Jealous Behavior in Chimpanzees Elicited by Social Intruders.

Despite increasing interest in animal emotions, jealousy has rarely been directly addressed in comparative research, except for studies of human-pet interactions. Jealous behavior emerges when a valuable social bond is threatened by a third-party, prompting aggression or intervention attempts to direct the partner's attention away from the rival. Emotional reactions that protect relationships are expected in species in which social relationships are important for fitness, including primates. Previous primate studies have alluded to this ultimate function, but never explicitly tested predictions corresponding to a proximate jealousy mechanism. We demonstrate jealous behavior in a long-established colony of chimpanzees ( = 17) during a socially disruptive period due to group introductions, which provided an ideal experimental opportunity to test predictions of a jealousy hypothesis. Specifically, we found that negative reactions (agonism and intervention attempts) towards social closeness between two groupmates were generally more common when the aggressor/intervener had a valuable relationship to one (as compared with both or neither) of the dyad's members, indicating that the other partner represented a potential social rival. In line with this suggestion, we found that negative reactions most often targeted dyads containing newly introduced individuals, especially when the social conditions for jealousy were met, and in particular during the socially unstable introduction period. Results underscore the potential adaptive role of jealousy in protecting fitness-enhancing relationships from social interlopers, by extension indicating that this emotion likely evolved in diverse animal societies.