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Horses and their Allies

Family Equidae

Mammalian Hybrids

EUGENE M. MCCARTHY, PHD GENETICS
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zeedonk
A zeedonk

Note: According to McClintock and Mochi (1976), the chromosome counts of zebras are 2n = 46 (E. grevyi), 2n = 44 (E. burchelli) and 2n = 32 (E. zebra). The domestic horse (E. caballus) has 2n = 64, and the ass (E. asinus), 2n = 62. The count for Przewalski’s Horse (E. ferus) is 2n = 66.

Note: Duff and Lawson (2004) list "wild ass" as the English common name of Equus africanus, stating that the range of the animal in question is northeastern Sudan, northeastern Ethiopia, and northern Somalia, the region where the animal often accorded the common name "Somali Wild Ass" resides. However, "wild ass" is a name traditionally associated with E. asinus, which is often equated with "domestic donkey." Actually, the wild animal most closely resembling the domestic donkey is often referred to as the "Nubian wild ass." It is often said to be extinct, but it's uncertain whether wild populations of the Nubian have been in existence since prehistoric times or whether currently existing wild populations are simply feral domestic asses (asses have been domesticated in Egypt and the Middle East for some 6,000 years). Its appearance differs from that of the Somali wild ass primarily in that it lacks the zebra-like leg striping of the latter. E. asinus was originally widespread in northern Africa and southwestern Asia. To avoid confusion, then, instead of following Duff and Lawson, I here will assign E. asinus the common names "Domestic Donkey | Nubian Wild Ass" and will call E. africanus "Somali Wild Ass."

Equus africanus [Somali Wild Ass]
× Equus asinus [Domestic Donkey | Nubian Wild Ass] CANHR. HPF(♀♀). Gray (1971, p. 94) notes two wild female hybrids "showed normal viability and fertility." Pocock (1911b, 993-994) says, "intermediate forms indeed, with very narrow spinal and shoulder stripes and a dusky patch on the ear, connect the Somali Ass with ordinary domestic varieties; and in all the many foals born in the [Zoological] Gardens [of London] between our Somaliland Ass and domestic asses of English and Spanish breeds, the ear-patch, shoulder and spinal stripes were present as in the dams." Natural hybridization is known to occur and is considered a threat to E. africanus. See: Equus asinus × E. grevyi. Rörig 1903 (p. 219); International Zoo Yearbook 1962 (p. 232), 1965 (p. 341). Internet Citations: ALWA.
× Equus burchelli (♀) [Zebra] In 1911, Pocock reported the cross Equus africanus × E. quagga as having occurred at the London Zoo. However, the alleged quagga mother he pictures (fig. 203, p. 992) is indistinguishable from an ordinary Burchell’s Zebra. See: Equus africanus × E. asinus; E. asinus × E. grevyi. Flower 1929a (p. 253); International Zoo Yearbook 1960 (p. 262), 1981 (p. 325); Pocock 1911b.
× Equus quagga [Quagga] See: Equus africanus × E. burchelli.
× Equus zebra (♀) [Mountain Zebra] CHR. DRS. Gestation in one case took about 390 days. A hybrid foal from an E. africanus stallion bred to an E. zebra mare had two transverse shoulder stripes, leg bands and zebra-like ear stripes. Flower 1929a (p. 253); McClintock and Mochi (1976); Pocock 1911b; Rörig 1903 (p. 219).

