and, in july and august, right when the hayden party was making their way down these hills, dust fills the air in the valley. [foreboding music] [bison grunting] the bison run is underway. [foreboding music] [bison grunting] female bison tend to mate with the most dominant bulls. [foreboding music] [bison grunting] this wallowing behavior by the males is a display of power and aggression. [foreboding music] during the mating season, each bull will lose around 200 pounds, dedicating all his time and energy to performing for the potential mates. [foreboding music] [bison grunting] it's an interesting ritual, but highly effective, because, in yellowstone, the bison are thriving, and, every year, more and more of these wooly giants roam the valley.