Where one sex (usually males) competes physically for the other sex, there is selection pressure for large size, musculature, weaponry like antlers and horns, territoriality, and aggression. The winners of these contests reap huge rewards in terms of mating (Bateman's principle), which is why sexual dimorphism is associated with polygyny. In monogamous species, there is much less incentive to invest energy into intrasexual competition.