As some of you now know, finally I have something that might be considered close to a dream job: I’m now a researcher for Impossible Pictures, the company that did Walking With Dinosaurs, Primeval and a host of other things (website here). This job isn’t going to be forever, but it’s a start, and it doesn’t make me feel any less bitter about being unable to get a job in academia. It means lots of expensive commuting (I don’t live in London, where the company’s based), and it also explains the recent lack of blog posts. But I’m not complaining. So, after turtle genitals and ostrich dinosaurs, I’ve decided to come back with a bang, and blog about… sheep.
Sheep are caprine bovids closely related to goats, and the two groups must have shared an ancestor highly similar to the Barbary sheep, Aoudad or Arrui Ammotragus lervia (Cassinello 1998). While caprines may have originated during the Late Miocene in the Mediterranean region (Ropiquet & Hassanin 2004), sheep might be ancestrally Asian given that the oldest sheep (Ovis shantungensis) is from the Pliocene of China. Unlike goats, sheep possess preorbital, interdigital and inguinal glands, they lack a beard, possess a short tail, and usually exhibit spiralling, supracervical horns. The true sheep belong to the genus Ovis: the bharals or blue sheep Pseudois aren’t true sheep, nor is the Barbary sheep. It’s been suggested at times that takin Budorcas and the recently extinct Balearic cave goat Myotragus balearicus are the closest relatives of true sheep.
Given that, in archaeological or palaeontological samples, it is difficult to distinguish the incomplete postcranial remains of sheep from those of goats, indeterminate sheep or goat remains are often dubbed ‘ovicaprin’. As Derek Yalden wrote in The History of British Mammals, this is a pretty daft name given that there is no such animal as Ovicapra. Indisputable sheep are usually divided into three genetic groups: the Asian argaliforms, the mouflon-like moufloniforms, and the mostly American pachyceriforms.
Argaliforms are relatively gracile sheep, suited for life in open, rolling habitats, and they have often been regarded as being represented by two Asian species: the Urial O. vignei and Argali O. ammon. It now seems, however, that these two are not close relatives, and that the Urial is a moufloniform (Bunch et al. 2006). Known from Iran in the west to Pakistan and India in the east, Urials are normally light brown, with a long, light-coloured neck ruff and – for a sheep – a rather long, thin tail. Like several of the sheep species, the species is polytypic, and experts remain undecided on which of the many named subspecies really are valid taxa. For the record, the most widely accepted forms are the Ladakh urial O. v. vignei, Transcaspian urial O. v. arkal [shown in adjacent image], Bukhara urial O. v. bocharensis, Afghan urial O. v. cycloceros and Punjab or Salt Range urial O. v. punjabensis. Most urial populations are declining and in danger of extinction, and none of the subspecies listed here have estimated populations exceeding 12,000. It used to be widely thoughts that urials were conspecific with mouflons (which explains why the urial is named O. orientalis in some sources), but this has been largely rejected on the basis of chromosome counts and other differences.
The largest and arguably most magnificent of wild sheep is the Argali, a wide-ranging species (occurring from Afghanistan to southern Siberia) that possesses prominently ribbed, enormous spiralling horns. Some forms, such as the Tibetan argali, possess a neck ruff. The large body size and hypertrophied display organs of argali have led some to regard them as hypermorphic compared to other sheep. A confusing array of argali subspecies (15 in total) have been named, and it has proved difficult to determine which represent valid taxa, which are hybrids, which are simply the result of phenotypic plasticity*, and even which are correctly assigned to the right species (Geist 1991). The Severtzov’s or Kyzylkum sheep O. a. severtzovi, for example, was conventionally classified as a form of urial** until chromosome counts showed that it was actually an argali (Bunch et al. 1998). Arkal sheep from Iran, Turkmenistan, Uzbekistan and Kazakhstan, conventionally regarded as the urial subspecies O. v. arkal, have recently been found to be genetically close to Kara Tau argali O. a. nigrimontana and are thus probably members of the argali species as well (Hiendleder et al. 2002).
* That is, members of some populations may reach larger body sizes and grow big horns because they have a particularly good diet, or inhabit a particularly favourable environment, compared to other populations.
** Though, actually, it was first named (in 1914) as a distinct species.
The seven most widely accepted argali subspecies are the Altai argali O. a. ammon, Kara Tau argali O. a. nigrimontana, Nyan or Tibetan argali O. a. hodgsonii [shown in image above], Pamir argali or Marco Polo argali O. a. polii, Tien Shan argali O. a. karelini, Gobi argali O. a. darwini and the probably extinct Northern Chinese or Shansi argali O. a. jubata. Of these, recent studies have supported the validity of at least Altai argali, Kara Tau argali, Tibetan argali, Marco Polo argali and Gobi argali and, among the additional subspecies, the Gansu argali O. a. dalailamae, Kuruktag argali O. a. adametzi, Sair argali O. a. sairensis, Severtzov’s or Kyzylkum sheep O. a. severtzovi, Kazakhstan argali O. a. collium and Littledale argali O. a. littledalei have also been regarded as valid taxa (Hiendleder et al. 2002, Wu et al. 2003) [adjacent picture shows Littledale argali. Unfortunately, many of the subspecies are rarely photographed except as hunter's trophies, so I'm sorry that photos of dead animals are the only ones we have available. While hunting of these animals does, arguably, add to the local economy, it is debatable whether sustained hunting is wise given the low numbers of some populations].