Equus asinus [Domestic Donkey | Nubian Wild Ass] (2n = 62)
× Bos taurus [European Domestic Cattle] See the separate article "Jumarts."
× Equus burchelli (↔ usu. ♂) [Zebra] CHR. ENHI. HPF(♀♀). This hybrid is known as a zeedonk (or zedonk, zebrass, zonkey, zebradonk, zebrinny, zenkey, zebronkey, or deebra). Usually a zebra stallion is paired with a she-ass, but the reciprocal cross is sometimes produced. Hybrids have striping on their legs, but usually not on their bodies. Individuals with body striping may represent backcrosses to zebra. In general,This account seems to indicate that male hybrids are sometimes partially fertile the stripes are narrower in hybrids than in the pure zebra parent. Grevy’s zebras, with their big heads, large and rounded ears, and thick, erect manes look more like mules than do other zebras. Indeed, the website of the National Zoo (Washington) asserts that many experts view Grevy’s zebras as striped asses, not closely related to zebras (NATZ). Z. grevyi is also geographically intermediate between asses and zebras and is capable of hybridization with both. These facts suggest Z. grevyi as a PHP of crossing between asses and zebras, but the matter should be further investigated. Darwin (1868, vol. II, p. 42) says, "Many years ago I saw in the Zoological Gardens a curious triple hybrid, from a bay mare, by a hybrid from a male ass and female zebra. This animal when old had hardly any stripes; but I was assured by the superintendent, that when young it had shoulder-stripes, and faint stripes on its flanks and legs. I mention this case more especially as an instance of the stripes being much plainer during youth than in old age." See figure in Geoffroy Saint-Hilaire and Cuvier (1824-1842, vol. 3, pl. 315). A skull of a hybrid of this type is in the British Museum (Gray 1873, p. 38). Ewart 1898, 1899; Flower 1929a (p. 253); Grubb 1981; International Zoo Yearbook 1960, 1961, 1962 (p. 232), 1968 (p. 304), 1969 (p. 232), 1971 (p. 279), 1972 (p. 332); King et al. 1966; Riley 1911 (p. 229); Treus et al. 1963; von Lehmann 1982.
× Equus caballus (↔ usu. ♀) [Domestic Horse] CHR. LFH. See the separate article "Mules (Donkey × Horse)."
× Equus grevyi (↔ usu. ♂) [Grevy’s Zebra] CHR. Hybrids between male Grevy’s zebras and female asses are easily obtained and grow quickly. However, the hybrids are of low fertility. The reciprocal cross also occurs. Certain facts suggest the animal known as the Somali Wild Ass (E. africanus) is this hybrid: The first thing to consider is that the putative parents come into contact in the geographic region where E. asinus is known to occur. True wild asses disappeared from most of their African range during Roman times, from Asia, even earlier (Groves 1986), but the ass remains a fairly common beast of burden throughout its original range and feral stocks are widespread. Thus, feral herds of E. asinus exist in northeastern Africa (and, until recently, what may have been primevally wild populations in the form of the Nubian Wild Ass). Southward dispersal of these wild or feral asses would be expected to bring them into contact with E. grevyi in the Horn of Africa. The Somali Wild Ass occurs only in this expected region of contact. Moreover, wild E. africanus are known to interbreed with feral E. asinus (See Internet citation ALWA) and feral donkeys run with zebra herds in eastern Africa (Kingdon 1979, p. 143). E. asinus × E. grevyi hybrids are well known and have the same appearance as E. africanus (in particular the zebra-like leg stripes). E. africanus is therefore geographically intermediate and seemingly identical to captive hybrids of known parentage. So it is a PHP of this cross. This view is further supported by the facts that the Somali Wild Ass is rare (classified as critically endangered by the IUCN) and variable—typical characteristics of hybrids (also reports of Somali wild asses in captivity note that they are difficult to breed, which suggests they suffer from the reduced fertility often associated with hybridity). The hypothesis can be tested both by genetic testing and by observing whether E. africanus populations become more grevyi-like as the range of E. grevyi is approached, which would strongly suggest the existence a hybrid zone. Riley (1911) states that the average gestation time for production of Grevy’s zebra-ass hybrids in an ass dam is 387 days. Antonius 1944b, 1951a; Churcher 1993 (p. 6); International Zoo Yearbook 1962 (p. 232); Flower 1929a (p. 253); Kaminski 1970; Mann 1938; Riley 1910, 1911; Roberts 1929; Rommel 1913; Rząśnicki 1930, 1936; Zuckermann 1953. Internet Citations: NATZ.
× Equus hemionus (↔) [Kulan | Mongolian Wild Ass | Dziggetai] CHR. Kulans probably come into contact with feral asses in Mongolia and northern China. Both male and female hybrids have been reported. Bannikov 1948; Brentjes 1969; de Lavison 1863; Flower 1929a (p. 254); International Zoo Yearbook 1990 (p. 468); Przibram 1910; Rörig 1903 (p. 218); Wagner 1863 (p. 85).
× Equus kiang [Kiang] A male hybrid was born in the London Zoological Gardens on Sept. 27, 1920. Flower 1929a (p. 254).
× Equus onager [Onager] CHR. Antonius 1944a; International Zoo Yearbook 1970 (p. 267).
× Equus zebra (↔ usu. ♀) [Mountain Zebra] CANHR. CON: southern Africa. HPF(♂&♀). Short (1967) notes that the ejaculates of some hybrids contain spermatozoa. This partial fertility suggests (by Haldane’s Rule) that female hybrids, too, will sometimes be partially fertile. Penzhorn (1985) reported a natural hybrid produced by a mating feral jackass and an E. zebra mare. Ewart (in Riley) says that in about 1775, Lord Clive bred the first recorded zebra hybrid by crossing a female Mountain zebra (which he brought with him on returning from India) with a common ass, and that about a quarter of a century later, in 1801, a similar hybrid was bred in Italy. Antonius 1934b, 1944a, 1951a; Benirschke 1964, 1967; Benirschke et al. 1964; Brown and Jenkins 1987; Chang et al. 1969; Geoffroy Saint-Hilaire 1806; Gray 1972; Flower 1929a (p. 253), 1931; International Zoo Yearbook 1966 (p. 398); MacClintock and Mochi 1976; Penzhorn 1985, 1988; Przibram 1910; Riley 1911 (p. 229); Rząśnicki 1930.
× Ovis aries [Domestic Sheep] Gesner (Historiae Animalium, Liber I, de Quadrupedibus viviparis, 1551, p. 19) writes, "At this time, in the palace of the king of France [probably Henri II (reigned 1547-1559)], an animal was raised, which in its anterior portion was like a donkey, but in its posterior parts, like a sheep. And, except in being of a dual nature, it followed each of these two distinct types of animals in all things." Translated by E.M. McCarthy. Original Latin: "Hoc tempore in aula regís Galliarum аnimal nutrírí aiunt ab anteriore parte asini posteríore ovis specíe. Sed praeter ínstitutum est bígenera hîc omnia persequi." This animal, if it ever truly existed, would probably have been kept in the menagerie that Henri created at the Chateau de Madrid, his palace in the Bois de Boulogne. For an extensive discussion of a related cross, see also the separate article “Jumarts.”

