Out of Tibet - Origins of the First Sheep

The argali is a species of wild sheep still found in Tibet

Sheep, as I have noted before, are members of the goat subfamily within the much larger group of the bovids. They are, in essence, a special kind of goat that has adapted to living in the foothills of higher mountains, rather than on the steeps-sided peaks themselves. The exact number of species of sheep is debatable, but there are at least five wild ones, plus the domestic animal, for a total of six.

When I discussed the evolutionary history of the goat subfamily a few years back, I talked primarily about the goats themselves, and about the muskoxen, and said very little about how the sheep became separated from the goats, and how they evolved since. A new paper, however, goes into much more detail about the origin of sheep and their fossil history. It is, in fairness, based on just one pair of fossilised horns, but, nonetheless, this is as good a starting point as any, and a sound excuse for me to summarise what we do know about sheep evolution.

Caprines: Go West, Young Sheep!

Bighorn rams

As we've seen, the domestic sheep is descended from one of what may be quite a large number of related species native, broadly speaking, to southern Asia. Back in the Pliocene, however, long before those species separated from one another, one stock of ancestral sheep headed north, into Siberia. When the Ice Ages arrived, and sea levels dropped, these sheep were among several animals that headed... well, from their perspective, they were going east, but where they ended up was western North America.

Sheep, in general, inhabit high hills and rugged terrain in the shadow of mountains. They don't live on the cliffs and the impassable heights themselves, but they do like precipitous rocks nearby, so that they can flee into them at the first sign of predators. Given those requirements, western North America is an absolutely ideal place for sheep to be, and the descendants of those first colonists spread far and wide. However, while there's much argument about the exact number of wild Asian species of sheep, it's pretty much agreed that, in America, there are only two.

Probably the more familiar of these is the bighorn sheep (Ovis canadensis). It's also easily the more widespread of the two, and inhabits a surprisingly broad range of habitats, from the mountains of southern British Columbia and the hills of North Dakota down to Baja California and the Sonora Desert. That obviously includes both hot, scrubby, deserts and cold, damp, pine forests, as well as much in between. Indeed, aside from the insistence on rugged terrain that prevents them from reaching as far east as, say, Kansas or Nebraska, bighorn sheep don't seem to have much in the way of requirements.

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.