Perhaps the best known of these animals (viz, the one usually mentioned and/or illustrated in books) is the remarkable Marco Polo argali from the Pamir Mountains of Afghanistan, Kyrgyzstan, Tajikistan, Pakistan and China. It possesses the longest horns of any argali, with individual horns recorded as reaching nearly 2 m (measured along the spirals) in some individuals. Despite frequent comments that it is the largest argali and thus the largest sheep, it is apparently exceeded in size by record-holding Altai and Gobi argalis. The size and fantastic appearance of these sheep makes them attractive targets for trophy hunters and, as an example, in Tadjikistan in 1995, hunters were able to kill argali for a fee of US$10,000-20,000.
North America is home to two species of wild sheep. The Bighorn O. canadensis occurs from northern Mexico to SW Canada, occurring in mountainous environments as well as deserts. Bighorns are not, apparently, named for their big horns. Rather, they are named for the Big Horn Mountains of Wyoming and Montana, though unfortunately I forget where I read this. Whatever, the horns are indeed big in the mountain-dwelling bighorns, and can have a combined weight of almost 20 kg, which is about equal to the weight of the rest of the skeleton. Fights between males [see adjacent pic] have been recorded as lasting over 25 hours, though this is not constant fighting of course and involves lots of resting and posturing. Bighorn do have accidents while climbing and fall to their deaths, but data indicates that such incidences are rare, with, for example, five deaths out of an annual count of 42 being due to falling (Kamler et al. 2003).
For whatever reason, bighorns in some populations seem to have, err, flexible sexual preferences and it has even been claimed that some bighorn populations are ‘homosexual societies’ where same-sex courtship and mating is routine among males (Bagemihl 1999). Courting males use the same routine of stylized gestures and postures as do male-female pairs, including a low-stretch, a head twist, a foreleg flick, and mutual grooming, head-rubbing and genital licking, and culminating in penetration. Apparently, some female bighorns morphologically mimic males and thereby solicit extra attention. Freemartins have been recorded among bighorns (Kenny et al. 1992) [freemartinism describes the condition where the female twin of a male develops intersexual characters: she may be born with both male and female genitalia, and display male appearance and behaviour].
You’ve probably heard that research on ‘gay’ sheep has been plagued by controversy (Steve Bodio covered this subject here), and I find it a difficult area to discuss as I just don’t know how far we should go in comparing this behaviour with that of our own species. It is, however, always worth reminding people (especially homophobes and right-wingers) that, despite what they say, homosexuality is not ‘unnatural’ given that it is pervasive and universal in the animal kingdom.
And on that note, I leave. More on sheep at another time.
Refs – –
Bagemihl, B. 1999. Biological Exuberance. Animal Homosexuality and Natural Diversity. St. Martin’s Press, New York.
Bunch, T. D., Vorontsov, N. N., Lyapunova, E. A. & Hoffmann, R. S. 1998. Chromosome number of Severtzov’s sheep (Ovis ammon severtzovi): G-banded karyotype comparisons within Ovis. The Journal of Heredity 89, 266-269.
– ., Wu, C., Zhang, Y.-P. & Wang, S. 2006. Phylogenetic analysis of Snow sheep (Ovis nivicola) and closely related taxa. Journal of Heredity 97, 21-30.
Cassinello, J. 1998. Ammotragus lervia: a review on systematics, biology, ecology and distribution. Annales Zoologici Fennici 35, 149-162.
Geist, V. 1991. On the taxonomy of giant sheep (Ovis ammon Linnaeus, 1766). Canadian Journal of Zoology 69, 706-723.
Hiendleder, S., Kaupe, B., Wassmuth, R. & Janke, A. 2002. Molecular analysis of wild and domestic sheep questions current nomenclature and provides evidence for domestication from two different subspecies. Proceedings of the Royal Society of London B 269, 893-904.
Kamler, J. F., Lee. R. M., DeVos, J. C., Ballard, W. B. & Whitlaw, H. A. 2003. Mortalities from climbing accidents of translocated bighorn sheep in Arizona. The Southwestern Naturalist 48, 145-147.
Kenny, D. E., Cambre, R. C., Frahm, M. W. & Bunch, T. D. 1992. Freemartinism in a captive herd of Rocky Mountain bighorn sheep (Ovis canadensis). Journal of Wildlife Diseases 28, 494-498.
Wu, C. H., Zhang, Y. P., Bunch, T. D., Wang, S. & Wang, W. 2003. Mitochondrial control region sequence variation within the argali wild sheep (Ovis ammon): evolution and conservation relevance. Mammalia 67, 109-118.