September 5, 2013 | 34
Come on, this is Tetrapod Zoology: you knew those asses would be of the equid kind, right? I don’t think there’s been much on Tet Zoo about equids yet, nor about perissodactyls at all (a crime, given my strong interest in fossil rhinos). See the links below, however. I’ve taken various wild ass photos at zoos over the years and today is a good chance to use a few of them.
The nomenclature used for asses differs according to which source you consult. All of the African forms – sometimes termed asinines – are included in the species Equus asinus, and indeed domestic donkeys are extremely close relatives of both the Nubian wild ass E. a. africanus and Somali wild ass E. a. somaliensis. Somali wild asses differ obviously from Nubian wild asses and domestic donkeys in their striped legs. While the Nubian wild ass is probably extinct in the wild, the Somali wild ass is critically endangered and reduced to a population of less than 600. A third form – the Atlas wild ass E. a. atlanticus – seems to have become extinct in early historic times.
Domestic donkeys belong to two genetic groups (termed Clade 1 and Clade 2) and it has sometimes been thought that both Nubian and Somali wild asses have contributed to the donkey gene pool, with the Somali wild asses contributing to the domestic donkey population in southern Europe (Clade 2) while donkeys elsewhere are mostly derived from the Nubian wild ass (Clade 1). Recent results indicate that Somali wild asses have little to no contribution to the donkey gene pool (Kimura et al. 2011), however. As with all stories of animal domestication, ideas on donkey domestication are hugely complex and I’m nowhere close to doing them justice here.
Incidentally, wild asses and donkeys are more like zebras than domestic horses in lacking ‘chestnuts’ (those weird, horny excrescences on the inside surfaces of the limbs) on the hindlimbs, an observation consistent with cladograms which show the caballine or caballid or caballoid lineage (domestic horses, European forest horses and so on) to be outside an ass + zebra clade. Nobody seems to know what the ‘chestnuts’ are: I’ve heard horsey people say that they’re vestigial remains of digits – Dent (1972) says this – but this can’t be true since they’re not associated with the metacarpus or metatarsus, the only places where digits occur.
The history of the donkey and the variation that we’ve bred into its various forms is really interesting (there are dwarf donkeys, giant donkeys, shaggy-furred donkeys and so on). I recently got hold of Anthony Dent’s amusingly titled Donkey: the Story of the Ass from East to West (Dent 1972) after being in quest of it for some years. It’s there on the shelf next to Dent and Goodall’s A History of British Native Ponies.
The Asian wild asses are the Kulan, Onager and Kiang, grouped together as the hemionines or hemionids. Today, the Kulan and Onager are generally considered subspecies of the same species – E. hemionus – while the Kiang (also called the Tibetan wild ass, Khyang or Gorkhar) of the Tibetan Plateau is E. kiang. It’s the largest of the asses, reaching a shoulder height of 1.4 m (that’s 13.3 hands, apparently) and with big males weighing 400 kg. As is typical for equids from seasonally cool places, its summer coat is far shorter than the much darker winter one. Kiang are chestnut coloured dorsally and with a dark brown dorsal stripe, and white muzzle, legs and underparts. However, there’s quite some variation in how dark even the summer coat is. This has, combined with differences in body size, led to the idea that there are three Kiang subspecies: the Western kiang (E. k. kiang), Eastern kiang (E. k. holdereri) and Southern kiang (E. k. polyodon), the last of which is so rare that it was thought extinct prior to the reported rediscovery of about 100 animals in 1996. There’s some suspicion that the three forms grade into one another and simply represent a cline: molecular work is needed on the different populations.
Kiangs are steppe-dwelling animals that live in female-led herds that are famous for their cohesion, the animals walking or running in file and sticking tightly in a group even when panicking. A peculiarity of their behaviour compared to that of domestic horses is that they indulge in little allogrooming. Adult males live alone for much of the year but form all-male bands during the winter (Nowak 1999). Protracted, bloody battles have been recorded as stallions have fought to defend their harems. As you might predict from the social behaviour associated with this lifestyle, sexual dimorphism is pronounced. I recently photographed the animals you see above at Edinburgh Zoo. Note that the male (the larger animal on the left) has a blockier-shaped head, with a more strongly downturned nose.
