Size and weight vary from species to species: Overall, the animals reach head-to-trunk lengths of 200 to 300 cm, the tail becomes 30 to 60 cm long. The shoulder height of the smaller species like the Asiatic ( Equus hemionus ) and the African donkey ( Equus asinus ) varies between 110 and 140 cm with a weight of 200 to 275 kg, the largest recent species, the Grevy's zebra ( Equus grevyi ) is at the withers up to 150 cm high and weighs between 350 and 430 kg, in exceptional cases up to 450 kg. The sexual dimorphism is only slightly developed, males only outperform females by around 10% in body weight. On the head, the facial area in particular is long. The eyes are on the side of the head, the ears are long and flexible. The fur is dense and often short, most species have longer hair, called long hair or mane , on the neck, forelock and tail . The fur color of some species is gray or brown on the top and whitish-gray on the underside. Stripes on shoulders and limbs can be present in several species. The three zebra species are characterized by their striking black and white fur dress. A striking feature are the chestnuts ( chestnuts ) or "night eyes" callus-like dark spots on the legs. Zebras and donkeys have these mainly on their front legs, wild horses and domestic horses often on all four legs. Size and shape vary individually. These may be stunted glands or remnants of a wrist or ankle pad. The front and rear feet end in wide hooves , of which horses only have one per limb ("equine"). The “hoof shoe” completely covers the last phalanx of the toe.

### Skull and dentition features

Asian donkey skull

Horses have a massive head, the facial skull is remarkably elongated and is mainly formed by the upper jaw . The intermaxillary bone is also elongated. The nasal bone has a long, narrow shape. The eye socket is set far back and lies behind the teeth. It is completely enclosed by bone. The posterior section of the skull is comparatively short, but the brain capsule is relatively large. A special feature of horses is the air sac , which is a bulging of the eustachian tube below the base of the skull. The paired openings each have a capacity of 350 to 500 ml. Originally interpreted as helping to cool the brain, according to Horst Erich König and Klaus-Dieter Budras, among others, the airbag probably serves - like the paranasal sinuses - to reduce the specific Weight of the skull. The lower jaw is also massive. The temporomandibular joint is high, the branch of the lower jaw is enlarged. At the rear end there is a strong angular process to which the masseter muscle is anchored.

Molars of the extinct Equus mosbachensis with the characteristic, strongly folded enamel

Horses have three incisors , one canine and six to seven rear teeth per half of their jaw . The dental formula is: . Overall, the dentition consists of 36 to 42 teeth. The incisors are chisel-shaped. Inside, they have indentations, the so-called infundibulum , which is surrounded by tooth enamel and emerges when chewing heavily. It is sometimes absent in the plains zebra ( Equus quagga ). Behind it there is a wide gap known as the diastema . The canine tooth of males is in this gap. In female animals it is either very small or completely regressed. The subsequent molars usually consist of three premolars (the foremost, also called wolf's tooth, is rarely present) and three molars . The anterior molars are very similar in structure to the posterior ones, i.e. they are molar-shaped . On the chewing surface there is a relief of twisted enamel strips , with layers of cement and dentin in between . The molars have a columnar shape with almost parallel side lines, the individual side surfaces being structured by edges and ripples. The extremely high ( hypsodontic ) tooth crowns are striking, but in principle only the top, actively working part protrudes from the tooth socket. The rest is hidden in the jaw and is gradually pushed out when worn. The tooth root itself is small and stays open until the tooth is almost chewed off. Only then does it close, at which point there is usually a noticeable increase in the size of the root. This root growth is apparently caused by the now stronger shear forces when chewing as a result of the lower tooth crowns. The space in the tooth socket that is freed up by pushing out the tooth is gradually filled with cancellous bone . The teeth represent an ideal adaptation to hard grass forage. ${\ displaystyle {\ frac {3.0-1.3-4.3} {3.0-1.3.3}}}$

### limbs

Anatomy of a stallion

One of the most characteristic features of horses is the reduction in the number of toes . All species living today have only one functional toe (monodactyly). It is the third toe, the remaining toes have receded and preserved on the skeleton of the limbs as rudimentary stylus legs . The stylus legs are not without function, however, as they have an important support function for the tendons that connect the lower limbs with the front and rear feet. The metacarpal bones are shorter than the metatarsal bones , which also affects the overall length of the front and rear legs. Like all unpaired ungulates, horses have a saddle-shaped talonavicular joint - the ankle joint between the ankle bone (talus) and scaphoid bone (navicular bone ) - which significantly restricts mobility. The ulna is greatly reduced and fused with the spoke in the lower half . Likewise, the lower end of the fibula completely fuses with the tibia . The thigh bone is comparatively short, but has a large bone process (trochanter tertius) on the upper shaft section below the joint head . The collarbone is missing.

### Internal anatomy

Heart of a horse - clarified preparation for the visualization of anatomical structures

The heart of the horses can like all vertebrates as a muscle pump blood throughout the body circulate. It is more globoid in shape than the human heart and is made up of four chambers: the left and right atria and the left and right ventricles. The average adult horse has a heart weighing 3.4 kg, which corresponds to about 0.6 to 0.7% of body weight. In domestic horses, it may increase slightly in size in response to conditioning. Circulatory capacity is determined in part by the functional mass of the heart and spleen. The heart rate in animals at rest is 15 to 45 beats per minute. It can increase threefold during strenuous activity. Studies on zebras show that young animals generally have a higher heart rate than older animals. In contrast, a larger muscle mass leads to slower heartbeats.

Like all unpaired ungulates , horses are rectum fermenters, which means that digestion takes place mostly in the intestinal tract . In contrast to that of ruminants , for example, the stomach is always simply built and single-chambered with a length of around 74 or 14 cm (measured over the outer and inner curvature) and a volume of 0.8 l in the house donkey . The fermentation takes place in the very large appendix and in the double-looped, 2 to 4 m long ascending colon (ascending colon). The pH levels in the small intestine rise from front to back and range from 6.3 to 7.5. In the subsequent large intestine , they fall back to around 6.7. The individual proportion of microflora in the small intestine also increases, in the front section it is about 2.9 × 10 6 per gram (wet weight), in the rear about 38.4 × 10 6 . In the appendix and colon, they are 25.9 and 6.1 × 10 6, respectively . The appendix can hold up to 33 l, the entire small intestine up to 64 l and the large intestine up to 96 l. Overall, the intestinal tract is up to 18 m long in the domestic donkey and up to 30 m in the domestic horse .

Horses differ from other mammals in the structure of the ovary : the ovarian tissue, usually referred to as "bark", with the follicles is located inside the organ, while the vascular medulla is outside. The ovarian cortex only reaches the surface at one point. This point is also visible from the outside as a retraction and is known as the "ovulation pit" (fossa ovarii), only at this point ovulation can occur. The mature follicle reaches a diameter of 5 cm and is therefore more than twice as large as that of a cattle. Males have a scrotum , but like all odd ungulates have no penile bone . The penis itself is around 35 cm long with a diameter of around 5 cm in a house donkey when it is not erect. The testicles of the donkey weigh between 123 and 136 g each. Their weight increases significantly towards the mating season, in the plains zebra, for example, it increases for both testicles combined from around 268 to 345 g. The increase in size is the stronger, the more strictly the reproductive phase is seasonally limited. The kidneys are located below the lumbar vertebrae and weigh 240 to 270 g in the domestic donkey, and double to three times the weight in the domestic horse.

