Bison are the largest land mammals in North America. Large bulls in their prime can weigh upwards of a tonne. There are two subspecies of bison in North America, the plains and the wood. Wood bison, found in the Yukon, are taller and shaggier than plains bison, with larger shoulder humps and long horns clear of hair. Wood bison evolved as a specific subspecies about five thousand years ago. During and after the last ice age, bison were among the most common large grazing animals in Beringia, the wide, grassy plains that covered parts of the Yukon, Alaska, and Siberia.
Climatic changes caused the glaciers to retreat and triggered a shift from grassland to forests, reducing the bison’s preferred habitat and causing a decline in bison numbers. The combination of shrinking habitat and increased hunting, particularly with the arrival of firearms, caused a drastic decline in bison populations. By the beginning of the twentieth century, only a few hundred wood bison were left. Since that time, however, North American governments have made a concerted effort to save and rebuild populations of both wood and plains bison.
Wood bison were reintroduced to the Yukon twenty years ago, when a small herd from Elk Island National Park were released. The herd flourished, and the Yukon's wild wood bison population has grown to about 1100, centered around the Aishihik area of southwestern Yukon. While this is above the targeted levels of 500, the wood bison still remain a threatened species in Canada.
As herd animals, bison co-operate and watch out for each other. This behaviour improves the their chances of survival when faced with challenges like extreme cold, patchy food supplies, and predators. Males are bigger than females, but both sexes have short black horns. Male horns are straight, while female horns curve slightly inward. Among the more unusual features of bison are their completely black tongues.