Sunday 3 February 2013

Caprines: Telling the Sheep from the Goats

Transcaspian urials
(Note that the rams of this species/subspecies have
particularly argali-like horns)
To most people, especially in the West, the most familiar member of the goat subfamily is probably... the sheep. Worldwide, domestic sheep are more common than goats, although not necessarily by as much as you might think. Goats are more popular than sheep as farm animals in places like India and Africa, but overall, sheep have the edge in numbers. That's probably no great surprise to those of you in most parts of Europe or America, and even less so if you're in Australia or New Zealand.

But perhaps I should back up there. Did I really just say that sheep are a kind of goat?

Well, yes I did. Kind of. Sheep are members of the goat subfamily, but really, they're goats only in the sense that ferrets are a kind of weasel. But why is it that way round at all?

The goat subfamily, or Caprinae, was given its name by the great Victorian zoologist John Edward Gray, quite early in his career, in 1821. From a scientific standpoint, there was absolutely no reason why he couldn't have named it the sheep subfamily, but instead, he chose goats as the best example of the group. I don't know his actual reasoning, but it certainly makes sense: far more members of the group resemble goats than resemble sheep. And so, from this, admittedly arbitrary, standpoint sheep are an odd kind of goat, and not the other way round.

Sheep and goats are quite closely related, and they do have a number of features in common. Like all caprines, they (at least in their wild forms) are adapted to living in marginal habitats. Both males and females have horns, something that, while not truly unusual in the cattle family, is by no means universal. Both species have scent glands on the face and feet, which they can use to leave scent marks that send signals to one another, and species of both genera live in tightly knit herds that defend a relatively small patch of resources from intruders - rather than spreading out over desolate plains.

But there are, of course, also clear differences. Goats generally inhabit precipitous cliffs and high mountain pastures, where few other animals of their size would dare venture, whereas sheep are more likely to be found at the foot of those cliffs, in hilly, but not mountainous, terrain. Sheep have less scent glands than goats, which have additional glands on the groin, as well as in slightly different positions on the face and near the feet. Goats also have bald patches on their knees, which sheep do not, and they have beards on their chins. In contrast, although the males of at least one sheep species do have beards further back, towards the throat, in general, sheep are entirely beardless.

There are six generally recognised species of sheep. One of these is the domestic sheep (Ovis aries), which unlike animals such as dogs or cats, is usually considered a distinct species from its wild ancestor. Thousands of years of unnatural selection have changed the domestic sheep significantly from its wild form (although, let's face it, no more so than dogs). Domestic sheep are docile, typically have hornless females, and, perhaps most obviously, have a thick woolly coat that gives their existence meaning. But what of the other five species?

The wild ancestor of the domestic sheep is...

Actually, that's not as simple a question as it might seem. Not because we don't know what the animal we're talking about is, but because we aren't sure what species it belongs to. It used to be thought that the wild ancestor was an animal called the mouflon, which was considered to be either a species in its own right (Ovis musimon) or a subspecies of its more widespread cousin, the urial (Ovis orientalis). Mouflon were originally found only on Corscia and Sardinia, and it turns out that they got there only because humans brought them. Which means that they are probably descended from very early domestic sheep rather than being the ancestors of them. So the ancestor of domestic sheep has to be the urial.

In the last few years, however, genetic analysis has shown that urials are far more diverse than we had thought. By this new understanding, there may be anything up to eight entirely separate species of urial. In which case the wild ancestor may be the Anatolian sheep (Ovis gmelini).

It gets worse. Some of the animals previously thought to be subspecies of urial turned out to instead be subspecies (or more likely full species) of a related animal, the argali. Furthermore, the type specimen of the urial - the single preserved individual that defines what it means to be a urial - may be a hybrid between two of the newly discovered species. Which means that the scientific name isn't valid, because it refers to something that doesn't really exist. You can't, after all, define a species by referring to a specimen that doesn't actually belong to a species in the first place.

All of this is controversial, and the source of much argument. Some scientists argue that there's only one species of wild sheep across the whole of Europe and southern and central Asia, that just happens to be quite variable. Others say that there are as many as seventeen, and there are many positions in between. Some still use the old scientific name for the urial, while others prefer a newer one (Ovis vignei), and others sidestep the question by just using the name for the domestic form. For that matter, some still call the urial a 'mouflon'.

Partly to keep the length of this post down, and partly for my sanity, I am going to use the definitions and names employed by the latest, 2008, version of the IUCN Red List. But you should be aware that there are other, equally valid, sources that I could have used, and that the story is likely a lot more complicated than I'm making out.


The wild ancestor of the domestic sheep is the urial (Ovis orientalis). Urials are most common today in Iran, but are also found further east and north, into Pakistan, Afghanistan, Turkmenistan, Uzbekistan, and small areas of neighbouring countries. One isolated subspecies, the Anatolian sheep - supposedly the one closest to the domestic form - lives today only in a small region of central Turkey.

