While hiking to Tower Fall in Yellowstone National Park last winter I came across this bison bull grazing above the trail. I slowly made my way past while watching for any signs of aggression. At one point I could see his breath as he exhaled into the cold air and I had my camera ready. Bison are the biggest land mammals found in North America, with bulls weighing up to 1 ton. They typically live for 12 to 15 years. But it's always a struggle to survive the harsh Wyoming winters, with up to 40% of calves succumbing before their their first year. They will forage for grass wherever the snow cover is thinner such as thermal areas or on steep hillsides as this bull was doing. If the snow is too deep, the hump on their back enables them to pivot their head from side to side, sweeping away snow like a plow. Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times. Their population dwindled to just two dozen in the early 1900's, but they now number at close to 5,000 animals.