Note: In regions where they interface, hybridization occurs between all of the various populations treated as races of Equus burchelli (Grubb 1981, Fig. 5). Kingdon (1979) suggested that certain populations treated as races of E. burchelli are actually the products of hybridization between other such populations. Thus, he suggests hybridization between boehmi and crawshayi produced zambeziensis, and that crossing between burchelli and crawshayi produced both antiquorum and chapmanni.

Historical note on zebra hybridization. Ewart (quoted in Riley 1911, pp. 229-230) provides the following brief summary of zebra hybrids produced prior to the twentieth century:

The first zebra hybrids bred were between the ass and the zebra mare, later came crosses between the zebra mare and the horse and later still, zebra mules—crosses between male zebras and ordinary mares.

Lord Clive seems to have bred the first zebra hybrid by crossing a female Mountain zebra (which he brought with him on returning from India) with a common ass. About a quarter of a century later (in 1801), a similar hybrid was bred in Italy and soon after this (in 1806) the first of a series of zebra-ass crosses made its appearance in Paris. Later still, zebra-ass hybrids were bred at Windsor Park (in the time of His Majesty George the Fourth) and at Lord Derby’s once famous menagerie at Knowsley. At least one of the Knowsley hybrids was a cross between an Asiatic ass (E. hemionus) and a Burchell’s zebra mare. A similar hybrid was bred in the Jardin des Plantes, Paris in 1875, and in 1893 a hybrid between a Burchell’s zebra mare (Chapman variety) and a white ass was bred in the Zoological Gardens, Melbourne. There is no record of zebra mules having been bred in the Zoological Gardens, London, but it is reported an effort is now being made in this direction. Some of the zebra-ass hybrids bred in Paris found their way about three years ago to England. One of these—evidently out of a Burchell’s zebra mare—I had the opportunity of studying through the kindness of the Hon. Walter Rothschild.

