Sunday 3 March 2013

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.

Physically, they're not that different from their Asian cousins, although the white patch on their rumps is larger, and more distinct. The rams, of course, also have big horns. Indeed, the skull of a male bighorn sheep can weigh anything up to 20 kg (44 lb). Having said that, they aren't quite as large as the horns of argalis, and while they typically curl round in almost a complete circle, they don't quite manage the full spiral shape that the latter species achieves.

Bighorn sheep live in herds of up to a hundred individuals, feeding on pretty much whatever plant matter they come across. Like many herd animals, however, adult males and females live apart outside of the rutting season. Herds will therefore tend to be either male-only, or maternal herds, including adult females, lambs, and sub-adult males that have yet to build up the courage (or, more accurately, the body mass and sexual maturity) to leave.

One reason for this is that, when it does come time to breed, any approaching males are unlikely to be close relatives of the females. But there's a little more to it than that, because males and females actually prefer different kinds of terrain.

For females, the priority seems to be safety. So what they want is land that has plenty of good look-out points from which they can keep an eye out for predators that might want to eat their lambs. The downside of this, bearing in mind that they're in rugged or even desert terrain to start with, is that the food in such places is generally pretty sparse. Apparently, the pay-off is worth it, although, especially in the north of their range, they often move between distinct summer and winter pastures to get the best of the plant life at each - and to avoid the worst of the winter snow. They may have once done something similar in the south, as well, (apart from the bit about the snow, obviously), but the spread of freeways, mines, and farmland has made that rather difficult.

For rams, however, the priorities are different. They don't have lambs to look after, so they're willing to risk flatter or more vegetated terrain if it means getting more food. This does put them at more risk of predation - although they'll still avoid heavily overgrown places - but that extra calorie intake means that they can bulk up muscles and horns alike, making themselves all the more masculine and desirable to the females. In short, they're willing to put up with the occasional attacks by wolves, coyotes, or cougars, if it means they get more sex at the end of the day.

Which is males for you.

In the north, the rut takes place in November and December, ensuring that lambs are born in late spring to early summer, giving them a proper run up to the harsh realities of winter. Further south, in the deserts, that's less of an issue, and the rut can last until June. This may be, in part, a response to unpredictable rain - at least some young are going to be born at a good time, even if you're not sure in advance when that will be.

The rut, naturally enough, is when all that bulking-up by the males pays off. They compete with one another both by making aggressive displays, and by clashing their heads together. The large horns have a somewhat spongy interior, which combined with large sinuses in the skull, act as shock absorbers, meaning that this sort of thing hurts rather less than you might imagine, and the rams don't suffer from concussion. They also practice fighting from a very young age, while the females are mainly playing at running away from imaginary predators.

The upshot of all this smacking each other about the head is that larger, stronger, males get to defend their chosen ewes from smaller ones. They're apparently rather less successful at this than they'd probably like, with some smaller males getting in when they're not looking. From the females perspective, at least as counted by the number of lambs she has, this really doesn't seem to make any difference, although selecting the larger males is probably better for them in the long term - and the latter do, in fact, get to mate more often than anyone else.

About six months later, the pregnant ewe will find somewhere secluded to hide while she gives birth, returning to the herd with her newborn lamb one to three days later. Lambs are usually singletons, and the birth of opposite sex twins can sometimes be a problem. That's because there's at least one recorded instance of freemartinism in bighorn sheep. Although far more common in cattle, this is also known in domestic sheep, and happens when the hormones and genetic material of the male foetus flood it's unborn sister, resulting in the birth of what's effectively a non-reproductive hermaphrodite.

Dall's sheep
Widely spread though it is, there comes a point where the winter weather is simply too cold for even northern populations of bighorn sheep. Here, they are replaced by the other American species, Dall's sheep (Ovis dalli). Named for naturalist W.H. Dall, and sometimes spelled without the 's, the animal is sometimes also called the "thinhorn sheep" to emphasise the difference from its southern cousin.

To complicate matters further, there are two visibly distinct subspecies of Dall's sheep. The more southerly one (O. d. stonei) is typically a dark greyish brown, with a white rump patch, and is commonly called "Stone's sheep", while the other (O. d. dalli) is almost pure white and, appropriately enough, is also called the "white sheep". In practice, there is some blurring between the two, and colouration isn't always a perfect guide as to which subspecies you're looking at.

Dall's sheep inhabit lands where winter temperatures plunge well below freezing point, living in the northern Rockies among alpine or subalpine vegetation, and only occasionally straying down into the true forests below. They are found in northern British Colombia, and most of Yukon and Alaska, but, like bighorn sheep, their preference for foothills below steep mountains prevents them from moving into northern Alberta, or into more than the most westerly stretches of the Northwest Territories. (Southern Alberta, on the other hand, is just too warm for them... which has got to say something).

