Sunday 24 April 2016

Bovines: Bison bison bison, the Bison

American bison
The only bovine native to the Americas is the bison (Bison bison). Indeed, it is one of only five species of bovid that fit that description - the other four all being members of the goat subfamily. To give at least some variety, however, there are two generally recognised subspecies of American bison, one of which, following the standard rules for naming subspecies, necessarily goes by the wonderfully tautonymous trinomial Bison bison bison.

This, in fact, is the plains bison, which was once found across pretty much the whole of what is now the contiguous United States, leaving aside only the Pacific and Atlantic coasts and the arid deserts of the Southwest; they also lived in far northern Mexico and up as far as central Alberta. The other subspecies, commonly known as the wood bison (Bison bison athabascae) is native to north-western Canada and to Alaska. Physically, it's slightly larger than its southern relative, which means that, in the absence of American rhinos, elephants, and so on, it is, in fact, the largest living, land-dwelling animal of any kind native to the Americas.

Because bison are, indeed, pretty big. A fully grown male can be anything up to 195 cm (6' 6") tall at the shoulder, and can weigh as much as a tonne (2,200 lbs). Females are, admittedly, quite a bit smaller, although at a maximum of about 180 cm (6') and 550 kg (1,200 lbs) they're still pretty big - and closer in size to the males than, say, those of yak. The horns are also about the same size in both sexes, although (relative to the rest of the animal) fairly small in comparison to those of most other bovines.

Unusually for bovines, bison are very much animals of open plains and wide grasslands. (In this respect, it's interesting to note that their closest relative may be the yak, which also lives in open terrain). However, in the more northern parts of their range, they commonly inhabit spruce forest, and, historically, they were also found in quite mountainous parts of Montana and Idaho. They are about as pure grazing animals as one could hope to find, indiscriminately shovelling up great quantities of grass and whatever else happens to be on the ground with their large, broad, mouths. Well over half of their diet consists of grasses or sedges of some kind, even in northern forests during winter, and it may approach 95% on grassy plains during summer.

Being so large, bison need to eat a lot of grass in order to prosper, and they can spend an average of ten hours a day just eating, and most of the rest standing around ruminating and digesting. Naturally enough, they will seek out areas with the best grass, and, in modern times, will often congregate where humans have used fire to manage the prairie - recently burned areas have more nutritious grass, even if there's less of it. It has also been suggested that bison and prairie dogs mutually benefit one another, each modifying the structure of grassland vegetation in ways that suit the other, so that bison graze more often where prairie dogs dig their burrows.

Bison also require quite a lot of water - although snow will do just fine - which explains why they were absent in places like Arizona and California. Not only do they drink it, but they spend quite a lot of time wallowing in the mud, when the opportunity to do so arises. This is mainly to get rid of biting insects and ticks, although it likely also helps them cool down in hot weather, and it may be relevant to their social interactions, or, for all we know, help to relieve stress by just being kind of fun.

The search for the best food probably also explains why bison used to migrate such large distances, moving from pastures that were verdant in the summer to those that were more productive in the winter. For most of the year, bison herds are dominated by the females, although they do include a number of younger adult males in addition to the calves. The older males live in groups of just two to six, and only join up with the main herds during the July to September rut. Today, these herds can have over 400 members, but they were clearly much larger in the past, when observers reported herds stretching as far as the eye could see (which is quite a distance on the open plains...)

Within the herd, males constantly compete for dominance with one another, with the fights often becoming very violent. The females also compete, but much less so, perhaps because they have less to gain, and more to lose if they are injured. Interestingly, it is typically the older females that dominate, even if they aren't obviously stronger or heavier than their younger relatives. This may be because they establish a social hierarchy early on in life, and never quite forget it - so that younger animals are more likely to submit to an established superior even if they are physically capable of winning a fight.

