In armor we see all of the processes that matter for the evolution of extreme weapons: individuals vary in the extent of their armament; these differences in weapon size affect the performance of their bearers (survival, growth, and reproduction in sticklebacks, and survival in soldiers); and, as a result, the sizes and shapes of these weapons evolve rapidly and dramatically over time. Weapons such as armor come at a price, and sometimes, when this cost is high enough, individuals with small weapons fare better than those with big ones. Indeed, most of the time, for most weapons, bigger is not better.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 63