Male red deer contend for female mating privilege, beginning with loud roars as an indication of fighting ability. Once out-called, a stag will sometimes withdraw without further ado. If not, roaring escalates, increasingly faster and fiercer. If one is not out-roared, fighting is likely.
Roaring is an honest indicator of power. Only a healthy, powerful buck can sustain a rage of roaring. Several deer species rely upon this same mating mechanism.
If roaring doesn’t end a red deer contest, push comes to shove. Male red deer lock horns and push each other a bit, whereupon one concedes defeat.