### Chromosome number

The number of chromosomes in horse species varies from 2n = 66 to 2n = 32:

 Przewalski's horse ( Equus przewalskii ): 2n = 66 Domestic horse ( Equus caballus ): 2n = 64 African donkey / house donkey ( Equus asinus ): 2n = 62 Asiatic donkey ( Equus hemionus ): 2n = 54-56 Tibetan wild ass ( Equus kiang ): 2n = 50-52 Grevy's zebra ( Equus grevyi ): 2n = 46 Plains zebra ( Equus quagga ): 2n = 44 Mountain zebra ( Equus zebra ): 2n = 32

The range in E. hemionus as well as in E. kiang is explained by the Robertson translocation .

## Distribution area and habitat

The wild forms of recent horse species still live in eastern and southern Africa and in the central regions of Asia . Horses prefer open terrain as a habitat. They are found in savannahs and steppes , but also in drier habitats such as semi-deserts and deserts . The habitats not only include uniform grassy areas, but also partly include bush and forest landscapes. The use of closed habitats depends in particular on how well the individual species can utilize leafy food. The influence of predators also plays a role. Furthermore, the different regions in the range of today's horses are subject to seasonal changes due to varying precipitation (rain in the more tropical and snow in the more moderate climatic zones). The availability of available water sources is also an important criterion for the presence of horses in a given area.

In the last millennia the range of the horses has decreased significantly. By the end of the Pleistocene , they were distributed over large parts of Eurasia , Africa and America . They became extinct on the American continent around 10,000 years ago for reasons that are not exactly clear. Attempts at explanation range from the hunted by the newly immigrated people to climatic changes after the end of the last ice age to epidemics or a combination of these factors. At least in South America, the range of the horses began to shrink rapidly in the late Pleistocene with the presence of the first early human settlers. In parts of Europe, too, individual stocks are likely to have died out around 10,000 years ago. In North Africa and West Asia they were probably exterminated in antiquity - only in Iraq and Iran did a population of the Asiatic donkey persist until the 20th century. In Eastern Europe, the last wild horse species - the tarpan ( Equus ferus ) - died out in the 19th century.

In contrast, domestic horses and house donkeys have been distributed worldwide by humans, and in some countries there are also feral populations of both forms. The largest number of feral domestic horses and donkeys live in Australia , but they can also be found in the USA and other countries.

## Way of life

### General

Although horses can also go looking for food during the day, they are predominantly crepuscular and nocturnal. This is especially true for the species in the tropical regions. In the more moderate latitudes, daytime activity can also predominate. As a rule, the animals spend between 60 and 70% of their active time eating. The rest of the time is spent on migratory movements and social interactions such as grooming, playing or fighting among stallions. When moving, horses only touch the last toe, so they are top walkers . There are several natural gaits that differ in speed and execution. They range from slow walking (step) with individual leg movements in a pass gait, to a faster trot (trot), in which two legs are moved diagonally at the same time, to fast running (gallop). In the latter, all four legs usually lift off the ground at the same time. The speeds reached are 6 to 10 km / h when walking, 6 to 19 km / h when trotting and 26 to 56 km / h and more when galloping. Sleep phases are rather short with an average of 2.5 hours a day. Short sleep breaks of just a few minutes to around a quarter of an hour predominate. However, periods of rest can be longer overall. Most of the time the animals stand by, only the young ones lie down. The typical sleeping pose is characterized by inclined rear legs, a lowered neck and drooping ears. The eyes are often open. These are characteristic features of escape animals. Sleeping or dozing while standing is made possible for the animals by the fact that they can firmly anchor the kneecap through the inclined position of the legs and thus prevent it from kinking.

### Social and territorial behavior

The Khur, a subspecies of the Asian donkey, lives mostly in individual mother-young animal groups.
The plains zebra form larger groups.

Horses have complex social behavior . In principle, two different basic types of group formation can be identified:

• The Grevy's zebra , the African donkey , the Kiang and some populations of the Asian donkey (including the Khur and the Onager ) show territorial behavior. The greatest bond is between the mother and the young. Male animals establish mating areas that can sometimes be over 10 km² in size. They mate with mares who cross their respective territories. Although animals sometimes form associations, there are no lasting relationships between adult animals in these species.
• The mountain and steppe zebra , different populations of the Asian donkey (such as the Kulan and the Dschiggetai ) as well as the Przewalski horse and feral domestic horses such as the mustangs , on the other hand, live in larger, stable associations. These consist of one to five pairs of mother and young animals and are accompanied by a stallion to watch and drive. The community structure is known as the "harem". Under certain circumstances - as with some feral domestic horse groups - several stallions can belong to one group. The size of such an association is heavily dependent on the regional food supply. The groups roam extensive spaces of activity that can overlap with those of other groups. A certain ranking can be established within the association , although this is not necessarily identical to the leadership role. In the case of feral domestic horses, the departure can be initiated by all group members regardless of gender, but the animals often follow higher-ranking individuals. The leadership role changes in the groups of mountain zebra, under certain circumstances the stallion leads the way (when reaching food or water sources) or a higher-ranking mare - usually the one with the youngest foal - signals the departure. Male animals that do not lead a group formation often form bachelor groups. Occasionally, very large herds also form, which are then composed of several groups. However, these are only of a temporary nature and disintegrate again after a certain time. With the plains zebra, this happens, among other things, in regions where large groups of bachelors occur. This enables the stallions of the individual group associations to better protect their harem from attacks.

The former type with mother-young animal groups and territorial stallions occurs largely in areas with a food supply that is more or less constant over the year. The migratory movements of the animals are not very rambling. The mating right lies exclusively with the stallion through whose territory a group of mother and young animals migrates. The second type with harem formation and supervising stallion is typical for landscapes with strong seasonal fluctuations and thus varying food supply over the course of a year. The group associations roam wide, open landscapes in search of food, but are subject to the constant risk of being threatened by predators. A larger group formation distributes the vigilance against predators among several animals, without a single individual having to sacrifice too much time, which it actually needs to eat. A stallion present in such a group, who is usually on the lookout for potential competitors, also reduces the time required for vigilance for the mares. Here the mating right lies with the stallion who leads a harem. Both basic types of group formation enable the stallion to mate with several mares and thus multiply polygynously . Mares in harem associations, on the other hand, live monandrically (mate only with one stallion), while those in mother-young animal pairs are polyandric (mate with several stallions). It is often assumed that the first basic type is the more original, which was also trained by numerous very early ancestors of today's horses. The second would then be a modified group type, which emerged with the emergence of open landscapes and a more seasonalization of the climate.

