When entering Yellowstone, the first large animals I saw were bison, and these are LARGE animals. The males weigh as much as 1800 lbs and the females 1,000 lbs. For 10 months or so out of the year, the males congregate in bachelor herds, but during the rut join the females. Unlike other herbivores in the park like elk, males do not try to establish separate groups of females.
As the females become receptive, males will associate with one female in a behavior called "tending". During the middle of the rut there are plenty of females for the males, but early and late in the rut males do compete with each other for the receptive females.
Groups of bison will defend each other from predators, unlike some of the other large herbivores in the park , which flee. Unrelated animals will respond to the alarm cry of a calf and protect it from wolves, bears, or other predators. In the winter, bison move snow with their large heads to eat the dried grasses beneath the snow. Males can plow through snow 3 feet deep with their massive heads. In addition to grasses, bison can also eat coarse sedges.
Bison often formed "bisonjams" on the roads in the park. Bison walk where they want to, when they want to, after all they are as big as some of the cars on the road. I saw a number of motorcyclists and bicyclists in the park. I would be a little wary of riding along the road if I knew I might come into contact with a 1800 lb animal.
If the winter is very harsh they will continue to move to lower elevations which take them out of the park where they are hunted, or rounded up and killed because they have been exposed to a disease called brucellosis. This bacterial disease causes bison and cattle to abort calves, and can be transmitted if an animal eats grass near where another has given birth. Local ranchers are concerned that the bison could spread the disease to their cattle. Ironic since cattle gave the disease to bison in the first place.