During the rut, the successful males gather harems of females, tending and defending them until they are ready to mate. Each typically mates only once, and over a very short period of just two weeks. Pregnancy lasts about nine and a half months, which is a week or two longer than it does in domestic cows, and is regular enough that birthing, too, takes place at about the same time for all members of a given herd. Although it has been suggested that this may help to ensure that there are simply so many calves about that predators can't possibly try and eat them all before most are too big to bother with, it seems more likely that it's just to take advantage of the best of the local weather.

Cows give birth to a single calf, typically about twice every three years. It takes a full year to wean them, and females aren't fully mature until the age of two, and males another year after that. Even then, the males are likely to have to wait a couple more years before achieving any success in the rut (unless, of course, there are no older bison around), and they don't reach their peak until about the age of seven. Bison can live for over twenty years, although, in the wild, most presumably don't.

Of course, one of the best known things about bison is that there are lot less of them now than there used to be. Quite how many that was is difficult to judge, although it's clearly in the tens of millions. While Native Americans had hunted bison for their meat for thousands of years, there just weren't enough of them to have any real impact in the long term. White hunters, on the other hand, who were mainly interested in leather, were far more successful, and there was also deliberate extermination of bison in the nineteenth century to make room for cattle ranches and the like. (It's not all 19th century though; bison had already vanished from Pennsylvania by 1735, for instance). One contemporary estimate, based on the number of hides sold, is that over three million bison were killed in the south of the US between 1871 and 1874 alone.

Whatever the real figure,the slaughter was clearly on a huge scale. By around 1900, only about a hundred bison remained alive. That they survived at all is largely due to efforts to domesticate them, something that the Native Americans had had no reason to attempt. Two genuinely wild herds - one of plains bison in Yellowstone Park, and one of wood bison in northern Alberta - do seem to have survived all the way through, and many have since been re-introduced to the wild from captivity elsewhere. Today, over 11,000 adult bison are thought to live in the open wild, although that number rises to 30,000 if you include bison living in fenced-off public land.

Far more live in semi-domesticated herds, where they are largely raised for their meat, which has been shown to be higher in protein and lower in fat than regular beef. Many of these animals, however, have at least some admixture of cattle genes, since the two species do interbreed to produce fertile young. Indeed, such hybrids have been bred deliberately since around 1880, with the resulting offspring said to be more docile than wild bison (which is possibly not very hard), yet hardier than cattle. They also have more meat over the rump than bison - a prime cut from beef cattle - and produce more milk into the bargain.

European bison
But, if the American bison is the largest land-dwelling animal found wild in America today, the largest in Europe is... the European bison (Bison bonasus). Sometimes known by its German name of "wisent" to avoid confusion with the American species, physically, the two animals look remarkably similar, and they're essentially the same size. In comparison to its American cousin, the European bison has a paler mane and beard, and a straighter back, due to heavier muscle over the hips, but otherwise, they are really quite hard to tell apart.

They are, however, different in habits, with the European bison inhabiting open woodland, and eating far more herbs and leaves from bushes and small trees than just grass - although, unsurprisingly, they'll readily eat the latter if they have to. They don't bother to migrate over the course of the year, although they do wander around a fair bit over areas of up to 150 km² (58 square miles) in search of food. Their herds are much smaller, too, with typically no more than twenty adults in the main matriarchal herds, and less in the small bachelor ones.

It is, however, very difficult to tell how much this is really "natural". That, of course, is because, while the white man did a lot of damage to the American bison, he got a considerable head start when it came to wiping out the European sort. I've related the sorry tale of the European bison, and the effects on its genetic diversity, before, but the summary is that they went extinct in the wild in 1927. All European bison today are descendants of just twelve zoo animals, whose descendants were re-introduced to the wild  starting in 1951. There are thought to be something like 2,000 wild European bison alive today, mostly in the Białowieża Forest on the border between Poland and Belarus. There are also at least 1,400 in captivity, enough that the species is no longer officially considered "endangered" by international standards.

