Autumn in Yellowstone is ungulate mating season (rut). Bull elk spend the summer eating and growing antlers in preparation for a chance to pass on their genes. The strongest bull elk control access to harems of about six cows with their calves. During the rut, dominant bulls have to work around the clock keeping their harems together and away from other bulls, bugling and driving off the competition. Would be usurpers test and challenge the dominant bull hoping to defeat him and take his place.
Bulls of similar strength battle, locking antlers and pushing vigorously to establish dominance. Sometimes they are injured fighting but bulls rarely die in the rut. The loser gives up and the winner goes back to patrolling his harem as long as he is able.
After the rut, the bulls gather together in small male groups for winter, spring and summer. The female groups congregate in larger herds for the winter, led by experienced matriarchs. These herds move to lower elevations in search of winter forage.
Elk cows give birth to single calves in May to late June. Newborn ungulates rely on staying hidden until they can keep up with their mothers and join the herd. Many calves are eaten by wolves, coyotes, black and grizzly bears, cougars, and golden eagles.