The skull of an African lion, in an undated handout image. Creatures that do violent things to their heads, like rams and woodpeckers among them, have very dense skulls, small brains and little fluid, all of which help reduce the chance of damage.
The brains of most animals that are prone to head banging -- these include deer and other antlered mammals, as well as various birds -- are relatively small; and they're bathed in only small amounts of cerebrospinal fluid, leaving little room for the brain to move and be shocked by the sudden decelerations and accelerations of their weaponized heads. ... Modifications of the skull are many, all produced by evolution to give each animal maximum advantage in adapting to its environment. ... Some skulls are narrow and delicate -- the gazelle's, say -- while others, like that of the lion, are squat and fat and powerful. Invariably the melding of form and function is displayed perfectly in an animal's skull -- so that one can quite easily deduce the manner in which the animal behaves, or the environment in which it lives, by examining ... the skull it leaves behind.
From Simon Winchester's "Skulls: An Exploration of Alan Dudley's Curious Collection."