Fighting plains zebra stallions

The horses communicate with their fellow species in a variety of ways using visual signs and tactile and olfactory signals and by means of sounds. The repertoire of gestures is very extensive and is expressed through the posture of the head, ears, jaw or tail and leg movements such as stamping. There is also a diverse range of sounds which, in addition to the well-known neigh, also includes various bells, snouts and blows. The sounds are most diverse with the feral domestic horses and with the wild horses. With the exception of the males, zebras and donkeys are usually calmer. Not all gestures and sounds have an aggressive character; some are to be understood as greetings, express well-being or, in species in group associations, also serve as contact calls. Communication via feces and urine , which act as information carriers over spatial and temporal distances, is of great importance for all horses, including domestic horses . This gives the individual animals information about other individuals. As a rule, females defecate on the spot, while males tend to be more strategic and place their excretions next to those of the females. Some species have permanent droppings, for example on well-traveled paths or at social assembly points. Tactile communication includes placing chins on each other's backs, which generally reduces aggressiveness and strengthens mutual social bonds. In addition, the animals lick and clean each other or nibble away parasites , which is very rare with zebras. Each of the mentioned forms of communication transmits its own information. Smells give clues to the individual and his or her personality. Sounds, in turn, reveal the status of an animal, since, among other things, dominant stallions often emit more persistent and higher-energy tone sequences than subordinate animals, which also end gutturally . All of this helps the animals to assess unfamiliar individuals when they meet. Research has shown that less than 15% of encounters between individuals or groups result in physical confrontation. If these cannot be prevented in spite of the various signaling, they are carried out with kicks in the front and rear legs and with bites.

### nutrition

Horses are exclusively herbivores and primarily eat grass . Due to the hard silica in these plants, the horses developed high-crowned molars with a high proportion of dental cement to counteract the increased abrasion when chewing. Softer parts of plants such as leaves or twigs are also eaten to varying degrees, and some extinct Equus species were also adapted to a mixed vegetable diet, such as the Cape zebra ( Equus capensis ). As a rule, the higher quality food is only available at certain times of the year. In the case of the plains zebra, different groups of groups can temporarily come together to form large herds if there is sufficient supply. As a rule, the food is plucked off with sensitive lips and pushed behind the incisors. As rectal fermenters, horses require almost twice as much food as ruminants of similar size . This also means that most of their daily activity is devoted to eating. In addition, they are less able to break down nitrogenous components and have to remove them through the urine. This results in a greater dependency on water to compensate for the loss of fluid, which is why migration movements are limited, especially in the dry periods of the year. Some species such as the Grevy's zebra or the African donkey can do without water for a long time if necessary, but after such a phase they balance their water balance with an intake of up to 30 l of water in a very short time. However, this largely only affects animals that do not raise any offspring. The behavior is also known from the house horse and led to the saying "drink like a horse". Since the horse's digestive system is generally less effective than that of ruminants, the droppings are correspondingly coarser. Young animals usually learn from their mother which food is rich in nutrients and useful for them. They may eat the mother's dung, which may help the learning process. Conversely, mothers also eat the boy's droppings. It is assumed here that the dams use this to identify pathogens and give their offspring appropriate T lymphocytes through breast milk as a defense.

### Reproduction

The horse species of the more temperate climates have a seasonally limited reproductive phase, in the more warm climatically influenced areas it can last all year round. However, there are also high birth rates here, which usually coincide with the rainy seasons. All horses are polyestrial , so that the sexual cycle repeats itself until fertilization or comes to a standstill in the northern latitudes due to the shorter days in winter. The cycle itself lasts around a week and starts again after two to three weeks if there is no fertilization. The intermediate phase is longer with zebras than with wild horses and donkeys. The ovulation occurs spontaneously. Stallions willing to reproduce sniff the mares' genitals and lay their heads on their torsos. With the help of the characteristic flehmens you can determine the estrogen status of a mare in the urine via the Jacobson organ . Male animals engage in a mating contest with one another, which is ritualized and begins with an arching of the neck, followed by various vocalizations and mutual sniffing. The final stage is a physical battle. Female animals express their own unwillingness by kicking their hind legs and thus preventing the stallion from climbing up. Mares who are ready to receive, however, usually stand still. Stallions in harem associations only perform one mating act per mare , whereas territorial stallions sit on a mare every 15 minutes and ejaculate about every hour . After orgasm , the stallion needs up to 20 minutes to recover.

Mother and young of the mountain zebra

The gestation period for horses is 330 to 390 days; it is longest in Grevy's zebra and shortest in domestic horses . Usually a single young is born. The birth usually takes place at night. The newborn is relatively heavy (it weighs between 25 and 40 kg, which is about 9 to 13% of the mother's weight) and precocious. It can follow the mother just a few hours after the birth and mostly stays close to her. On average, it sucks milk for about a minute every hour. Father animals do not participate in the rearing of the offspring. Occasionally, however, infanticide occurs. After 7 to 18 months, the young animal is weaned. The sexual maturity occurs two to six years, with young stallions due to the social structures can usually reproduce at advanced ages than mares. In harem communities, both male and female animals then leave the parental group, which is relatively rare in mammals. The life expectancy of horses in the wild is 21 to 40 years.

In principle, mares are physically able to reproduce every year, but there is often a period of several years between two births. The oestrus cycle starts again three to four weeks after the birth. The sex ratio at birth is 1: 1, but this can be very different for the mares. In some Asian donkey populations, young and older mares are more likely to give birth to female offspring, whereas middle-aged animals are more likely to give birth to male foals. The reasons for this could be the highly variable reproductive chances of the adult stallions. Mother animals therefore have to invest more time and energy in rearing the male offspring so that they can later successfully prevail against their sexual mates and then mate. Middle-aged mares usually have enough experience and the physical requirements for more intensive rearing of male animals. Young mares, on the other hand, are mostly inexperienced, while older mares often show a deteriorating constitution. The predominant birth of female offspring by young and old mares promotes the vitality of a population in this case and increases the number of female animals that can later bear young animals.

### Enemies and enemy behavior

Horses have a number of natural enemies , including primarily large predators such as hyenas , wolves , wild dogs, and big cats . Like many ungulates, they are flight animals. The build of horses is designed for fast and persistent running, so they flee when threatened. If cornered, horses can also kick their hooves or inflict painful bite wounds on attackers. Their most powerful weapon are their heavily muscled hind legs. In harem communities, the stallion defends the group. The female animals move away from the source of danger at around half the escape speed in order to give the stallion the opportunity to unlock.

## Systematics

### External system

Internal systematics of the Equidae according to Prado and Alberdi 1996 and Mihlbachler et al. 2011
Equinae

Equini
Protohippina

Pliohippina

 Equus

The horses ( Equus ) form a genus from the family of the same name in German, the horses (Equidae). The family originated in the Lower Eocene 56 million years ago and has been widespread in North America and Eurasia since then , in the transition from the Lower to the Middle Miocene around 16 million years ago, the first early representatives also reached Africa . Today Equus is the only member of his family, which makes it monotypical . The closest living relatives are the tapirs and the rhinos , together they form the order of the odd ungulate (Perissodactyla). However, tapirs and rhinos are more closely related to each other and together form the suborder Ceratomorpha , to which numerous forms that are now extinct are assigned. The horses traditionally face the Ceratomorpha. Together with their extinct ancestors, they belong to the suborder Hippomorpha (equine relatives). Within this subordination, a distinction is made between the superfamily Equoidea , which is composed of the horse family and the extinct family of the Palaeotheriidae . Sometimes the Brontotheriidae , a group from the Eocene that also includes fossil, partly very large forms, are referred to the suborder Hippomorpha and are therefore also more closely related to the horses. The separation of the line of horses from that of rhinos and tapirs took place according to molecular genetic studies at least 54 to 56 million years ago.