Kiang have become increasingly rare as their steppe habitat has been taken over for grazing by domestic horses, and also for agriculture and industry. Nowak (1999) says that there are only 52 captive Kiang outside of China. I’m not sure if there are Kiang x domestic donkey hybrids. Onagers have certainly been crossed with donkeys, despite the major chromosomal differences (in Onager, 2n = 54, in donkeys, 2n = 62). But then, hybridisation of just about any sort goes across Equidae: a review of this subject is in fact titled ‘Interspecific and extraspecific pregnancies in equids: anything goes’ (Allen & Short 1997).
Wild ass evolution is more interesting that you might think since there have been suggestions that the fossil species E. conversidens of the North American Pliocene and Pleistocene is especially close to the Kiang, in which case the Kiang lineage may have a distinct origin and history relative to those of the Onager and Kulan. This is inconsistent, however, with genetic data which indicates that E. hemionus and E. kiang are very close kin that only separated less than 500,000 years ago (Ryder & Chemnick 1990). If you’re wondering, molecular studies don’t find asinine and hemionine asses to group together: George & Ryder (1986) found E. hemionus to be closer to zebras than to African wild asses and Orlando et al. (2009) found the hemionine and asinine clades to be well separate. Then there are the fossil asses of Africa, some of which (like E. tabeti) have been regarded as closer to hemionines than to asinines by some authors. I’m not endorsing this, it might be erroneous (see Churcher 1982).
Zebras? Don’t get me started: I’ve been trying to finish an article on them since 2006. ‘Stripes do not a zebra make’ (Bennett 1980), and while some studies do find zebras to form a clade (George & Ryder 1986), others find them scattered all about the equid tree (Bennett 1980, Orlando et al. 2009). Specifically, Orlando et al. (2009) found Grevy’s zebra E. grevyi to belong to a clade that also included the hemionines while Hartmann’s mountain zebra E. hartmannae belonged to a clade that also included asinines. The Quagga and plains zebras E. quagga were off on their own.
Oh, then there’s E. hydruntinus and the sussemiones. They’ll have to wait to another time – this was meant to be a quick picture-led post!
For previous Tet Zoo articles on perissodactyls, see…
Refs – -
Allen, W. R. & Short, R. V. 1997. Interspecific and extraspecific pregnancies in equids: anything goes. The Journal of Heredity 88, 384-392.
Bennett, D. K. 1980. Stripes do not a zebra make, part I: A cladistic analysis of Equus. Systematic Zoology 29, 272-287.
Churcher, C. S. 1982. Oldest ass recovered from Olduvai Gorge, Tanzania, and the origin of asses. Journal of Paleontology 56, 1124-1132.
Dent, A. 1972. Donkey: the Story of the Ass from East to West. George G. Harrap & Co., London.
George, M. & Ryder, O. A. 1986. Mitochondrial DNA evolution in the genus Equus. Molecular Biology and Evolution 3, 353-546.
Kimura, B., Marshall, F. B., Chen, S., Rosenbom, S., Moehlman, P. D., Tuross, N., Sabin, R. C., Peters, J., Barich, B., Yohannes, H., Kebede, F., Teclai, R., Beja-Pereira, A. & Mulligan, C. J. 2011. Ancient DNA from Nubian and Somali wild ass provides insights into donkey ancestry and domestication. Proceedings of the Royal Society B: Biological Sciences 1702, 50-57.
Nowak, R. M. 1999. Walker’s Mammals of the World, Volume II. The Johns Hopkins University Press, Baltimore and London.
Orlando, L., Metcalf, J. L. Alberdi, M. T., Telles-Antunes, M., Bonjean, D., Otte, M., Martin, F., Eisenmann, V., Mashkour, M., Morello, F., Prado, J. L., Salas-Gismondi, R., Shockey, B. J., Wrinn, P. J., Vasil’ev, S. K., Ovodov, N. D., Cherry, M. I., Hopwood, B., Male, D., Austin, J. J., Hänni, C. & Cooper, A. 2009. Revising the recent evolutionary history of equids using ancient DNA. Proceedings of the National Academy of Sciences 106, 21754-21759.
Ryder, O. A. & Chemnick, L. G. 1990. Chromosomal and molecular evolution in Asiatic wild asses. Genetica Dordrecht 83, 67-72.