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Members of the genus Bison are large, even-toed ungulates within the subfamily Bovinae. Two extant and four extinct species are recognized. The surviving species are the American bison, also known as the American buffalo (although it is only distantly related to the true buffalo), Bison bison, found in North America, and the European bison, or wisent (Bison bonasus), found in Europe and the Caucasus. The North American species is composed of two subspecies, the plains bison, Bison bison bison, and the wood bison, Bison bison athabascae. While all bison species are usually grouped into their own genus, they are sometimes included in the closely related genus Bos, together with cattle, gaur, kouprey and yaks, with which bison have a limited ability to interbreed.
The American bison and the European wisent are the largest terrestrial animals in North America and Europe. Bison are good swimmers and can cross rivers over half a mile (1 km) wide. Bison are nomadic grazers and travel in herds. The bulls leave the herds of females at 2 or 3 years of age, and join a male herd which is generally smaller than the female herds. Mature bulls rarely travel alone. Both sexes get together for the reproductive season, towards the end of the summer. American bison are known for living in the Great Plains. Both species were hunted close to extinction during the 19th and 20th centuries, but have since rebounded. The American plains bison is no longer listed as endangered, but the wood bison is on the endangered species list in Canada.
Although superficially similar, there are a number of physical and behavioural differences between the American and European bison. The American species has 15 ribs, while the European bison has 14. The American bison has four lumbar vertebrae, while the European has five. (The difference in this case is that what would be the first lumbar vertebra in wisent has ribs attached to it in American bison and is thus counted as the 15th thoracic vertebra, compared to 14 thoracic vertebrae in wisent.) Adult American bison are not as rangy in build, and have shorter legs. American bison tend to graze more, and browse less than their European cousins. Their anatomies reflect this behavioural difference; the American bison's head hangs closer to the earth than the European's. The body of the American bison is typically hairier, though its tail has less hair than that of the European bison. The horns of the European bison point through the plane of their faces, making them more adept at fighting through the interlocking of horns in the same manner as domestic cattle, unlike the American bison, which favours butting. American bison are more easily tamed than their European cousins, and breed with domestic cattle more readily.
Evolution and Genetic history
The bovine family (Taurids and Bisonids) diverged from the common ancestral line with Water Buffalo and African Buffalo about 5 to 10 million years ago. Thereafter, the family lineage of bison and taurine cattle does not appear to be a straight forward ‘tree’ structure as is often depicted in much evolution, because there is evidence of interbreeding and crossbreeding between different species and members within this family, even many millions of years after their ancestors separated into different species. This cross breeding was not sufficient to conflate the different species back together, but it has resulted in unexpected relationships between many members of this group, such as Yak being related to American Bison, when such relationships would otherwise not be apparent.
A 2003 study of mitochondrial DNA indicated four distinct maternal lineages in tribe Bovini:
However, Y chromosome analysis associated wisent and American bison. An earlier study using amplified fragment length polymorphism (AFLP) fingerprinting showed a close association of wisent and American bison and probably with yak, but noted that the interbreeding of Bovini species made determining relationships problematic.
The Steppe Bison (Bison priscus) diverged from the lineage that led to cattle (Bos taurus) approximately 2 to 5 million years ago. The Bison genus is clearly in the fossil record by 2 million years ago. The Steppe Bison spread across Eurasia and was the bison that was pictured in the ancient cave drawings of Spain and Southern France
The European Bison or Wisent arose from the Steppe Bison, without fossil evidence of other ancestral species between the Steppe Bison and the European Bison, though the European Bison might have arisen from the lineage that led to American Bison if that lineage backcrossed with the Steppe Bison. Again, the web of relationships is confusing, but there is some evidence that the European Bison is descended from bison that had migrated from Asia to North America, and then back to Europe, where they crossbred with existing Steppe Bison.
At one point, some Steppe Bison cross bred with the ancestors of the modern Yak. After that cross breeding, a population of Steppe Bison (Bison priscus) crossed the Bering Land Bridge to North America. There is evidence of multiple crossings of bison to and from Asia starting before 500,000 years ago and continuing until at least 220,000 years ago. The Steppe Bison spread through the northern parts of North America and Steppe Bison lived in Eurasia until approximately 11,000 years ago and North America until 4,000 to 8,000 years ago.
Bison latifrons (Giant Bison or Longhorn Bison) is thought to have evolved in midcontinent North America from Bison priscus, after the Steppe Bison crossed into North America. Giant Bison (Bison latifrons) appeared in the fossil record appoximately 500,000 years ago. B. latifrons was one of many species of North American megafauna which became extinct during the transition from the Pleistocene to the Holocene epoch (an event referred to as the Quaternary extinction event). It is thought to have disappeared some 21,000–30,000 years ago, during the late Wisconsin glaciation.