When the first hybrid between a zebra mare and a horse was bred is uncertain. But for the untimely death of a zebra mare, F. Cuvier would have succeeded in obtaining a hybrid of this kind in 1808. In the Jardin d’Acclimatisation several horse-zebra crosses seem to have been obtained prior to 1880, and between 1880 and 1890 three were bred by Lady Meux at Theobalds Park, Hertfordshire.

In 1815, a hybrid of some historic interest was bred by Lord Morton by crossing the often-referred-to seven-eighths Arabian chestnut mare with a quagga. A similar hybrid Darwin tells us was bred by Lord Mostyn. Later (about 1870), it is said a cross was obtained in the Jardin des Plantes between a pony mare and a male Mountain zebra.

Note: In crosses of Equus burchelli with other zebras, the striping patterns of the hybrids are generally intermediate between those of the parents (Grubb 1981). In crosses with the horse or ass, the striping in the hybrids is narrower than in the E. burchelli parents (Grubb 1981).

zorse
Cuvier and Lacépède's illustration of "Equus quagga," which has all the appearance of being a typical zorse (Cuvier and Lacépède 1808)

Equus burchelli [Burchell’s Zebra] (2n = 44)
See also: Equus asinus.
× Equus caballus (↔ usu. ♀) [Domestic Horse] CHR. HPF(♀♀). This hybrid is known as a zorse. This cross occurs in captivity, but it seems likely that the quagga, an animal treated as a species (E. quagga) and now said to be extinct, was actually this hybrid. It is classified as extinct by the IUCN, but Lowek (1999, vol. 2, p. 1024) notes that "although E. quagga sometimes is described as having much less pronounced striping than other zebras, it actually demonstrates considerable variation. Some individuals are almost without stripes, but others have a pattern nearly the same as seen in the neighboring South African subspecies, E. burchelli burchelli, with stripes covering most of the body except for the hindquarters, belly and legs." Lowek goes on to say (p. 1025, citing Harley 1988) that there has been some investigation of the possibility of using selective breeding of quagga-like E. burchelli individuals to restore Equus quagga. Grubb (1981, p. 7, citing Antonius 1951a; Gregory 1926; Lyon 1907; Rau 1974, 1978), too, notes that pelage patterns supposedly diagnostic of E. quagga are present in specimens of E. burchelli burchelli (which is itself classified as extinct). These facts suggest the former existence, of extensive gene flow between E. burchelli burchelli and E. quagga. In this connection it is of interest that Barrow (1801-1804), in his An Account of Travels into the Interior of Southern Africa, in the Years 1797 and 1798, observed mixed herds of quagga and Burchell’s Zebra. On the other hand, some evidence suggests the quagga was more of a wild horse than a zebra. Thus, in an analysis of skulls and dentition, Bennett (1980) concluded quaggas are more like horses than zebras. In a similar study, Azzaroli and Stanyon (1991) concluded E. burchelli and E. zebra are more closely related to each other than either is to E. quagga. Lowek 1999 (p. 1023) pictures an intermediate between quagga and E. burchelli. Kingdon (1979, p. 139) notes that "although only 17 mounted skins [of quaggas] are known to exist (Harper, 1945), plus a few pictures, the range of variation was evidently far greater than that of living zebra populations." A high level of variability is characteristic of hybrid populations. Kingdon also says (p. 139) that quaggas and horses have thicker winter coats, but not zebras. Hence, the quagga was rare even when extant (given Kingdon’s comment on the rarity of skins), highly variable, and morphologically intermediate between E. burchelli and E. caballus, two equids for which partially fertile hybrids are known in captivity. Moreover, it occurred in a region where horses introduced by early Europeans (17th century) would have come into contact with native zebras. Also Keast (1965, p. 58) notes that natural hybrids between horse and zebra have been repeatedly reported. For all these reasons, the quagga is a PHP of this cross. All extant pictures of quaggas held in the London Zoo are collected in Edwards (1995). Anonymous 1901; Breen and Gill 1991; International Zoo Yearbook 1969 (p. 233), 1970 (p. 266), 1977 (p. 322); Flower 1929a (p. 253); Grubb 1981; King et al. 1966; Kingdon 1979 (plate facing p. 139); Rörig (1903, p. 218); Treus et al. 1963. Internet Citations: GREE1.
× Equus ferus [Przewalski’s Horse] CHR. DRS. Treus et al. 1963.
× Equus grevyi [Grevy’s Zebra] CANHR. CON: Kenya. HPF(♀♀). A probable natural hybrid was reported by Keast (1965) Striping of hybrids is complete down to the hooves, though E. burchelli has white shanks and pasterns. Male hybrids produce abnormal spermatozoa. Keast (1965, p. 58) says a Kenyan taxidermist possessed the skin of a probable hybrid, which may have been of natural origin. Grubb (1981, p. 6) notes that these hybrids resemble E. zebra. These zebras differ with respect to karyotype. Benirschke 1977; Grubb 1981 (p. 6); International Zoo Yearbook 1990 (p. 469); Keast 1965; Ryder et al. 1978.
× Equus hemionus [Kulan | Mongolian Wild Ass | Dziggetai] CHR. DRS. Gray (1873, p. 38) said there was a stuffed specimen of this hybrid in the British Museum. Flower 1929a (p. 253); International Zoo Yearbook 1973 (p. 336), 1975 (p. 380); Kingdon 1979 (p. 138); Wagner 1863 (p. 85).
× Equus quagga [Quagga] See: Equus burchelli × E. caballus.
× Equus zebra (♀) [Mountain Zebra] CHR. CON: Namibia, Angola. Most hybrids are female. LFH. Hybrid foals resemble E. burchelli, except for their larger ears and their hindquarters pattern. They have no dewlap. Wilson and Reeder (2005) state that "Equus wardi Ridgeway, 1910 is a hybrid between E. burchelli and E. zebra." Shortridge (1934), quoting Flower, says such a hybrid survived several years in the Paris Botanic Gardens. Flower 1929a (p. 253); McClintock and Mochi (1976); International Zoo Yearbook 1979 (p. 371), 1989 (p. 336); Pocock 1909.

