During the winter the bison are split into two separate groups. Cows and calves make up one group, bulls make up the other. The cow/calf group consists of 20-70 bison. The bull group has 5- 20 members.
Both groups search for exposed grass to feed on. Bison use their heads, horns, and powerful necks to clear away the snow that covers the food supply. Bison can eat grass buried under snowdrifts 4 feet deep! When water is difficult to find, they eat snow.
Their thick coats and heavy layers of fat protect them from the cold temperatures. During blizzards, bison huddle together for warmth. Bulls often take shelter in trees, but the other group often tries to resist the wind in the open where they are more likely to find nourishing grass.
As the temperature starts to warm, the bison move off their winter range. Now their heavy layers of fat have been depleted, and the search for new grass keeps the bison on the move. The thick coats are no longer needed, so the bison shed (molt)their heavy hair (pelage). Tattered patches of hair often cling to the bison.
The cows are still separate from the bulls. The pregnant bison tend to separate themselves a small distance from the cow/calf group. After approximately 275 days of gestation, the cows bear their calves.
The calves are born without humps and weigh about 50 pounds. Their coats are the color of cinnamon and will darken with age.
The bulls and the cows finally join together as a herd during the summer. The mating season, known as the rut,begins and lasts until early fall.
The calves are really growing. Dark horns break the skin and their coloring darkens. The baby hair falls out and molting occurs as the darker hair pushes from beneath. The hump is starting to develop.
The adults rub against any available object and wallow on the ground to rid themselves of their molting pelage, and to scratch themselves when insects make them itch. When telegraph lines first crossed the prairies, many were knocked down by bison rubbing and scratching.
At this time, the bison are building fat for the winter. They graze much more, spending less time on the scratching, napping, and rolling that is popular in the summer. They are also in the process of growing pelage for the cold weather. Because they are in prime condition, this is often considered the best time for hunting. By November, the bulls have left the herd.
The calves are looking more like adult bison with their coloring and shape.
Their hair is also thickening into a long, shaggy appearance. Calves at this age weigh about 400 pounds. Bison in the wild live 12-15 years, but hardy animals may live 30 years or more.