The purpose of aggressiveness differs depending on the opponent at which it is directed. Aggression against other species is action for the purpose of obtaining food, while aggression against the same species is action aimed at suppressing prolonged and escalating competition over resources.
Therefore, if there is no competition, there is no need for aggression within species, and such aggression may have a role in coordinating social relationships within a group.
■References (Books, papers, Web articles, etc.)
How Humans Evolved (Eighth Edition) Robert Boyd, Joan B. Silk W. W. Norton & Company; Eighth edition (December 1, 2017)
『暴力はどこからきたか——人間性の起源を探る』 第1章 攻撃性をめぐる神話 山極寿一（NHKブックス、2007）
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