Equus caballus [Domestic Horse] (2n = 64)
See also: Equus asinus; E. burchelli and the separate article "Jumarts."
× Alces alces (♂) [Moose | Elk] See the separate article "Moose × Domestic Horse."
× Cervus elaphus [Red Deer | Elk | Wapiti] See the separate article “Deer × Horse.”
× Equus ferus (♀) [Przewalski’s Horse] CANHR. CON: Mongolia. HPF(♀♀). Common in captivity. Also, wherever these horses come into contact in the wild, hybridization is likely. The IUCN (Internet Citations: FEREQ) says that for wild, reintroduced populations of Przewalski’s horse, which is critically endangered, hybridization is the primary threat to their survival. Moreover, it is suspected that captive stocks of so-called "E. ferus" are broadly affected by interbreeding with E. caballus. A backcross female (to an E. ferus male) had 2n = 65 and was very like E. ferus. A backcross of the same type was indistinguishable in appearance from E. caballus (with the exception of a prominent dorsal stripe). E. caballus and E. ferus, which have often been treated as conspecific, are now usually treated separately. Ahrens and Stranzinger 2005; Allen and Short 1997; Boyd and Houpt 1994; Hatami-Monazah and Pandit 1979; International Zoo Yearbook 1962 (p. 232), 1967 (p. 315), 1967 (p. 304) , 1969 (p. 232), 1970 (p. 266), 1971 (p. 278), 1973 (p. 336); Koulischer and Frechkop 1966; Ryder et al. 1978; Trommerhausen-Smith et al. 1979; Volf 2003; Wilson and Reeder 2005 (p. 1018).
× Equus grevyi (♂) [Grevy’s Zebra] CAONHR. CON: eastern and southern Africa. Grevy’s stallions do not readily mate with mares, and more services are required to achieve pregnancy than in the case, say of a jackass serving a mare. There is a high rate of spontaneous abortion, and surviving hybrids of both sexes are of very low fertility (sterile?). However, Keast (1965, p. 58) notes that natural hybridization has been repeatedly reported. These hybrids are said always to be chocolate brown, whatever the color of the parent horse, with the Grevyi striping pattern. They also have the zebra’s tufted tail. Antonius 1944b, 1951a; Breen and Gill 1991; Crew and Smith 1930; Churcher 1993; Gray 1972; King 1967; King, et al. 1966; Kingdon 1979 (pp. 138; 143); Lus 1938; Mann 1938; Riley 1911; Roberts 1929; Rommel 1913; Rząśnicki 1936; Short 1967; Webb 1952.
× Equus hemionus (♂) [Kulan | Mongolian Wild Ass | Dziggetai] CHR. Hybrids resemble kulans more than horses, but their tails and manes are long. Ackermann 1898 (p. 45); Antonius 1932a; Bannikov 1948; Przibram 1910; Rörig 1903 (p. 218).
× Equus onager (↔) [Onager] CHR. Brentjes 1969; Rörig 1903.
× Equus quagga [Quagga] CHR. Bell 1837 (p. 392) states that "the Earl of Morton being desirous of obtaining a breed between the Horse and the Quagga, selected a young Mare of seven-eighths Arabian blood and a fine male of the latter species; and the produce was a female hybrid. The same Mare had afterwards first a Filly and afterwards a Colt by a fine black Arabian Horse. They both resembled the Quagga in the dark line along the back, the stripes across the forehead and the bars across the legs; in the Filly the mane was short stiff and upright, like that of the Quagga; in the Colt it was long, but so stiff as to arch upwards and hang clear of the sides of the neck; in other respects they were nearly pure Arabian as might have been expected from fifteen-sixteenths Arabian blood."
× Equus zebra (↔) [Mountain Zebra] CHR. CON: Africa. Rörig 1903 (p. 219).
× Odocoileus virginianus [White-tailed Deer] See the separate article “Deer × Horse.”