Within the horse family, the genus Equus is placed in the subfamily of Equinae . Their representatives are characterized by better adaptation to grass food and therefore developed high-crowned ( hypsodontal ) teeth. Here, in turn, Equus belongs to the tribe of the Equini and the sub- tribus of the Pliohippina . The Pliohippina comprise the single-hoofed ( monodactyl ) horses, a characteristic that is typical for all modern representatives of the genus Equus . They in turn represent the sister group of the Protohippina, which are designed as three-hoofed ( tridactyle ) animals a bit more primitive. The Equini, for their part, face the Hipparionini (sometimes the Protohippina are also led on the tribe level (Protohippini) within the Equinae and then form the sister group to the Hipparionini). For anatomical reasons , the closest relative to Equus is Dinohippus , who lived in North America during the transition from the Miocene to the Pliocene .

### Internal system

Internal system of the genus Equus according to Vilstrup et al. 2013
Equus
non-caballines

Equus quagga
 E. q. quagga E. q. chapmani

caballines

Internal classification of the genus Equus according to Price et al. 2009
Equus
non-caballines

caballines

Lower molars of the genus Equus : left domestic donkey (stenonin), right domestic horse (caballin)

The number of recent horse species is still controversial, usually six, seven or eight today's species are distinguished. Likewise, the relationships between the individual species have not been fully clarified, so various molecular genetic investigations show partly contradicting results. Traditionally, modern horses are divided into two large groups of forms: the caballine (also caballoid) group, whose name goes back to today's domestic horse Equus caballus , and the stenonine (also stenonid, zebroid or non-caballine) group, named after the extinct Equus stenonis from the villa franchium . The clearest difference between the two forms is the linguaflexid of the lower molars. This clearly curved enamel strip on the rear (tongue-side) tooth edge, located between two prominent protrusions (metaconid and metastylid), is V-shaped (stenonine) on the one hand, and U-shaped (caballin) on the other. All today's zebras and donkeys are added to the stenonine horses, while the caballines include today's wild and domestic horses and are also referred to as "real horses".

The following are the most commonly accepted species today:

• Wild horse or tarpan ( Equus ferus Boddaert , 1785); extinct in the 19th century;
• African donkey or wild donkey or real donkey ( Equus asinus Linnaeus , 1758); Eritrea , Ethiopia and Somalia ; Ancestral form of the house donkey ; Highly endangered population in the wild;
• Asiatic donkey or half donkey or horse donkey ( Equus hemionus Pallas , 1775); several subspecies (onager, kulan and others) from Central Asia through South Asia to East Asia ; Survival endangered;
• Kiang or Tibetan wild ass ( Equus kiang Moorcroft , 1841); Tibetan highlands and adjacent areas; larger and more "horse-like" than other donkeys; Existence not endangered;
• Grevy's zebra ( Equus grevyi Oustalet , 1882); Kenya , Somalia and Ethiopia ; particularly narrow striped pattern; Survival endangered;
• Mountain zebra ( Equus zebra Linnaeus , 1758); two subspecies in Namibia and South Africa ; smallest zebra species with horizontal stripes on the croup to the base of the tail; Existence endangered;
• Plains Zebra ( Equus quagga Boddaert , 1785) several subspecies, including the extinct in the 19th century quagga , from southern Sudan to South Africa; Belly striped and between the strips often lighter “shadow strips”; Existence not endangered;
The extinct Syrian donkey is sometimes viewed as a form of the Asian donkey, but is sometimes also considered a separate species.

In addition to these commonly recognized species a revision raised the ungulates of Colin Peter Groves and Peter Grubb from 2011 and the Przewalski's horse ( Equus przewalskii ), the Hartmann's Mountain Zebra ( Equus hartmannae ), the Khur ( Equus khur ) and the Syrian wild ass ( Equus hemippus ) in an independent species status. For a long time, the Przewalski horse was considered a subspecies of the wild horse and was regarded as the ancestral form of the domestic horse, but recent genetic studies have interpreted the horse representative as a re-feral domesticated form. It became extinct in the wild in the 1960s, and reintroduction attempts are now underway in Mongolia , China and other countries. The population is estimated at 2000 animals. The domestic and wild horses as well as the Przewalski horse are assigned to the subgenus Equus as caballine horses . The African and Asian donkeys (Asian donkey, Kiang, Khur and Syrian half donkey) are sometimes in the subgenus Asinus , and sometimes the Asian donkeys are also included in the subgenus Hemionus . The same applies to the various forms of zebra, for which on the one hand the common subgenus Hippotigris exists, on the other hand the Grevy's zebra is also listed in the subgenus Dolichohippus .

Originally the genus Equus contained more than 230 described, only fossil-known taxa in addition to the recent species, 58 of them from North America alone. Most of these extinct forms were based on only fragmented fossil material or were poorly described. For this reason, numerous taxa were synonymous in 1985 and 1989 .

## Tribal history

### origin

The horse family is a very old group of odd ungulates, whose tribal history goes back around 56 million years. The genus Equus includes the modern horses and is the youngest link in this development. It is embedded in a group of other monodactyl horses, including Pliohippus , Dinohippus , Astrohippus , Haringtonhippus and Hippidion . The origin of modern horses is in North America. According to molecular genetic analyzes, Equus split off from the line of the other horses in the Pliocene around 4.5 to 4.0 million years ago . The genetically closest relatives, Hippidion and Haringtonhippus , separated from 5.2 to 7.7 million years ago and 4.9 to 5.7 million years ago, respectively. Equus itself most likely emerged from Dinohippus , although it is still difficult to distinguish between this genus and the earliest modern horses. Presumably, this process took place in the southern part of North America through cladogenesis .

### The stenonine horses

Skeletal reconstruction of Equus simplicidens , one of the earliest known members of the genus Equus

The molecular genetic data obtained on the splitting off of the horses agree relatively well with the oldest paleontological evidence, which comes from the Ringold Formation in Washington State and is older than 3.4 million years. These are assigned to the species Equus simplicidens , an early member of the Stenonine horse group, similar to the findings from the Hagerman fauna of the Horse Quarry in Idaho , where more than 150 skulls of this horse species have been found, their age estimated at around 3.7 million years becomes. This earliest representative of Equus is sometimes assigned to the subgenus Plesippus . Other important finds of Equus simplicidens have come down to us from northern and central Mexico , for example from Jalisco . In the later course of the Pliocene, the relatively slender and delicate Equus cumminsi appeared, which is based on individual finds from Texas and is often referred to as being similar to donkeys.

