[15] The change may have come about through a tendency to avoid pejorative terms in speech and may be comparable to the substitution in North American English of rooster for cock, or that of rabbit for coney, which was formerly homophonic with cunny (a variation of the word cunt).

The genus appears to have spread quickly into the Old World, with the similarly aged Equus livenzovensis documented from western Europe and Russia.

[18] Molecular phylogenies indicate the most recent common ancestor of all modern equids (members of the genus Equus) lived ~5.6 (3.9–7.8) mya.

[19] The oldest divergencies are the Asian hemiones (subgenus E. (Asinus), including the kulan, onager, and kiang), followed by the African zebras (subgenera E. (Dolichohippus), and E. (Hippotigris)).

Donkeys were probably first domesticated by pastoral people in Nubia, and they supplanted the ox as the chief pack animal of that culture.

The domestication of donkeys served to increase the mobility of pastoral cultures, having the advantage over ruminants of not needing time to chew their cud, and were vital in the development of long-distance trade across Egypt.

By the Gold Rush years of the 19th century, the burro was the beast of burden of choice of early prospectors in the western United States.

[32] The Domestic Animal Diversity Information System (DAD-IS) of the FAO listed 189 breeds of ass in June 2011.

The rapid increase is attributed to attention paid to identification and recognition of donkey breeds by the FAO's Animal Genetic Resources project.

[37][38] Donkeys vary considerably in size, depending on both breed and environmental conditions, and heights at the withers range from less than 90 centimetres (35 in) to approximately 150 cm (59 in).

[6] Although jennies come into heat within 9 or 10 days of giving birth, their fertility remains low, and it is likely the reproductive tract has not returned to normal.

Some large donkey breeds such as the Asino di Martina Franca, the Baudet du Poitou and the Mammoth Jack are raised only for mule production.

[50] Likely based on a stronger prey instinct and a weaker connection with humans, it is considerably more difficult to force or frighten a donkey into doing something it perceives to be dangerous for whatever reason.

[51] Although formal studies of their behaviour and cognition are rather limited, donkeys appear to be quite intelligent, cautious, friendly, playful, and eager to learn.

Of the more than 40 million donkeys in the world, about 96% are in underdeveloped countries, where they are used principally as pack animals or for draught work in transport or agriculture.

In their native arid and semi-arid climates, donkeys spend more than half of each day foraging and feeding, often on poor quality scrub.

[71] The donkey has a tough digestive system in which roughage is efficiently broken down by hind gut fermentation, microbial action in the caecum and large intestine.

If they are worked long hours or do not have access to pasture, they require hay or a similar dried forage, with no more than a 1:4 ratio of legumes to grass.

[78] In temperate climates the forage available is often too abundant and too rich; over-feeding may cause weight gain and obesity, and lead to metabolic disorders such as founder (laminitis[79]) and hyperlipaemia,[77] or to gastric ulcers.

[citation needed] Feral donkeys can also cause problems, notably in environments that have evolved free of any form of equid, such as Hawaii.

[82] There is a small community of feral donkeys on St. John, U.S. Virgin Islands, that descend from the animals brought by Danish colonists for agricultural work.

While they add to the island's charm, they also cause issues like vegetation damage and road hazards, leading to population management efforts.

They may compete with livestock and native animals for resources, spread weeds and diseases, foul or damage watering holes and cause erosion.

[86] The earliest documented donkey hybrid was the kunga, which was used as a draft animal in the Syrian and Mesopotamian kingdoms of the second half of the 3rd millennium BCE.

A cross between a captive male Syrian wild ass and a female domesticated donkey (jenny), they represent the earliest known example of human-directed animal hybridization.

They were produced at a breeding center at Nagar (modern Tell Brak) and were sold or given as gifts throughout the region, where they became significant status symbols, pulling battle wagons and the chariots of kings, and also being sacrificed to bury with high-status people.

They fell out of favor following the introduction of the domestic horse and its donkey hybrid, the mule, into the region at the end of the 3rd millennium BCE.

It is commonly believed that mules are more easily handled and also physically stronger than hinnies, making them more desirable for breeders to produce.

[citation needed] The offspring of a zebra–donkey cross is called a zonkey, zebroid, zebrass, or zedonk;[89] zebra mule is an older term, but still used in some regions today.

The foregoing terms generally refer to hybrids produced by breeding a male zebra to a female donkey.