The extreme similarity between European and American bison led to the understandable assumption that the two species were very closely related. Indeed, as indicated in my previous post on European bison, this was supported by genetic analyses. However, not least because they're capable of interbreeding, it can be difficult to untangle the true relationships between the various species of bovine. Since I wrote the post, two new studies have shown a different relationship, with American bison closely related to yak, and European bison closer to domestic cattle (presumably via the extinct aurochs).

For the reasons already given, this may yet be reversed again, and bison restored to their original position on the family tree. But, if not, then the European bison is not really a "bison", and even if it is, bison are still closer to domestic cattle than are, say, gaur, which are currently placed in the domestic cattle genus Bos. There are a number of ways this inconsistency can be dealt with, depending not least on where various fossil species happen to be placed - Pelorovis, for instance, while generally thought to be a kind of African buffalo, has been proposed as a possible cattle-ancestor. But the simplest option is to ditch the genus Bison altogether, and put everything that's closer to cows than to buffalo into Bos.

This suggestion is hardly new, and the fact that bison of both species can crossbreed with cattle without much difficulty would tend to support it anyway. Since about 2000, use of the scientific name Bos bison has begun to replace Bison bison in the scientific literature, and is now quite common. As it happens, this is the original name they were given when first described in 1758, with the genus Bison only being erected almost seventy years later, in 1827. (As an aside, the original description said that bison were from "Mexico", but this was 1758, and Linnaeus meant the bit of Mexico that we now call "Kansas").

I've stuck with the older name, partly because it's still the more common one, but also because it gave me a better title for the post. Such is life as a blogger.

The word "bison" itself is Latin, which explains what it's doing in the scientific name, but in American English, the animals are often called "buffalo", which is said to originally be a pre-modern Portuguese word that has the same ultimate word-root as "beef". (And, as we all know, Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo). In the rest of the world, however, the buffalo is an entirely different sort of animal, one that likely diverged from the bison/cattle line over 5 million years ago. I'll turn to look at buffalo in a couple of months, but, before I do so, there's just a couple of other species to look at...

[Photos by "Mongo" and "Grottesdehan", from Wikimedia Commons. Cladogram adapted from Bibi et al 2013 and Yang et al 2013.]


  1. Buffalo is not Portuguese. It's an Italian form /bufalo/ of a Greek Bubalus or Bubalis.

    1. The OED strikes out again! Although it did at least agree that the (alleged) Portuguese word came from the Greek "bubalus".

  2. Concerning bison relationships and similar questions, the first paper is in open access and contains this paragraph:

    "Conflict is also present with regard to taxonomic revisions made at the species level. Relationships based on mitochondrial genomes benefit from independent confirmation by nuclear genes e.g. [97]. For example, the mitochondrial sister relationships of Capra sibirica to Hemitragus jemlahicus and Bos (Bison) bonasus to Bos taurus, which result in the polyphyly of Capra and the bisons, are contradicted by both nuclear genomic and morphological analyses that both support the monophyly of these two taxa [97–102]. Analyses such as those presented here or by Hassanin et al. [8] similarly support previous findings [103] regarding mitochondrial polyphyly within the bushbuck (here labelled T. scriptus 1 and T. scriptus 2 + 3 in Figure 1). However, these mitochondrial relationships contradict morphological evidence, and, in light of the examples of the bisons and Capra, the elevation of T. scriptus sylvaticus to the level of a separate species (T. sylvaticus) unrelated to T. scriptus should await further confirmation from the nuclear genome [103]. The possibility of incomplete lineage sorting, or mitochondrial lineage introgression among geographically proximate tragelaphin species, as probably occurred within Caprini and Bovini, has not yet been ruled out."

    1. Oh, indeed. As I say, there's no strong reason to suppose this will stand. It depends a lot on which genetic/morphological factors you happen to consider most significant. It's entirely possible, for instance, that there was some admixture between aurochs and European bison in the relatively recent past, evolutionarily speaking, and that that's muddying the waters.