Skull of Equus eisenmannae , an early Eurasian Stenonine horse

Around 3 to 2.5 million years ago in the end of the Pliocene , the stenonine horses reached Eurasia and spread relatively quickly in the landscapes that were originally inhabited by Hipparion representatives. It was the first single-hoofed horse to set foot on Eurasian soil. Very early finds from the Linxia Basin in East Asia, dating to around 2.6 million years, are assigned to Equus eisenmannae . Other stenonine forms that occur almost simultaneously in East Asia are Equus huanghoensis , Equus qingyangensis and Equus yunnanensis . Since there are sometimes clear morphological differences between these horse members, some scientists assume that there are several waves of immigration. Equus livenzovensis from Montopoli in southern Europe is similarly old . These various original representatives may form the basis for the subsequent radiation of the stenonine horses, from which well-known forms such as Equus stenonis , Equus sanmeniensis , Equus sivalensis and Equus suessenbornensis arose . The animals often showed characteristics of today's zebras and donkeys, which is why they were originally grouped together to form the taxon Allohippus . The genus is mostly not recognized, but some scientists advocate its independence due to its morphological peculiarities. In Eurasia, the stenonine horses show a wide variety of relationships with one another. In general, a particularly large group of shapes around Equus suessenbornensis and a more variable one around Equus stenonis can be worked out morphologically . It is noteworthy that at many Eurasian sites of stenonine horses two sympatric species occur, which differ in terms of body size. For example, at the eponymous Middle Pleistocene site in Süssenborn, but also in Voigtstedt (both Thuringia ), in addition to the large Equus suessenbornensis , which weighs up to 590 kg, there is also the smaller Equus altidens , which weighed only around 260 kg. The common occurrence of different Stenonine horses at a site is most likely to be associated with a stronger niche use . For both Equus suessenbornensis and Equus altidens , a reference is made to the subgenus Sussemionus , due to some special tooth properties , whose character form Equus coliemensis from the Kolyma River in Yakutia . The majority of the stenonine horses of Eurasia disappeared again in the course of the Middle Pleistocene. One of the few descendants is Equus ovodovi , which was described on the basis of some finds from the Proskuriakova cave in southwestern Siberia . The fossil remains, which show typical characteristics of the subgenus Sussemionus , belong to the Young Pleistocene with an age of 40,000 years . Additional finds of the species came to light in northeastern China. Another late form is Equus hydruntinus , the European wild ass, which occurred over large areas of western Eurasia in the late Middle Pleistocene and New Pleistocene and only became extinct during the Holocene 5000 to 3000 years ago. Morphologically, it shows mixed features that are reminiscent of both today's Asiatic donkey and the relatives of Sussemionus . From a genetic point of view, Equus hydruntinus is closer to the Asian donkey and probably represents an extinct side branch, while Sussemionus belongs more to the distant family circle of donkeys and zebras.

At least 2.5 to 2 million years ago, the stenonine horses had also reached northeast Africa, where they were first recorded in the Omo region with an age of around 2.3 million years . The form named Equus oldowayensis was about the size of today's Grevy's zebra. Similar to Eurasia, the representatives of the genus Eqqus in Africa displaced early hipparion-like horses that had spread across the continent as early as the Middle Miocene . Here separate lines of development emerged that produced species such as Equus koobiforensis , Equus numidicus , Equus tabeti or Equus capensis . The latter, also known as the "Cape Zebra", is relatively common in southern Africa. As a comparatively massive animal, it had a reconstructed shoulder height of around 150 cm and its body weight was an estimated 400 kg.

### The caballine horses

Skeletal reconstruction of Equus conversidens , a caballines horse from North America

The ancestor of the caballine horses can also be found in North America for the first time 2.5 to 1.8 million years ago and is commonly referred to as Equus scotti . Finds of this sturdy horse weighing around 550 kg were recovered with several skeletons from Rock Creek in Texas . Two groups of forms were generally described for the Middle and Young Pleistocene of North America: on the one hand, a robust form with broad limbs, which is morphologically and genetically assigned to the Caballine horses and is often equated with Equus conversidens , on the other hand, a slender, more delicate species, which is due to its distinctive foot anatomy in English it is also called stilt-legged horse ("stilt -legged horse "). The latter was partly led under the species name Equus francisci . Originally thought to be close relatives of the donkey, the exact relationships between the stilt-legged horses and other horses remained unclear for a long time. Several DNA examinations then showed that these form an endemic group of their own in America, with possibly closer ties to the caballine forms. In 2017, the stilt-legged horses were referred to the independent genus Haringtonhippus based on further genetic studies , but other authors consider this genus to be synonymous with Equus . Various horse species are recorded in the Young Pleistocene. The large Equus occidentalis is relatively important and occurs in large numbers in the asphalt pits of Rancho la Brea and in the Diamond Valley Lake Local Fauna , both in California . Much smaller, however, were Equus mexicanus and Equus cedralensis , both of which were described from sites in central Mexico .

With the creation of the Panama Land Bridge , horses also made their way into South America. One of the first horse representatives on this continent was the genus Hippidion , which immigrated from North America about 2.5 million years ago. About 2 million years ago, Equus reached South America, where the genus soon spread widely. Once considered with at least five species to be quite rich in forms in South America, today only Equus neogeus is largely recognized as a valid form, but it showed considerable variations in size. Across the American continent , horses (both Equus and Hippidion and Haringtonhippus ) became extinct from unknown causes in the transition from the Pleistocene to the Holocene around 10,000 years ago.

The first appearance of caballine horses in Eurasia is not certain, very old finds from the Villafranchium are known from Beresti ( Moldova ). The oldest clear representative of the caballine horses in Eurasia is usually associated with Equus mosbachensis . A significant number of finds was first documented in the early Middle Pleistocene . Fossil remains are not only passed down from the eponymous Mosbacher Sands in Hesse, they are spread over large parts of Europe such as Fontana Ranuccio in Italy or the Arago Cave in France. In the period that followed, the caballine horses largely replaced the stenonines in Eurasia. A possible explanation for this is a larger ecological range of the former compared to the latter. It is also noteworthy in this context that, in contrast to the stenonine horses, the caballine very rarely appear sympatric at individual sites. Comparable to the stenonine horses, the caballine forms in Eurasia also split into a diverse group with numerous representatives, known species include Equus steinheimensis , Equus taubachensis and Equus chorsaricus . The Young Pleistocene caballine horses of Central Europe are mainly referred to as Equus germanicus . What is striking is a noticeable reduction in size, which began in the late Middle Peptocene. The extremely strong Equus mosbachensis initially reached a weight of 610 to 740 kg with a shoulder height of around 165 cm, while the younger Equus steinheimensis weighed around 470 kg. The significantly larger dimensions compared to today's species are explained, among other things, by a higher growth rate caused by the more extensive food supply in the warm climatic sections of the Pleistocene. In the following Young Pleistocene to the Holocene, the size reduction continued. However, a strong fragmentation of the horse population is to be expected for the period after the maximum freezing of the last glacial period (about 20,000 years before today), which among other things led to significantly varying sizes depending on the geographical distribution. It is under discussion whether the size variations of the caballine horses of the Middle and Young Pleistocene express an independent speciation. Sometimes different skeletal proportions can also be grasped, recognizable among other things by animals with strong or slender legs and narrower or wider snouts. Some authors suspect that this clear plasticity of the horse's body is adapted to corresponding warm or cold climatic conditions and thus to more closed or largely open landscapes. According to this view, the different horse forms of Eurasia could be more “eco-morphotypes” than separate species.

