For instance, this rule tells us that humans must belong to the great ape family, since chimps are closer to us than they are to gorillas, but there's nothing in it to say that gibbons can't also belong. We could, in other words, have an "ape family", and the only reason we don't is that we figure gibbons are sufficiently different from great apes that we ought to give them a family of their own. (In fact, "apes" as a whole are considered a superfamily).
So it is with the goat subfamily, the caprines. Animals like gorals are clearly goat-like, but it turns out that goats are closer to sheep than they are to gorals. So we put them all - goats, sheep, gorals, and all the rest - into one subfamily. But where, exactly, does that subfamily end? How much are we going to include before we decide that, no, this animal is different enough from goats that we're going to call it something else entirely? It is, of necessity, a completely arbitrary decision.
It is for this reason alone that the chiru (Pantholops hodgsoni) is not a caprine. It is the closest living relative of the goat subfamily, but most (though not all) researchers consider it just different enough that it's over the line and into a separate subfamily. Indeed, with no close relatives of its own, it gets a subfamily all to itself - the one and only living species of pantholopine.
The chiru is often called the "Tibetan antelope", and that's a fair description. An antelope, after all, is really just any member of the cattle family that is neither caprine nor bovine (and for a suitably narrow definition of "bovine", at that) so it really gets that description by default. Nowadays, we prefer the name for the animal in the Tibetan language - or at least, as close to the correct pronunciation of "གཙོད" as we can manage in English.
As the other part of their name suggests, chiru live only on the Tibetan Plateau, occasionally reaching as far south as Ladakh in northern India, but rarely staying there for long. Adapted to living on high, barren, plains, rather than on mountain slopes, they do look much more 'antelope-like' than goat-like, although they're about the same size as a typical goat. Their fur is grey to brownish, with black markings on the flanks, a black face and white muzzle, and a pale belly. During the breeding season, the males become a much purer white, their coats contrasting with their black faces.
However, their shape alone does not disqualify them from being caprines - a group that, after all, includes takin and muskox. One key difference is in their horns. Male chiru have long, lyre-shaped horns, with a series of rings round the base that fade away to a smooth point higher up. You might think that the rings would be a useful tool for ageing the animals, but that's only true to a limited extent, since they stop growing after four years. As a result, there are no age rings on the inside of the horn, either.
What's significant about the horns isn't that, though: it's that the females don't have any at all. This is not unusual within the cattle family, but the caprines are an exception, with wild females normally having horns, albeit smaller ones than the males. (As you may have noticed, in domesticated sheep and goats, this is less often the case).
Another difference is in the scent glands. Caprines have scent glands in front of the eyes and just above the hooves, and these are particularly large in males - and responsible for the smell of, for example, billy-goats. In chiru, the scent glands are instead on the rump, and there is nothing in the place that we'd expect them in true goats. Finally, chiru have two fewer cheek teeth than caprines do. This is presumably something to do with their diet, especially since saiga antelope, which eat the same kind of food, show the same unusual feature. But, if so, it's not clear why this would be, and contrary to earlier expectations, we no longer think that saiga and chiru are closely related.
Living on a chilly plateau between 3250 and 5000 metres (10,600 to 18,000 feet), chiru have enlarged nasal cavities, that help to warm the meagre air at that elevation. The same cavities include inflatable sacs that act as a resonating chamber, and which are larger in the males, allowing them to make loud growls, and even roars, during the mating season.
That mating season lasts from November to December, with the young being born about six months later. During this time, males gather harems of females, but for the rest of the year, the sexes travel alone, in herds of up to two dozen individuals, feeding on the sparse grasses and whatever else they can find in what is almost an Arctic landscape.
In the spring, female chiru, and to a lesser extent, males migrate northwards to their traditional calving grounds. Such migration routes may stretch for 300 km (190 miles) or more, all to reach an area where they will stay for three weeks at most, before returning home again. Why they should bother doing this isn't really clear. If nothing else, northwards is generally not the direction to head in if you're looking for higher quality food - at the time of year when they're doing it, many of the plants haven't yet sprouted in the calving grounds, even though they already have in the place they've just left.
One possibility is that this made more sense in the distant past, perhaps when the climate was slightly different. Another possibility is that, because the breeding grounds are relatively small, and herds gather together into much larger groups during the migration, that it helps to maintain genetic interchange, with some animals heading back south with a different group than the one they left with. Certainly, the overall population does seem well-mixed genetically. Interestingly, the same studies also show evidence of a large increase in numbers, not at the end of the last Ice Age, as we might expect, but at the end of the second-to-last, around 0.6 million years ago.
Today, however, the chiru's population is in decline. Given their homeland, their life has never been particularly easy, of course. They get eaten by wolves and snow leopards, and food can be very difficult to find beneath the snow in winter. Botflies are also a problem; one species burrows into their skin to eat their flesh, while another likes to climb up their nose to do so. Apparently, during the summer, chiru dig themselves into soft sand until they're almost hidden in order to escape the flies. Doubtless this helps against the snow leopards too, at least when they're hunting by sight, rather than scent.
But it's probably no great surprise that humans are an even bigger problem than botflies. The construction of the Qinghai-Tibet Railway, between 1984 and 2006, and of its accompanying highway, have both cut right across the chiru's migration routes. Fortunately, the chiru seem to have got the hang of crossing them when traffic is light, and they have caused less problems than might have been expected. The roads do, however, cause problems for the chiru in another way: they make it easier for hunters to reach these remote regions.
Chirus have long been hunted, and their original population began to decline as a result thousands of years ago, but hunting has really taken off since the railway was completed. The population is believed to have roughly halved since 1983, and, with no more than 100,000 or so left alive, is continuing to drop at an alarming rate. The reason for isn't that they are tasty - it would be a long way to go for food - or even that their horns are particularly valuable as ornaments or ingredients in traditional Chinese medicine (although they are).
No, the problem is their fur. The wool of chirus is remarkably soft and warm, and is regarded as one of the highest quality wools of any animal - somewhat to the irritation of the legal cashmere industry. All trade in the wool, known as shahtoosh, is banned internationally. Hunting chiru, for any purpose, is also banned in both China and India, but the extreme remoteness of the area makes enforcement difficult, and the potential profits are huge.
There is, of course, no need to kill the chiru to get the wool off of it, and there have been at least token gestures towards domesticating the animals. But for hunters, that's not a realistic option, especially since they have to break the law to sell their pickings anyway. Today, the chiru remains an endangered species, and has been classified as such since 2000, but there are at least some signs that the law enforcement is improving, and that the current downward trend in the population might one day be reversed.
[Painting by Philip Sclater, copyright expired.]