Evolutionary aspects of aggression the importance of sexual selection

Aggressive behaviors in animals, for example, threat, attack, and defense, are commonly related to competition over resources, competition over mating opportunities, or fights for survival. In this chapter, we focus on aggressive competition over mating opportunities, since this competition explains much of the distribution of weaponry and large body size, but also because this type of competition sheds light on the sex skew in the use of violence in mammals, including humans. Darwin (1871) termed this type of natural selection, where differences in reproductive success are caused by competition over mates, sexual selection. Not all species have a pronounced competition over mates, however. Instead, this aspect of sociality is ultimately determined by ecological factors. In species where competition over mates is rampant, this has evolutionary effects on weaponry and body size such that males commonly bear more vicious weapons and are larger than females. A review of sexual selection in mammals reveals how common aggressive competition over mating opportunities is in this group. Nearly half of all mammal species exhibit male-biased sexual size dimorphism, a pattern that is clearly linked to sexual selection. Sexual selection is also common in primates, where it has left clear historical imprints in body mass differences, in weaponry differences (canines), and also in brain structure differences. However, when comparing humans to our closest living primate relatives, it is clear that the degree of male sexual competition has decreased in the hominid lineage. Nevertheless, our species displays dimorphism, polygyny, and sex-specific use of violence typical of a sexually selected mammal. Understanding the biological background of aggressive behaviors is fundamental to understanding human aggression.