In Africa, on the other hand, the caballine horses could never really spread far. Individual finds are documented from the Young Pleistocene of Allobroges in Algeria and are assigned to Equus algericus .

### About the origin of today's horses

Skeleton of a Grevy's zebra

The phylogenetic origin of the horse species still alive today is difficult due to the multiple species names used by Pleistocene equus representatives. It is possible that today's wild horse ( Equus ferus ) is the only caballine form that goes back to the wild horses of the Young Pleistocene . In western Eurasian areas these are mostly associated with Equus germanicus . In the North Asian region, the Late Pleistocene and Early Holocene horses are mostly known under the name Equus lenensis , of which some ice mummies , including the Yukagir horse from the Kondratiev River in northern Yakutia , have survived. The Przewalski horse , originally considered a wild horse shape can be determined by genetic studies, the Domestizierungsversuche of the late Neolithic belonging Botai culture traced in Central Asia. There could be a genetic relationship to Equus lenensis . The ancestors of today's domestic horse and the Przewalski horse had already separated around 117,000 years ago in the transition from the Middle to the Young Pleistocene.

The ancestors of donkeys and zebras are still largely unclear. In terms of molecular genetics, the separation between caballine and stenonine horses dates back around 3.7 to 4.4 million years. The donkeys and zebras split up 1.7 to 2 million years ago. A diversification of zebras into steppe ( Equus quagga ), mountain ( Equus zebra ) and Grevy's zebras ( Equus grevyi ) first began 1.6 million years ago with the mountain zebra. The other species followed over a period of around 200,000 years. The African and Asian donkeys separated at a roughly similar point in time 1.5 to 1.8 million years ago. Finally, the Kiang was formed, which probably happened in the Middle Pleistocene. The dates given are much younger than originally assumed, as a separation of donkeys and zebras was assumed around 2.8 million years ago, with the donkey line going back around 3 million years ago. To what extent there are relationships to individual extinct forms, such as the donkey to the very early North American Equus cumminsi, as was often postulated earlier, remains unclear. According to phylogenetic studies from 2019, however, the zebras could be derived from Equus simplicidens of North America and the very early stenonine horses of Eurasia. This is supported by similarities in the structure of the teeth and in the comparative dimension of the metacarpal and metatarsal bones .

## History of Taxonomy, Nomenclature and Etymology

The genus Equus was scientifically named by Linnaeus in 1758 as part of his important work Systema Naturae . Linnaeus defined the genus according to its teeth and the limbs with only one toe. The name Equus , which is Latin in origin and means "horse", was used before. It can be found, among other things, in John Ray's review of the quadruped and snake-like animals from 1693, to which Linnaeus also referred in his Systema Naturae . Linnaeus differentiated three types of horses: Equus caballus (domestic horse), Equus asinus (domestic donkey) and Equus zebra (zebra). Its subdivision of the genus was adopted by various naturalists and scientists in the transition from the 18th to the 19th century. John Edward Gray then propagated in an extensive revision of the horses in 1824 a division into two genera and separated Asinus from Equus . In the former he also included the zebras. Gray justified the division into two genera with the formation of stripes in donkeys and zebras and the different distribution of chestnuts on the legs. About a decade and a half later, in 1841, Charles Hamilton Smith lifted an "asinine group" with the donkeys from a "hippotigrine group" with the zebras. He differentiated the donkey and zebra at the genus level and united the latter in the newly created taxon Hippotigris . In the meantime, some new species had already been described, above all the Asian donkey in 1775, the plains zebra in 1785 (as quagga) and 1824 (as Burchell's zebra) and the wild horse also in 1785. In the further course of the 19th and early 20th centuries, additional genera created. Dolichohippus is of noteworthy importance for the Grevy's zebra by Edmund Heller in 1912. As early as 1823, Frédéric Cuvier had used the term Hemionus as a higher taxonomic unit, but it is regarded as a noun nudum . Therefore Wilhelm Otto Dietrich is considered to be the first name of the genus Hemionus , who united the Asian donkey under her in 1959. In 1945, George Gaylord Simpson , in his general taxonomy of mammals, doubted the generic independence of the genera Asinus , Hippotigris and Dolichohippus (as well as Hemionus according to F. Cuvier) and shifted them to the rank of subgenus due to the clear similarities to Equus . The view is still largely held today, but the exact number of sub-genres - variable figures are between three and five - is still under discussion.

A first comprehensive description of the teeth of horses was provided by Richard Owen in his Odontography in 1845 . Building on this, in 1899 Marcellin Boule recognized a clear dichotomy within horses with the domestic horse on one side and the zebra on the other, taking into account numerous fossil forms based on the dental structure. In the extensively illustrated article, Boule traced the zebras back to Equus stenonis . The shape had previously been introduced by Igino Cocchi in 1867 using a skull from Valdarno in Tuscany (Italy). Later authors adopted this branch and worked it out in more detail. Above all, Paul O. McGrew discussed in a 1944 contribution the various tooth features of fossil and recent horses for their pros and cons. So he came to the opinion that, among other things, the pli caballin , a tight enamel loop between two main tubercles of the maxillary molars (hypoconus and protoconus) is not an exclusive feature of the domestic horse. On the other hand, he emphasized the linguaflexid of the lower molars with the course reminiscent of a U (caballin) or V (stenonin) as supporting. With the emergence of new scientific investigation methods in the last third of the 20th century, this already morphologically determined dichotomy within the genus Equus was confirmed on a genetic and biochemical basis. Among other things, Ann Forstén , but also María T. Alberdi and others therefore continued the division of the genus into the informal groups of caballine and stenonine horses (after Forstén "caballoid" and "stenonid"), with Forstén also the donkey in the stenonine form a castle.

Equus caballus , the domestic horse as the nominate form of horses

The nominate form of the genus Equus is Equus caballus when it is first mentioned in Linnaeus' Systema Naturae . Here he distinguished the domestic horse with Equus caballus and the domestic donkey with Equus asinus . In 1785 Pieter Boddaert named the wild horse or the Tarpan with Equus ferus . Likewise, in 1858 Leopold Fitzinger led the African donkey under the scientific name Asinus africanus . From these different naming conventions an inconsistent use of the names for the wild and domesticated forms followed. Some researchers therefore also use Equus caballus and Equus asinus as species names for the wild horse and the African donkey (often including a subspecies name for the wild form, i.e. E. c. Ferus or E. a. Africanus ), while others used the later on Scientific names established by wild populations. However, it was more difficult than with the currently living domestic and wild animals with the phylogenetic predecessors or transitional forms, in which a reliable assignment to one or the other group is not always possible. In general, in the modern zoological system, pets do not come under the existing naming conventions. Exceptions are the species names given by Linnaeus, which have been in use for over 200 years. In 2003 the International Commission on Zoological Nomenclature therefore decided, at the request of some scientists ( Opinion 2027 , Case 3010 ), to preserve the names Equus caballus and Equus asinus (together with 13 other names of domesticated mammals) and make them usable in principle. Scientists and authors can therefore choose the name for a wild or domesticated form, provided two species names are available. However, the decision does not override Opinion 271 of 1954, in which the type species of Equus was determined to be Equus caballus . It is also not above the priority rule of the International Code for Zoological Nomenclature , according to which the species name given first is also the legitimate one. Accordingly, when considering the wild horse and the domestic horse, the former would be the only species to be assigned to the latter and not vice versa. The same can be said of the African donkey and the house donkey. In this case, the priority rule also applies to the plains zebra, which was scientifically established in 1824 by John Edward Gray with Asinus burchelli . Almost 40 years earlier, however, Pieter Boddaert had introduced the extinct quagga with Equus quagga . For a long time, both species were considered independent. However, genetic studies have shown a close relationship. Due to the subsequent synonymization of both species, Equus quagga is now the correct species name for the plains zebra.