Out in this particularly frozen and desolate terrain, they are apparently quite good at avoiding predators by dashing up nearby cliffs, and few fall prey to wolves or coyotes. They eat little but grass and sedges, not least because there isn't much else that they could eat. Like bighorn sheep, they move between summer and winter pastures, which, in their case, means heading downhill in winter to the only place where there's food, and then moving onto higher slopes in the spring to grab the tastiest shoots as they begin to emerge. While snow isn't exactly something they can avoid, they do at least stay away from areas with truly heavy snow cover - presumably because it's hard to run in a snowdrift, especially when you have hooves.

Breeding in Dall's sheep follows much the same pattern as in their bighorn cousins, although the harshness of the landscape does make it difficult for the males to travel far, resulting in more naturally inbred and isolated populations. As their alternative name suggests, the horns of rams are thinner than those of the southern species, but they're still quite impressive and much the same length and shape. One study shows that there's a trade-off in horn growth: males that grow large horns rapidly tend to wear themselves out and not to live as long as those who grow them more slowly, although the fact that such variation exists implies that both tactics must have their pros and cons.

Although males and females do live apart, the different sex herds are more likely than bighorn sheep to live close to one another. Even so, females are careful about where they choose to do their lambing, adjusting their choices from year to year depending on the weather.

Snow sheep
When the ancestors of bighorn and Dall's sheep crossed the Bering land bridge during the Pleistocene, they left some of their relatives behind. When sea levels eventually rose again as the ice caps melted, this original population became separated from its American kin. Those sheep are still there, representing the first link in the chain of travel that would lead through Alaska and western Canada, and ultimately all the way to Baja California. These are snow sheep (Ovis nivicola) and, while they live in Asia, they are much more closely related to the two American species than they are to their neighbours further south. This is despite, rather oddly, the species having a smaller number of chromosomes than any other sheep, on either side of the Pacific.

Snow sheep physically resemble bighorn sheep, although with white patches on the legs, underparts, and sometimes also on the forehead. Indeed, such is the similarity that they are sometimes called "Siberian bighorn sheep". They inhabit tundra landscapes close to steep cliffs, and only occasionally venture into even sparse forests, preferring the truly cold wastes further north. As might be expected, they feed almost entirely on grasses and sedges, although they do take some willow shoots when those are available, and, when they get hungry enough, they'll feed on moss, lichen, or just about anything else that they can find.

They are remarkably widespread. The majority are found in eastern Siberia, especially around Kamchatka and the extreme northeast, but one isolated population lives over a 600 miles away from its closest neighbours, way out in the Putoran Mountains of north-central Siberia. With their homeland often even colder than that of Dall's sheep, they are an excellent example of the ability of caprines to tolerate marginal terrain where little else will live. They are less concerned by deep snow than their kin, perhaps because they have little choice in the matter, and frequently scrape through it to reach the frozen plants underneath.

Compared with the American species, snow sheep have been far less studied, but what we do know suggests that they're fairly similar. They rut between November and December, just as their kin do (at least, outside of the American southwest), during which time rams may travel for miles in the hope of finding anyone that will mate with them. Twins represent only about 5% of births, although that's probably more than in Dall's sheep, and the females seem to rear them in much the same way. In such a harsh environment, of course, many lambs don't survive their first winter, but, again, that's not so different from Dall's sheep, where over a quarter of lambs don't even reach their second week of life. One difference is that males apparently segregate into herds based on their age, with older and younger individuals living apart - something that, so far as I can tell, has not been recorded in the American species.

For all that wild sheep inhabit desolate landscapes, and for all that they like nearby rocky slopes into which they can escape, they generally don't live on top of those slopes, in the actual mountains. But their closest relatives do exactly that. They, of course, are the true goats, and it is to them that I will turn next.

[Pictures by Kim Keating, "Denali Nature Preserve", and Joseph Smit, from Wikimedia Commons]

1 comment:

  1. Pneumonia is the biggest threat to Bighorn Sheep populations. Wildlife health researchers across the west have found that pneumonia in bighorns is most often associated with a bacterium named Mycoplasma ovipneumoniae, although other bacteria in the family Pasteurellaceae can take advantage of its presence and cause death.

    Domestic sheep can carry both M. ovipneumoniae and Pasteurellaceae, usually accompanied by few or no signs of disease. Wild sheep are not adapted to these bacteria and, when exposed, often develop severe pneumonia and die. Following these outbreaks, some bighorn sheep become carriers of M. ovipneumoniae. This can cause disease and death in lambs that prevents population recovery. These carriers can also be a threat to nearby uninfected herds.
    These outbreaks have led to regional extinctions and have made re-introduction efforts more difficult. The disease can linger for many years in a herd until a bad season causes a large die-off. Many times the survivors end up getting culled to prevent the disease from spreading.
    In March 2018 Dall's Sheep in Alaska were found to be carrying M. ovipneumoniae for the first time. I'm guessing climate change will be a factor in the spread of these type of diseases northwards.