Temporal range: 2–0 Ma Early Pleistocene – Recent
Hamilton Smith, 1827
Two extant and six extinct species are recognised. Of the six extinct species, five went extinct in the Quaternary extinction event. Bison palaeosinensis evolved in the Early Pleistocene in South Asia, and was the evolutionary ancestor of B. priscus (steppe bison), which was the ancestor of all other Bison species. From 2 MYA to 6,000 BC, steppe bison ranged across the mammoth steppe, inhabiting Europe and northern Asia with B. schoetensacki (woodland bison), and North America with B. antiquus, B. latifrons, and B. occidentalis. The last species to go extinct, B. occidentalis, was succeeded at 3,000 BC by B. bison.
Of the two surviving species, the American bison, B. bison, found only in North America, is the more numerous. Although commonly known as a buffalo in the United States and Canada, it is only distantly related to the true buffalo. The North American species is composed of two subspecies, the Plains bison, B. b. bison, and the Wood bison, B. b. athabascae, which is the namesake of Wood Buffalo National Park in Canada. A third subspecies, the Eastern Woodland Bison (B. b. pennsylvanicus) is no longer considered a valid taxon, being a junior synonym of B. b. bison. References to "Woods Bison" or "Wood Bison" from the eastern United States confusingly refer to this subspecies, not B. b. athabascae, which was not found in the region. The European bison, B. bonasus, or wisent, is found in Europe and the Caucasus, reintroduced after being extinct in the wild.
The American bison and the European bison (Wisent) are the largest surviving terrestrial animals in North America and Europe. Bison are good swimmers and can cross rivers over half a mile (800 meters) wide. They are nomadic grazers and travel in herds. The bulls leave the herds of females at two or three years of age, and join a male herd, which are generally smaller than female herds. Mature bulls rarely travel alone. Towards the end of the summer, for the reproductive season, the sexes necessarily commingle.
American bison are known for living in the Great Plains, but formerly had a much larger range including much of the eastern United States and parts of Mexico. Both species were hunted close to extinction during the 19th and 20th centuries, but have since rebounded; the Wisent owing its survival, in part, to the Chernobyl Disaster, ironically, as the Chernobyl Exclusion Zone has become a kind of wildlife preserve for Wisent and other rare megafauna such as the Przewalski's Horse, though poaching has become a threat in recent years. The American Plains bison is no longer listed as endangered, but this does not mean the species is secure. Genetically pure B. b. bison currently number only ~20,000, separated into fragmented herds—all of which require active conservation measures. The Wood bison is on the endangered species list in Canada and is listed as threatened in the United States, though there have been numerous attempts by beefalo ranchers to have it entirely removed from the Endangered Species List.
Although superficially similar, physical and behavioural differences exist 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 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 less slim in build and have shorter legs. American bison tend to graze more, and browse less than their European relatives. Their anatomies reflect this behavioural difference; the American bison's head hangs lower 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 tribe (Bovini) split about 5 to 10 million years ago into the buffalos (Bubalus and Syncerus) and a group leading to bison and taurine cattle. Thereafter, the family lineage of bison and taurine cattle does not appear to be a straightforward "tree" structure as is often depicted in much evolution, because evidence of interbreeding and crossbreeding is seen between different species and members within this family, even many millions of years after their ancestors separated into different species. This crossbreeding 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 fingerprinting showed a close association of wisent with American bison, and probably with the yak, but noted that the interbreeding of Bovini species made determining relationships problematic.
The Bison genus diverged from the lineage that led to cattle (Bos primigenius) at the Plio-Pleistocene boundary in South Asia. Two extant and six extinct species are recognised. Of the six extinct species, five went extinct in the Quaternary extinction event. Three were North American endemics: Bison antiquus, B. latifrons, and B. occidentalis. The fourth, B. priscus (steppe bison), ranged across steppe environments from Western Europe, through Central Asia, East Asia including Japan, and onto North America. The fifth, B. schoetensacki (woodland bison), inhabited Eurasian forests, extending from western Europe to the south of Siberia.