The German name "horse" is derived from the Middle Latin name paraveredus for a courier horse on secondary routes. This in turn is based on the Celtic - late Latin word veredus for "courier horse" and the Greek prefix παρά ( pará ) for "beside" or "with". The word "donkey" was conveyed via the Old High German esil from the Latin asinus (or asellus as a diminutive). The origin is assumed to be from a language in Asia Minor . The origin of the word "zebra" is unclear. Possibly it can be found in the word zecora of the Oromo language of northeastern Africa. For the first time the Portuguese named striped animals of central Africa with zebra in the 15th century . Later this was also carried over to similar animals of southern Africa.

## Horses and people

### prehistory

The importance of horses for humans goes back to the Paleolithic . The animals were mainly used as raw materials and food resources. Remnants of horses can be found at numerous sites from the Old to the Upper Palaeolithic in Europe alone. Miesenheim in the Neuwied Basin or Ehringsdorf near Weimar in Thuringia are the only examples . Horses are also relatively common at the sites in the Geiseltal from the Middle to Young Pleistocene . While the use of the animals can be determined with comparative certainty by cutting marks and impact marks on the bones, evidence of direct hunting is far less common at this time. One of the most impressive comes from Schöningen in Lower Saxony, where in an area of ​​around 1200 m², in addition to eight airworthy wooden spears up to 250 cm long, countless remains of horses were discovered. According to analyzes, almost all of these horse remains belong to Equus mosbachensis , and also to Equus hydruntinus . The finds date to the late Middle Pleistocene and are thought to be between 300,000 and 400,000 years old. At around 50,000 years old, the skeleton of an African donkey was found in Umm el Tlel in Syria and had a splintered Levallois tip stuck in its cervical spine as a relic of a former hunting event.

Horse figure from the Vogelherd cave in the Lone Valley , around 35,000 years old

In the Upper Palaeolithic, the outstanding position of horses is particularly evident in cabaret and cave painting , which appeared around 35,000 to 40,000 years ago. In the cave art of the Franco-Cantabrian area alone , at least two dozen sites with depictions of horses are known. There are colored drawings as well as engravings, incisions and reliefs. The sometimes very individually designed portraits not only reveal the early ancestors of the wild horse, different donkey shapes were shown - albeit less often - with interpretations ranging from the “European wild ass ” ( Equus hydruntinus ) to the Asian donkey. Horses represent the animals most frequently depicted. They account for around 27% of all animal representations, which puts them ahead of the horned bearers and deer . The frequency of horses is very different from cave to cave. The oldest cave works of art include those of the Chauvet grotto with around 40 portraits of wild horses from around 32,000 to 26,000 years ago. In Lascaux, on the other hand, over 360 representations of horses were counted, which means that they represent around 60% of all animal images. At around 17,500 years old, they are only about half as old as those of the Chauvet Grotto. In addition to the horse representations of the Franco-Cantabrian cave art, individual images from the Kapova cave in the Urals have also been described. Horses have a similarly extensive share in the Upper Paleolithic small art. Here you can distinguish between carvings in stone or bone or fully and semi-sculptural figures. The only 4.8 cm long horse figure with a curved neck from the Vogelherd Cave in the Lone Valley in the Swabian Alb , for example, is of particular importance , as it is one of the oldest art objects in the world together with other animal figures with an age of around 35,000 years.

### Domestication

House egg line according to Kimura et al. 2011
House line

 E. a. somaliensis (Somali wild ass) E. a. asinus (house donkey clade 2 )

E. a. africanus (Nubian wild ass) / E. a. asinus (house donkey clade 1 )

Pack donkey

In their domesticated form as house horses and donkeys , horses made an important contribution to the history and cultural development of mankind. They mainly had the function of riding , working and pack animals . The timing of domestication of both species is under discussion and is being investigated using different approaches. It is estimated that this happened to the domestic donkey around 3000 BC. In ancient Egypt . The oldest unambiguous evidence from this period includes some well-preserved skeletal finds from Abydos . After anatomical examinations by a team led by Stine Rossel and Fiona B. Marshall in 2008, the animals were primarily used as load carriers. But there are also indications that the house donkey was already in predynastic times around 4000 or 4500 BC. Occurred. This is supported by individual finds of small animals from Tell el-Iswid or from El Omari in Lower Egypt and from Nagada in Middle Egypt. According to genetic evidence, the African donkey is the wild form of the house donkey . In the first analyzes from 2004, the Nubian subspecies (Nubian wild ass) could be identified as the likely initial form. However, the domestic donkey may have been domesticated multiple times, as these early studies showed. In later investigations, at least two clades of the house urchin could be worked out, each going back to independent domestication processes. Clade 1 largely represents today's domestic donkey and probably has its origin in northern Africa. In its mitochondrial DNA it is indistinguishable from the Nubian wild ass. In contrast, Clade 2 is closer to the Somali wild ass, but is not identical to this subspecies. It represents the second form of domestication, but in contrast to Clade 1, it is based on a smaller starting group. Due to its peculiarity, the ancestral form and the place of origin of Clade 2 could not yet be identified more precisely.

