Evolutionary forces at work

WHEN two drunken men fight over a woman, alcohol and stupidity may not be the only things at work. Sadly, evolution may have shaped men to behave this way. Almost all of the traits considered to be masculine—big muscles, facial hair, square jaws, deep voices and a propensity to violence—evolved, it now seems, specifically for their usefulness in fighting off or intimidating other men, allowing the winner to get the girl.

That, at least, is the contention of David Puts, an anthropologist at Pennsylvania State University, in an upcoming paper in Evolution and Human Behavior. Dr Puts is looking at how sexual selection gave rise to certain human traits. A trait is sexually selected if it evolved specifically to enhance mating success. They come in two main forms: weapons, such as an elk's horns are used to fight off competitors; and ornaments, like a peacock's tail, which are used to advertise genetic fitness to attract the opposite sex.

Researchers have tended to consider human sexual selection through the lens of the female's choice of her mate. But human males look a lot more like animals designed to battle with one another for access to females, says Dr Puts. 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.

The heavier brow and jaw of males might have developed to withstand blows from other males. (In prehistoric times, a broken jaw could have ended up being a fatal injury.) Heavy eyebrows, facial hair and deep voices all could serve to make a man more imposing to other men. Dr Puts does not dismiss mate selection outright. Women are attracted to some of the same traits that are good for dominating other men because they signal that the man will sire sons who will also be successful at mating. But female choice probably is not the primary cause of the traits.

It is a disturbing idea to modern minds, harking back to old stereotypes about violent cavemen battling with clubs while a passive woman, fetching in furs, waits helplessly to see who will win her. But Dr Puts emphasises that evolutionary biology is not destiny. Regardless of our evolutionary past, in modern societies men and women freely choose their mates. However, understanding the evolutionary pressures that made men the way they are could help us better understand male violence, including murder, domestic abuse, gang violence and perhaps even warfare.