In a sense, males compete with rival males regardless of whether sexual selection proceeds through female choice or male competition and, in many respects, the process—the intensity, consistency, and social nature of selection—is the same regardless. Why, then, do some species embark on a trajectory of overt competition leading to the evolution of weapons, while others end up dancing or singing with displays? Here is where the ingredients for arms races come into play, for these are the pieces that must fall into place if sexual selection is to trigger evolution of extreme weapons. The first ingredient is competition, the essence of all sexual selection. The second is economic defensibility.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 191