House horse line according to Gaunitz et al. 2018 and Fages et al. 2019
House horse line

original Iberian wild horses

Equus lenensis (Siberia)

 Botai horses (domesticated) Equus przewalskii (Przewalski's horse)

 Equus caballus (domestic horse; Neolithic, Bronze Age) Equus caballus (domestic horse; present)

Double-deck car from Dresden's first horse-drawn tram

Domesticated horses appeared around the same time period. Mainly in Central and West Asia , some of the locally existing archaeological cultures of the Late Neolithic and Early Bronze Age were largely based on the use of horses as a source of raw materials, not just for food but also for tool making. The animals also found their way into ritual acts, their great importance is reflected, among other things, in numerous art objects with horse motifs. Among the best known cultural groups include the Khvalynsk culture in Russia , the ocher grave culture in the Ukraine and the Botai culture in Kazakhstan . For a long time it was discussed whether the horses used represented wild animals or domesticated animals. At least the horses of around 3500 BC. The botai culture that formed in the BC show signs of wear on the premolars , which are characteristic of bridles . Accordingly, they were tamed animals that, in addition to being used as a source of food, were also used for riding, according to some considerations, which possibly increased the mobility of the steppe peoples. However, a 2018 study on horse remains from the Botai culture showed that these early domesticated horses do not belong in the line of today's domestic horses. Instead, they form the basis of the Przewalski horse , which has long been considered the original wild form. Today's domestic horse must therefore have been domesticated again. This view is also supported by the long genetic separation between Przewalski's horse and domestic horse, which dates back to the Eem warm period. So far nothing is known about the founding group of the house horse. During the period in question, individual wild horse populations lived in Eurasia, including one in Siberia and one on the Iberian Peninsula , both of which, however, contributed little to the domestic horse gene pool. It is also unclear where this took place. Possibly the new taming took place in the Western Eurasian steppe landscapes. A more dynamic process can be assumed, in which regional wild forms were repeatedly crossed. DNA analyzes on finds of Pleistocene and Early Holocene wild horses and on domestic horses of the Neolithic as well as the Bronze and Iron Ages revealed a relatively high variety of colors, which probably only developed in domestication and breeding. Another study from 2019 came to a similar conclusion. However, it indicates that there is a significant influence of Persian animals in today's domestic horses, which only developed in the last millennium due to the sometimes strong Islamic character of some regions of Europe. In addition, modern breeding practices led to a decrease in the diversity of domestic horses. Occasionally this happened in the distant past, as shown by the example of the "leopard-spotted" horses (mainly white animals with black spots). These have been known genetically since the end of the Pleistocene and found their way into the gene pool of early domestic horse populations. In the subsequent period, such as the late Bronze Age, they disappeared several times and were obviously reintroduced. One reason for the multiple breeding of this trait could be that the offspring may be night-blind and could therefore be more easily captured by predators .

Bronze Age snaffle gag made from deer antlers as a reference to domestic horses

Similar to the domestic donkey, the first domestic horses in the western part of Eurasia were probably first used as carrying and draft animals. Since the Bronze Age, they have been increasingly used as mounts, as suggested by some rock art in Sweden, including the equestrian rock image of Tegneby . When the domestic horse appeared in western Eurasia is not clear. Generally one assumes the early Bronze Age, which among other things suggests finds of bridles . On the other hand, individual findings from the middle Neolithic lead to the assumption that domestic horses may have been used here much earlier. This includes, for example, a horse's skull that was intentionally laid down in the earthworks of Salzmünde in Saxony-Anhalt from the time of the funnel cup . It dates from around 3400 to 3100 BC. Other very early references to domesticated horses in Central Europe were reported from Vyškov in South Moravia , among others . Two horse skulls lay in a grave with a human corpse from the time of the bell-cup culture .

Mustangs as feral domestic horses of North America

Both the domestic donkey and the house horse achieved worldwide distribution as companions and farm animals of humans. They reached areas in which wild horses had previously become extinct (America) or which they had never colonized (Australia and numerous remote islands). The wild horses of America, for example, largely come from European breeding. The first modern horses came to America in 1492 in the wake of Christopher Columbus . The animals came from the province of Seville , mainly from the salt marshes of the Guadalquivir river . According to historical reports, a group of 70 horses were stationed on Hispaniola as early as 1503 . In the course of the colonization of America, the Spaniards introduced Iberian horses to numerous regions. In 1553, around 10,000 wild specimens were living in the Mexican state of Querétaro alone . The ancestry of the first domestic horses in America can also be proven genetically, as studies on mustangs and European and Arab breeding lines show. According to this, almost a third of all mustangs examined have genetic connections to Iberian horses. Some American breeding lines also have their origins in Iberian horses, since the North American Sulfur and Spanish Mustangs , for example, share common and sometimes very original haplotypes with the Menorcans and Sorraias . The North American pedigree horses generally have less variability than their European relatives, which is due to the only small founder groups. The same can be said for some South American lines. On the other hand, the Iberian horses do not form a closed unit either, as they were subject to the influence of various breeding lines, for example from North African horses during the rule of the Moors . Subsequently, the Europeans also brought other breeding lines to America, some of which also went wild there.

Due to the motorization of agriculture and the spread of automobile traffic, the use of horses and donkeys in the western industrialized countries for passenger and goods traffic has decreased significantly, riding is mostly only practiced as a hobby or sport. In the underdeveloped regions of the world, the use of animals as a means of transport is still widespread. Another important area of ​​use is horse meat as food. The mare's and donkey's milk are also used, and the skin of both types is made into leather, with horse leather being of particular importance in the manufacture of elaborate shoes. In contrast to other farm animals, however, these purposes always played a subordinate role. There are also many uses for horsehair .

In contrast to wild horses and donkeys, zebras were never domesticated, only individual animals were tamed. Reasons may lie in the higher aggressiveness of the stallions, who fight more often with each other with possibly more serious injuries. This is especially true for the group-forming plains zebra, which come together to form larger communities for longer migrations in the annual rhythm. In addition, zebras have a better side view than the other representatives of the genus, so that it is more difficult to catch.

### hybrid

The descendants of a donkey stallion and a horse mare are called mules ; the opposite case, descendants of horse stallions and donkeys, as mules . Crossbreeds between horse and zebra or donkey and zebra are called zebroids . Most hybrids of the genus Equus are born in human captivity and in some cases are specifically bred because the offspring are stronger and sometimes more resistant than the parent forms. However, they can also occur in the wild if the distribution areas of two horse species overlap. A natural hybridization is currently only the case in the zebra and the African wild ass. In the phylogenetic past, there had been several gene exchanges between the individual species.

### Danger

The quagga became extinct at the end of the 19th century.

Most horse species are endangered. Hunting and habitat restriction have pushed many species to the brink of extinction . The quagga , a subspecies of the plains zebra , was extinct at the end of the 19th century. The Przewalski horse is considered extinct in the wild and only lives in nature thanks to reintroduction projects . There are only a few hundred specimens left of the wild African donkey , the IUCN lists it as critically endangered . The Asian donkey and Grevy's zebra are endangered , the mountain zebra are endangered ( vulnerable ). The plains zebra and the kiang are currently not endangered ( least concern ). With regard to the two forms of domestic animals, the existence of the house urchin is endangered. The skin of animals in particular is used in traditional Chinese medicine when it is ground into powder or jelly ( ejiao ) , for which it is estimated that up to 4.8 million individuals are slaughtered each year. In China alone, the population decreased from 11 million house-eggs in 1992 to 2.6 to 4.6 million in 2017. Some experts suspect that the global population could collapse in half within five years (as of 2019).

## literature

• Stephen Budiansky: The Nature of Horses: Exploring Equine Evolution, Intelligence, and Behavior . The Free Press, New York 1997, ISBN 0-684-82768-9 .
• Jens Lorenz Franzen: The primeval horses of the dawn . Elsevier, Spektrum Akademischer Verlag, Munich 2007, ISBN 3-8274-1680-9 .
• Ronald M. Nowak: Walker's Mammals of the World . The Johns Hopkins University Press, Baltimore 1999, ISBN 0-8018-5789-9 .
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