Equus ferus [Przewalski’s Horse] (2n = 66) See: Equus burchelli; E. caballus. E. ferus is often treated as a subspecies of Equus caballus.

Equus grevyi [Grevy’s Zebra] (2n = 46)
See also: Equus africanus; E. asinus; E. burchelli; E. caballus.
× Equus zebra (♀) [Mountain Zebra] CANHR. CON: eastern Africa. According to Groves (1974), attempts to breed Grevy’s stallions to mountain zebra mares are associated with high rates of abortion. However, entirely viable hybrids have been produced. For example, Gray (1972, p. 108) says a hybrid was born without difficulty at Schönnbrunn Zoo. It developed rapidly, and had "a lively temperament." King (1965, Fig. 10) describes a probable natural hybrid. International Zoo Yearbook 1973 (p. 336); Kingdon 1979 (p. 138); McClintock and Mochi (1976); Rząśnicki 1930, 1951.

Equus hemionus [Kulan | Mongolian Wild Ass | Dziggetai] (2n = 54)
See also: Equus asinus; E. burchelli; E. caballus.
× Equus kiang (♀) [Kiang] CHR. CON: northern China? HPF. International Zoo Yearbook 1978 (p. 391), 1984/1984 (p. 545).
× Equus onager (↔) [Onager] CHR. DRS. According to the IUCN (Internet Citation: ONAG1), supposed onagers reintroduced in Israel are actually hybrids of this type. Flower 1929a (p. 254); International Zoo Yearbook 1973 (p. 336), 1975 (p. 380), 1978 (p. 391), 1980 (p. 437), 1981 (p. 324), 1982 (p. 427), 1984/1984 (p. 545), 1988 (p. 474).
× Equus zebra (♀) [Mountain Zebra] CHR. Hybrids were bred at London Zoo in the 1830s. Rörig 1903 (p. 218); Flower 1929a (p. 253).

Equus kiang [Kiang] See: Equus asinus; E. hemionus.

Equus onager [Onager] See: Equus asinus; E. hemionus.

Equus quagga [Quagga] See: Equus burchelli× E. caballus.

Equus zebra [Mountain Zebra] (2n = 32) See: Equus africanus; E. asinus; E. burchelli; E. burchelli × E. grevyi, E. grevyi; E. hemionus.


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