Males mature sexually at about four years but keep growing until they reach about eight years. While young males often roam together in loose association, older males tend to be solitary. The older, larger bulls are dominant and devote much of their time to patrolling for females in heat. A dominant bull will approach a herd, draw himself up to his full height and stare at a potential rival. While this is usually enough to make the adversary leave, once in a while a subordinate bull stands his ground and a fight ensues.
Giraffes fight by charging and swinging their heads at each other as hard as they can.
At up to 25 pounds, their heads are substantial weapons, and the thud of a blow is audible at least 100 yards away. Although violent, these fights normally do not result in injury because the males' nine-inch horns are blunt and covered with skin, and the skin on their necks can be an inch thick. Moreover, their brains are well-protected by skulls that are quite thick and extensively pocketed with sinuses. When one of the bulls tires of the rapid rain of blows, he flees, leaving the victor free to inspect the cows. Like many animals, including cats and horses, male giraffes can tell a female is in heat by testing her urine.