Bison are the most reliable beasts in Yellowstone. Nearly 5,000 in number, they are commonly seen from the valleys and overlooks, by the rivers and by the roads. They scratch their great heads upon trees to deal with insect pests, rub off the bark, and leave scars across the forest, which everyone misattributes to bears or elk. On a misty morning in August, the bellows of the bulls echo across the plains like a rumbling volcano. As the mist rolls away in the rising sun, the battlefield is revealed. Hundreds of bison in the valley below. A bull trots beside a cow, sniffs her rear end, waits for her to come into heat, roars across the plains, telling other bulls that this one is MINE. His hold lasts only as long as he can keep other bulls away, by intimidation or by force. The cows swear fealty to no partner. Their criteria is simple: the best fighters are the most attractive. He who dominates others of his kind will sire many children whom he will not know. For the life of the bull bison is solitary, walking with the herd when convenient, walking alone when convenient, eating grass all day, tolerating others of his kind when sharing the meadow. In the prime of health, he has little to fear. At 2,000 pounds, with a battering ram for a forehead, swords in his horns, and knives in his hooves—grizzly bears and wolves keep their distance and search for easier prey. The bull eats and walks and stays out of trouble, until the next mating season. Then the fury of testosterone consumes him again. All of his weapons are at the ready, but his opponents are equally armed. Corpses litter the field at mating season’s end, the result of those fights in which both adversaries refused to back down. And this attracts beasts that are normally harder to see. Grizzly bears appear on the field, thankful for the scavenged feast, in time to prepare for winter hibernation.