The sixth, B. palaeosinensis, evolving in the Early Pleistocene in South Asia, is presumed to have been the evolutionary ancestor of B. priscus and all successive Bison lineages. The steppe bison (B. priscus) evolved from Bison palaeosinensis in the Early Pleistocene. B. priscus is seen clearly in the fossil record around 2 million years ago. The steppe bison spread across Eurasia, and all proceeding contemporary and successive species are believed to have derived from the steppe bison. Going extinct in 6,000 BCE, outlasted only by B. occidentalis, B. bonasus and B. bison, the steppe bison was the predominant bison pictured in the ancient cave paintings of Spain and Southern France.
The modern European bison is likely to have arisen from the steppe bison. There is no direct fossil evidence of successive species between the steppe bison and the European bison, though there are three possible lines of ancestry pertaining to the European wisent. Past research has suggested 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. However, more recent phylogenetic research points to an origin either from the phenotypically and genetically similar Pleistocene woodland bison (B. schoetensacki) or as the result of an interbreeding event between the steppe bison and the aurochs (Bos primigenius), the ancestor of domesticated cattle, around 120,000 years ago. The possible hybrid is referred to in vernacular as the 'Higgs bison' as a hat-tip to the discovery process of the Higgs boson.
At one point, some steppe bison crossbred with the ancestors of the modern yak. After that crossbreeding, a population of steppe bison crossed the Bering Land Bridge to North America. The steppe bison spread through the northern parts of North America and lived in Eurasia until around 11,000 years ago and North America until 4,000 to 8,000 years ago.
The Pleistocene woodland bison (B.schoetensacki) evolved in the Middle Pleistocene from B. priscus, and tended to inhabit the dry conifer forests and woodland which lined the mammoth steppe, occupying a range from western Europe to the south of Siberia. Although their fossil records are far rarer than their antecedent, they are thought to have existed until at least 36,000 BCE.
Bison latifrons (the "giant" or "longhorn" bison) is thought to have evolved in midcontinent North America from B. priscus, after the steppe bison crossed into North America. Giant bison (B. latifrons) appeared in the fossil record about 120,000 years ago. B. latifrons was one of many species of North American megafauna that 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.
B. latifrons co-existed with the slightly smaller B. antiquus for over 100,000 years. Their predecessor, the steppe bison appeared in the North American fossil record around 190,000 years ago. B. latifrons is believed to have been a more woodland-dwelling, non-herding species, while B. antiquus was a herding grassland-dweller, very much like its descendant B. bison. B. antiquus gave rise to both B. occidentalis, and later B. bison, the modern American bison, some 5,000 to 10,000 years ago. B. antiquus was the most common megafaunal species on the North American continent during much of the Late Pleistocene and is the most commonly found large animal found at the La Brea Tar Pits.
In 2016, DNA extracted from Bison priscus fossil remains beneath a 130,000-year-old volcanic ashfall in the Yukon suggested recent arrival of the species. That genetic material indicated that all American bison had a common ancestor 135,000 to 195,000 years ago, during which period the Bering Land Bridge was exposed; this hypothesis precludes an earlier arrival. The researchers sequenced mitochondrial genomes from both that specimen and from the remains of a recently discovered, estimated 120,000-year-old giant, long-horned, B. latifrons from Snowmass, Colorado. The genetic information also indicated that a second, Pleistocene migration of bison over the land bridge occurred 21,000 to 45,000 years ago.
During the population bottleneck, after the great slaughter of American bison during the 19th century, 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. 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. 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. The U.S. National Bison Association has adopted a code of ethics that prohibits its members from deliberately crossbreeding bison with any other species. In the United States, many ranchers are now using DNA testing to cull the residual cattle genetics from their bison herds. 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%.
There are also remnant purebred American bison herds on public lands in North America. Three herds are in Yellowstone National Park, Wind Cave National Park in South Dakota, Elk Island National Park in Alberta, Canada and Grasslands National Park in Saskatchewan, Canada. In 2015 a fourth purebred herd of 350 individuals was identified on public lands in the Henry Mountains of southern Utah via genetic testing of mitochondrial and nuclear DNA. This study, published in 2015, also showed the Henry Mountains bison herd to be free of brucellosis, a bacterial disease that was imported with non-native domestic cattle to North America.
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.
Bison 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 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 what is a typical weight of 2,000 pounds (900 kg) (can be up to 2700 lbs) moving at 30 mph (50 km/h). The hind legs can also be used to kill or maim with devastating effect. In the words of early naturalists, they were dangerous, savage animals 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.
