Quite how they're related, and which ones are on the goat side of the tree, and which on the sheep side, has been a matter of some debate, and depends largely on whether or not you think that physical appearance is more important than genetic similarity. Even if you look only at the genetics, it seems that they're all close enough to one another that it makes a fair difference which genes you happen to consider most important.
Still, we can say that some look more like goats, and some look more like sheep. On the goat-like side are the tahrs - not to be confused with turs, which really are goats. There are three species of tahr, all of which look fairly similar to one another, and have traditionally been placed together in the genus Hemitragus - a word that literally means "half-goat". In the traditional, appearance-based, scheme it's thought that these are the closest animals to true goats, without actually being them. Genetic analysis, however, shows that this is just an illusion, and the three animals aren't especially close relatives. Two of them, therefore, have been removed from the genus, and given new scientific names to reflect their distinct nature.
The one that remains, however, really does seem to be a very close relative of the true goats. This is the Himalayan tahr (Hemitragus jemlahicus), which lives on the southern slopes of the Himalayas, in Nepal and either side of the Indo-Chinese border. Physically, they do look quite goat-like, although the males are larger than typical goats, and the females quite a bit smaller. However, while the males do have shaggy dark hair, they do not have beards. The horns are also quite different from those of wild goats, being much shorter, and with a sharp edge running down the front.
Tahrs live around the treeline, moving lower in winter, and higher in summer. In the Himalayas, this is a high altitude, typically about 3 to 4 km (10,000 to 13,000 feet) above sea level, and tahrs have been spotted as high as 5,500 metres (18,000 feet) - higher than the summits of the highest peaks in Europe or the contiguous United States. They feed, like other goats, mainly on grasses, but they seem to be able to subsist on just about anything. Their digestive tract is adapted for such indiscriminate feeding, and can change in structure depending on the food available. Populations of tahr have been introduced to New Zealand for hunting, and have also escaped from ranches in both South Africa and California. Where this has happened, they have made quite a nuisance of themselves, quickly adapting to whatever food was available and leaving little left over for native animals.
Even so, back in their native land, their numbers are declining, especially in Nepal, although not enough for them to be considered a threatened species. It's the usual story of advancing agriculture and hunting, although, in the case of the tahrs, enough of them live in habitats that no human would want for this not to be a major issue. Other than humans, their main threat comes from leopards on the lower slopes, and snow leopards further up. Indeed, at some times of the year, as much as 50% of a snow leopard's diet may consist of Himalayan tahrs - mainly focussing on kids and sub-adults. Presumably in an effort to reduce such predation, Himalayan tahrs are rarely found on slopes of less than 40° (4:5).
On the other hand, an animal that they apparently don't mind are red-billed choughs, which regularly pluck their hair for nest material, and eat any parasites that they find there. They live in herds of up to twenty individuals, although four to six is more usual. The herds are usually mixed-sex, even outside the October to January rut. Pregnancy lasts for around six months, and, unlike true goats, twins are rare.
Siberian ibex prefer to avoid areas with significant tree cover unless they really need to avoid heavy snow. Much of their homeland is so arid and desolate that they don't need to travel to high elevations to find the rocky open terrain that they crave, and they can be found in hills as "low" as 700 metres (2,300 feet) in the Gobi Desert. Often, however, they are higher - much higher. Indeed, living at elevations of up to 6,700 metres (22,000 feet), they probably inhabit higher terrain than any comparably sized herbivore. Even when they're in a zoo, rather than on a mountainside, like those of many other goats, their kids will play on the steepest surfaces they can find - a risky tactic that nonetheless must pay off when they reach adulthood.
In most respects, Siberian ibexes are very similar to goats in their behaviour, as well as their appearance. There are a few subtle differences, however, such as twins being less common. They're so widespread, and live in such terrible terrain that, as a species, they don't seem to be much threatened by man. Although, of course, there are always local exceptions to that.
However, when we look at its behaviour and biology, its easier to see the differences. To be fair, it isn't much like a sheep, either, and the differences are largely a reflection of where it happens to live. Indeed, the Nilgiri tahr is unique in being the only caprine to live well inside the tropics. Specifically, it inhabits the southern end of the Western Ghats mountain range where they form the border between the Indian states of Kerala and Tamil Nadu.
Compared with the homelands of other caprines, this is a very wet area, at least during the monsoon season, where the slopes are covered with lush grasslands between deep, forested valleys. At altitudes of up to 2600 metres (8,500 feet), the tahrs range sufficiently high up the mountains that the climate arguably isn't tropical any more where they live, but even so, unlike most other goat-like animals, they don't have to experience winters.
Whereas regular goats, and most other caprines, breed in the winter and give birth in the summer, Nilgiri tahrs synchronise their breeding with the rain. They breed at the height of the monsoon season, between June and July, giving birth from January to February, when the tropical weather is as cool as it ever gets. While they are occasionally eaten by local predators, such as leopards and dholes, they seem to be fairly good at avoiding this, and relatively few end their lives in this way.
