I’ve been to Yellowstone twice, both times with our oldest son Slade, a professional photographer/videographer. Each time I swore I wouldn’t take another image of a bison. But when I saw this one rolling in the dirt, I couldn’t resist. Wallowing is when bison roll on the dry ground in a “dust bath” to relieve skin irritations caused by shedding a winter coat or seeking relief from biting insects. Rolling on the ground also creates a layer of dirt on the bison’s body that forms a defensive barrier from ticks and lice. The wallowing behavior of bison creates wallows; bowl-like depressions. Wallows were once a common feature of the Great Plains before European settlement and changes in land use. Across the Great Plains, it is estimated that there may have been five or more bison wallows per acre. With a historic extent of 500,000 square miles, the Great Plains could have once been marked with over 1.5 billion bison wallows.