Aggressive behaviors in animals, for example, threat, attack, and defense, arecommonly related to competition over resources, competition over matingopportunities, or fights for survival. In this chapter, we focus on aggressivecompetition over mating opportunities, since this competition explains muchof the distribution of weaponry and large body size, but also because this type ofcompetition sheds light on the sex skew in the use of violence in mammals,including humans. Darwin (1871) termed this type of natural selection, wheredifferences in reproductive success are caused by competition over mates, sexualselection. 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 effectson weaponry and body size such that males commonly bear more vicious weaponsand are larger than females. A review of sexual selection in mammals reveals howcommon 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 alsocommon in primates, where it has left clear historical imprints in body massdifferences, 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 thehominid lineage. Nevertheless, our species displays dimorphism, polygyny, andsex-specific use of violence typical of a sexually selected mammal. Understandingthe biological background of aggressive behaviors is fundamental to understandinghuman aggression.
San Diego: Academic Press, 2011. 7-22 p.