Physically, it's their hair that most easily distinguishes urials from their domestic descendants, which is hardly surprising given one of the main purposes of sheep husbandry. These wild, ancestral sheep, have brown hair over most of the body, with a blackish band across the lower flanks and upper legs, white underparts and lower legs, and often a whitish saddle-shaped patch on the mid-back. The males have large curving horns, of the kind we traditionally expect to see on rams. Females have much smaller and narrower horns, with a slight curve and a sharp point; many are entirely hornless.

Urials live in hilly and rugged terrain, often among the foothills of higher mountains. Their primary food is grass, although they will certainly nibble shrubs when they come across them. They are generally active during the day, although, across much of their range it becomes too hot to do that for much of the summer, leading them to do most of their grazing in the morning and evening, when it is cooler, but still light.

Kazakhstan argalis
Further east, we come to the argali (Ovis ammon), inhabiting the high foothills and rugged terrain of central Asia. They are found across the Tibetan plateau, north of the Himalayas, and through much of the land beyond, into Mongolia, eastern Kazakhstan, and parts of the small central Asian republics of Tajikistan and Kyrgyzstan. Much of this land is near desert, so that many of them feed on shrubs as much as on grasses, especially in winter. On the other hand, the cooler climate means that there is no need to take shelter from the noonday heat, so that they are active throughout the hours of sunlight. They are also more likely than urials to travel between distinct summer and winter pastures, thus avoiding both the worst of the snow and of the summer heat, depending on the time of year.

Argali look very similar to urials, so much so that some have considered them to belong to the same species in the past. However, they don't seem to be able to cross-breed to produce fertile offspring, apparently because, like horses and donkeys, they have a different number of chromosomes. They tend to be slightly darker and more uniform in colour than urials, although at least one subspecies is surprisingly pale. Their horns are also larger, with those of older males often curling round in a spiral, rather than simply forming a curving arc. While most are smaller, the horns of males can be anything up to 190 cm (75 inches) long, and with a remarkably thick and heavy base, as much as 50 cm (17 inches) in circumference. Even the horns of females are longer and more curved than those of female urials.

Both species are, of course, herd animals, although, given their reduced numbers in modern times, it's difficult to say just how large those herds would be if they were left to themselves. Probably about thirty to fifty animals is typical, although it likely depends on how fertile the land they are living in happens to be. For most of the year, males and females inhabit separate herds, with the females looking after the lambs and living alongside non-reproductive yearling males. The males have a clear dominance hierarchy, enforced as much by simply showing off the size of their horns with a simple head-shake as by actually having to use them. Since the horns of caprines grow throughout life, the dominant individuals, with the biggest horns, will also be the oldest, and likely the most muscular.

The herds come together for the rut some time between October and December, with the larger males forcibly preventing their younger kin from getting a look-in. Unlike some other herd animals, they only show interest in one female at a time, jealously guarding her until they are sure she is pregnant. However, the rut lasts long enough that they can mate with several females before it's all over. Lambing occurs between April and June, and, as with domestic sheep, commonly results in the birth of one or two young. There is relatively little difference between the two species in this respect; argalis have a slightly longer pregnancy of around 160, rather than 155, days, and are less likely to give birth to twins than are urials.

Neither species is entirely safe from human activity, although argalis, living in more remote regions, are faring slightly better than their western kin. The land that they live in is, after all, suitable for grazing domestic sheep, so they have to find areas outside of farmland to inhabit. If, as I've indicated above, they aren't just two species, but seventeen, then some of those species could reasonably be described as endangered, being restricted to relatively small regions. Whether they are depends a lot on what you consider a species to be. At the very least, some, and probably all, of the seventeen are "evolutionarily significant units", a somewhat vague term that boils down to "they're worth preserving".

On that front, there has been what we could generously describe as mixed success. That scientists are so confused as to what the heck we're talking about is unlikely to be the core of the problem, but it's not helping much either. Fortunately, there are no such issues for the remaining three species of sheep, to which we'll turn next.

[Pictures by "Altaipanther" and S. Reznichenko, from Wikimedia Commons]


  1. Should Pachyceros be recognized as a subgenus for bighorns?

    1. I'm not really a fan of subgenera (not that such things are up to me!) But I can see a case for distinguishing the (bighorn+Dall)+snow clade from the urial+argali clade, as /Pachyceros/ is intended to do. They seem to have diverged quite a while ago, and I can see that one might want to recognise that.

    2. I don't like subgenera too. If Linnean taxonomy was created as a "binomial" system, it should be binomial, with genera as the lowest rank taxa. If a genus is too big, or includes many branches, let's split it.