Fighting off rivals may be responsible for masculine traits

Almost all of the traits considered to be masculine such as big muscles, facial hair, square jaws, deep voices and a propensity to violence seem to have evolved specifically for their usefulness in fighting off or intimidating other men, allowing the winner to get the girl. On average, men have 40% more fat-free mass than women, which is similar to the difference in gorillas, a species in which males unquestionably compete with other males for exclusive sexual access to females. In species whose males do not fight for access to females, males are generally the same size as, or smaller than, females.