This is a full grown adult male bison rolling or wallowing in a pit dug out and rolled in for many years. Bison wallow covering their hide with layers of dust and dirt for protection against the hot sun in the high Montana altitude, it also helps to protect against biting insects. This photo was taken in late spring early summer, in autumn the bison will not only urinate but often times will ejaculate in the pit then wallow in it. Hoping the added smell will attract suitable mates during the rut.