In an interdisciplinary perspective, aggression is regarded as “an ensemble of mechanism formed during the course of evolution in order to assert oneself, relatives or friends against others, to gain or to defend resources (ultimate causes) by harmful damaging means [...] These mechanisms are often motivated by emotions like fear, frustration, anger, feelings of stress, dominance or pleasure (proximate causes) [...] Sometimes aggressive behavior serves as a stress relief or a subjective feeling of power." Predatory or defensive behavior between members of different species may not be considered aggression in the same sense.