On my last trip to Yellowstone in October, we encountered this bull Bison resting in the middle of a Bison wallow, apparently tuckered out after wallowing. He had just finished coating himself with dirt which is what Bison do to shed excess fur, relieve skin irritations, and stay cool. These Bison wallows are quite common throughout the Yellowstone area where Bison roam and it is. not uncommon to see them take turns in the wallow.