American bison live in river valleys, and on prairies and plains. Typical habitat is open or semiopen grasslands, as well as sagebrush, semiarid lands, and scrublands. Some lightly wooded areas are also known historically to have supported bison. They 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 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.
Throughout most of their historical range, landowners have sought restrictions on free-ranging bison. Herds on private land are required be fenced in. In the state of Montana, free-ranging bison on public lands may be shot, due to concerns about transmission of disease to cattle and damage to public property. In 2013, Montana legislative measures concerning the bison were proposed and passed the legislature, but opposed by Native American tribes as they impinged on sovereign tribal rights. Three such bills were vetoed by Steve Bullock, the governor of Montana. The bison's circumstances remain an issue of contention between Native American tribes and private landowners.
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Bison are ruminants, which allows them to derive their energy from cell walls. Bison were once thought to almost exclusively consume grasses and sedges, but are now known to consume a wide-variety of plants including woody plants and herbaceous eudicots . Over the course of the year, bison shift which plants they select in their diet based on which plants have the highest protein or energy concentrations at a given time and will reliably consume the same species of plants across years . Protein concentrations of the plants they eat tend to be highest in the spring and decline thereafter, reaching their lowest in the winter . In Yellowstone National Park, bison browsed willows and cottonwoods, not only in the winter when few other plants are available, but also in the summer . Bison are thought to migrate to optimize their diet , and will concentrate their feeding on recently burned areas due to the higher quality forage the regrows after the burn . Wisent tend to browse on shrubs and low-hanging trees more often than do the American bison, which prefer grass to shrubbery and trees.
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Due to their size, bison have few predators. Four notable exceptions are humans, the grey wolf, brown bear, and coyote. The grey wolf generally takes down a bison while in a pack, but cases of a single wolf killing bison have been reported. Brown bear also consume bison, often by driving off the pack and consuming the wolves' kill. Brown bear and coyotes also prey on bison calves. Historically and prehistorically, lions, tigers, Smilodon, Homotherium, cave hyenas and Homo sp. had posed threats to bison.
Infections and illness
For the American bison, the main cause of illness is malignant catarrhal fever, though brucellosis is a serious concern in the Yellowstone Park bison herd. Bison in the Antelope Island bison herd are regularly inoculated against brucellosis, parasites, Clostridium infection, infectious bovine rhinotracheitis, and bovine vibriosis.
The major concerns for illness in European bison are foot-and-mouth disease and balanoposthitis, which affects the male sex organs; a number of parasitic diseases have also been cited as threats. The inbreeding of the species caused by the small population plays a role in a number of genetic defects and immunity to diseases, which in turn poses greater risks to the population.
The term "buffalo" is sometimes considered to be a misnomer for this animal, as it is only distantly related to either of the two "true buffalo", the Asian water buffalo and the African buffalo. Samuel de Champlain applied the term buffalo (buffles in French) to the bison in 1616 (published 1619), after seeing skins and a drawing shown to him by members of the Nipissing First Nation, who said they travelled forty days (from east of Lake Huron) to trade with another nation who hunted the animals. Though "bison" might be considered more scientifically correct, as a result of standard usage, "buffalo" is also considered correct and is listed in many dictionaries as an acceptable name for American buffalo or bison. Buffalo has a much longer history than bison, which was first recorded in 1774.
Humans were almost exclusively accountable for the near-extinction of the American bison in the 1800s. At the beginning of the century, tens of millions of bison roamed North America. American settlers slaughtered an estimated 50 million bison during the 19th century. Railroads were advertising "hunting by rail", where trains encountered large herds alongside or crossing the tracks. Men aboard fired from the trains roof or windows, leaving countless animals to rot where they died. The overhunting of the bison reduced their population to hundreds. Attempts to revive the American bison have been highly successful. Farming of bison has increased their population to nearly 150,000. The American bison is, therefore, no longer considered an endangered species. The extinction of four species of bison (B. antiquus, B. latifrons, B. occidentalis, and B. priscus) was due to natural selection (see section Evolution and genetic history).
As of July 2015, an estimated 4,900 bison lived in Yellowstone National Park, the largest U.S. bison population on public land. During 1983–1985 visitors experienced 33 bison-related injuries (range = 10–13/year), so the park implemented education campaigns. After years of success, five injuries associated with bison encounters occurred in 2015, because visitors did not maintain the required distance of 75 ft (23 m) from bison while hiking or taking pictures.