Unfortunately, because India is so densely populated, and because the Western Ghats are much greener and more hospitable than the mountains of Central Asia, this ability to avoid predators has not helped the Nilgiri tahrs as much as it might. Active hunting, and the simple loss of wild land has meant that the total population now probably numbers less than two thousand individuals, mostly in the Anamalai Hills nature reserve, and while they can live up to nine years in captivity, their life expectancy in the wild is little more than three years. As a result, they are formally considered to be an endangered species.
Arabian tahrs are noticeably smaller than the other two species, with smoother, more rounded, horns. Their facial markings are similar to those of the Nilgiri tahrs, while their longer hair is more reminiscent of the Himalayan species. Compared with the other species, they live in very small herds. The Nilgiri species, for example, can have herds dozens strong, while a herd with just five adults is unusual for Arabian tahrs, and simple pairs, or even solitary animals, are common. While it is illegal to hunt them in Oman, it's hard to actually enforce that law, and the tahrs also suffer from diseases that they catch from domestic goats, which are common livestock in the area.
While Nilgiri tahrs are apparently close relatives of sheep, and Himalayan tahrs are close relatives of goats, the Arabian species is neither. Instead, their closest relative is a North African species. Often called the Barbary sheep (Ammotragus lervia), in recent years, use of the local name aoudad has become more common, to emphasise the fact that it really isn't a sheep. Indeed, while they can interbreed with domestic goats, Barbary sheep are unable to do so with domestic sheep, suggesting that, as genetic and reproductive studies also imply, they are at least somewhat closer to the former.
Barbary sheep like steep and fairly arid terrain, but other than that, they seem quite flexible. They are found from open forest to rocky semi-desert, at elevations as high as 4000 metres (13,000 feet). Although their numbers and range are declining fast enough for them to be considered a threatened (though not 'endangered' species) species, they still inhabit patches of territory across a wide swathe of northern Africa. Their native lands extend from Morocco to northern Mauritania in the west, right across to Egypt in the east, taking in much of the less sandy parts of the Sahara on the way.
They have also been introduced successfully to a number of equally arid places, because they seem to be able to survive for some considerable time without water. There are now free-roaming populations of Barbary sheep in southern Spain, in California, Texas, and New Mexico, and in parts of northern Mexico. They are widely hunted in many of these areas, which is perhaps a good thing, since they put pressure on the native wildlife. On the other hand, they are also hunted in places where they are the native wildlife, and there has been a particularly steep population decline in Egypt. Some studies have even looked into cryopreserving their sperm for conservation purposes, using the same techniques employed in domestic sheep husbandry.
At least in their native habitat, Barbary sheep eat leaves and buds in the summer and autumn, and switch to grassier fodder in the winter and spring. Their herds typically number about fifteen or so adults, although they can swell to four times that size. As with many other caprines, there is a dominance hierarchy within the herd. Oddly, though, at least among females, this seems to have little to do with physical strength, and more to do with social factors such as the number of offspring and relationship with herd-mates.
Like the Barbary sheep, blue sheep are closer to goats than they are to real sheep, and, they too, are able to crossbreed with the former but not the latter. The scientific name of their genus, Pseudois, reflects this, literally meaning "false sheep". They are found across the high altitude, western, parts of China, and in neighbouring countries such as Nepal, Bhutan, and parts of northern India and even Burma.
The greater blue sheep (Pseudois nayaur), sometimes called by the local name of "bharal" to emphasise its lack of sheepishness, is by far the more common of the two species. They're about the same size as sheep or goats, with black markings and a blue-grey coat that turns brown in the summer. The horns of the rams stretch outward and to the side, forming a semi-circle with slightly upturned tips, quite different from the coiled spirals of true sheep.
|Greater blue sheep|
Just as the Siberian ibex is a main staple of snow leopard diet in the west, and Himalayan tahrs in the south, so blue sheep constitute their main prey elsewhere in the mountains. Indeed, it has been estimated that, in some areas, the average blue sheep has about a one in four chance of being eaten by a snow leopard in any given year.
Yet, despite this, and despite the fact that they largely eat the same food as domestic livestock, greater blue sheep remain remarkably numerous. So numerous, in fact, that they're probably the single most common hoofed herbivore across the whole of the Tibetan plateau. The same cannot be said of the endangered dwarf blue sheep (Pseudois schaeferi), which inhabits only one small area of the upper Yangtze Gorge in Batang County, China.
Dwarf blue sheep are noticeably smaller than their greater cousins, weighing only about half as much. Their coats are paler, with less markings, and their horns slightly less dramatic. Inhabiting the gorge, rather than the surrounding mountain slopes, their home is more forested than open, and their predators are wolves and true leopards, not snow leopards. It's possible that this restricted homeland of less than 300 square kilometres (115 square miles) is the main factor responsible for their small size. In fact, the two species are so similar that it's recently been argued that they're not really separate at all.
Regardless of what they are, dwarf blue sheep have dramatically declined in numbers over the last few decades, and as few as 200 remain alive today. Even compared with some of the tahrs, they are an animal on the brink of extinction.
[Pictures by "4028mdk09", Rami Radwan, VR Vinayaraj, Colin Hepburn, "Kuribo", and "reurinkjan", from Wikimedia Commons. Cladogram adapted from Lalueza-Fox et al 2005, Yang et al 2013, and Ropiquet & Hassanin 2005.]