The Bison latifrons (Giant Bison or Longhorn Bison) species was replaced by the smaller Bison antiquus. Bison antiquus appeared in the North American Fossil record approximately 250,000 years ago. Bison antiquus in turn evolved into the Bison occidentalis, then into the yet smaller Bison bison — the modern American bison — some 5,000 to 10,000 years ago. Some researchers consider Bison occidentalis to be a sub-species of Bison antiquus
During the population bottleneck, after the great slaughter of American Bison during the 1800s, the number of bison remaining alive in North America declined to as low as 541. During that period, a handful of ranchers gathered remnants of the existing herds to save the species from extinction. These ranchers bred some of the bison with cattle in an effort to produce "cattleo" (today called "beefalo") Accidental crossings were also known to occur. Generally, male domestic bulls were crossed with buffalo cows, producing offspring of which only the females were fertile. The crossbred animals did not demonstrate any form of hybrid vigor, so the practice was abandoned. The proportion of cattle DNA that has been measured in introgressed individuals and bison herds today is typically quite low, ranging from 0.56 to 1.8%. In the United States, many ranchers are now utilizing DNA testing to cull the residual cattle genetics from their bison herds. The U.S. National Bison Association has adopted a code of ethics which prohibits its members from deliberately crossbreeding bison with any other species.
Some cattle breeds are intentionally bred with bison to produce, for instance, Beefalo hybrids. Wisent-American bison hybrids were briefly experimented with in Germany (and found to be fully fertile) and a herd of such animals is maintained in Russia all the time. A herd of cattle-wisent crossbreeds (Zubron) is maintained in Poland. First-generation crosses do not occur naturally, requiring caesarean delivery. First-generation males are infertile.
Wallowing is a common behavior of bison. A bison wallow is a shallow depression in the soil, either wet or dry. Bison roll in these depressions, covering themselves with mud or dust. Possible explanations suggested for wallowing behavior include grooming behavior associated with moulting, male-male interaction (typically rutting behavior), social behavior for group cohesion, play behavior, relief from skin irritation due to biting insects, reduction of ectoparasite load (ticks and lice), and thermoregulation. In the process of wallowing, bison may become infected by the fatal disease anthrax, which may occur naturally in the soil.
The bison's temperament is often unpredictable. They usually appear peaceful, unconcerned, even lazy, yet they may attack anything, often without warning or apparent reason. They can move at speeds of up to 35 mph (56 km/h) and cover long distances at a lumbering gallop.
Their most obvious weapons are the horns borne by both males and females, but their massive heads can be used as battering rams, effectively using the momentum produced by 2,000 pounds (900 kg) moving at 30 mph (50 km/h). The hind legs can also be used to kill or maim with devastating effect. At the time bison ran wild, they were rated second only to the Alaska brown bear as a potential killer, more dangerous than the grizzly bear. In the words of early naturalists, they were a dangerous, savage animal that feared no other animal and in prime condition could best any foe (except for wolves and brown bears).
The rutting, or mating, season lasts from June through September, with peak activity in July and August. At this time, the older bulls rejoin the herd, and fights often take place between bulls. The herd exhibits much restlessness during breeding season. The animals are belligerent, unpredictable and most dangerous.
The American bison live in river valleys, and on prairies and plains. Typical habitat is open or semi-open grasslands, as well as sagebrush, semi-arid lands and scrublands. Some lightly wooded areas are also known historically to have supported bison. Bison will also graze in hilly or mountainous areas where the slopes are not steep. Though not particularly known as high altitude animals, bison in the Yellowstone Park Bison Herd are frequently found at elevations above 8,000 feet and the Henry Mountains Bison Herd is found on the plains around the Henry Mountains, Utah, as well as in mountain valleys of the Henry Mountains to an altitude of 10,000 feet.
European bison (Wisent) tend to live in lightly wooded to fully wooded areas and areas with increased shrubs and bushes, though they can also live on grasslands and plains.
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Bison have a fairly simple diet. The bison's main food is grass. Bison also eat the low-lying shrubbery that is available. In the winter, bison forage in the snow looking for grass. If there is little grass available, bison have to resort to eating the twigs of shrubs. They will also consume sedges. One difference between bison and domestic cattle is that bison are notably better at browsing than cattle, since cattle are more obligate grazers.
Wisent are noted to browse more often on shrubs and low hanging trees than their American bison cousins that generally prefer grass, if given a choice.
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Due to their large size, few predators attack bison. However, wolf packs can take down a bison. There are even documented cases of a single wolf taking down bison. Brown bears will also prey on calves, and commonly drive off wolves to take over their kills.
Infections and illness
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The main cause of illness in bison is malignant catarrhal fever (MCF). However, American bison, especially the Yellowstone Park Bison Herd are noted to also get infected with Brucellosis. Bison in the Antelope Island Bison Herd are regularly inoculated against brucellosis, parasites, clostridium, infectious bovine rhino tracheitis and bovine vibriosis.
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