BISON

BISON 

Bison or buffalo are large, even-toed ungulates in the genus Bison within the subfamily Bovinae.

Two extant and four extinct species are recognized. Of the four extinct species, three were North American: Bison antiquus, B. latifrons, and B. occidentalis. The fourth, B. priscus, ranged across steppe environments from Western Europe, through Central Asia, and onto North America.

Of the two surviving species, the American bison, B. bison, found only in North America, is the more numerous. Although sometimes referred to historically as a "buffalo", 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. The European bison B. bonasus, or wisent, is found in Europe and the Caucasus, reintroduced after being extinct in the wild.


While all bison species are classified in their own genus, they are sometimes bred with domestic cattle (genus Bos) and produce fertile offspring called beefalo or zubron.




Evolution and genetic history


The bovine family (Bovidae) 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 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:
Taurine cattle and zebu
Wisent
American bison and yak and
Banteng, gaur, and gayal


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 steppe bison (B. priscus) diverged from the lineage that led to cattle (Bos taurus) about 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 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 some evidence shows 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 crossbred with the ancestors of the modern yak. After that crossbreeding, a population of steppe bison crossed the Bering Land Bridge to North America. Evidence exists 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 lived in Eurasia until around 11,000 years ago and North America until 4,000 to 8,000 years ago.







Skulls of European bison (left) and American bison (right)


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 500,000 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 was replaced by the smaller B. antiquus, which appeared in the North American fossil record around 250,000 years ago. B. antiquus, in turn, evolved into B. occidentalis, then into the yet smaller B. bison, the modern American bison, some 5,000 to 10,000 years ago. Some researchers consider B. occidentalis to be a subspecies of B. antiquus.


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 inSouth Dakota and Elk Island National Park in Alberta, 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.[24]