Individual vocal recognition and dear enemy effect in the black-and-gold howler monkey (Alouatta caraya)

Many animal species respond less aggressively to calls emitted by neighbors in comparison with strangers, an asymmetry known as the “dear enemy” effect. The adaptive significance of having “dear enemies” would be to minimize defensive costs towards less-threatening individuals (like neighbors). The opposite situation, in which known neighbors become untrustworthy, representing an even greater menace than strangers, is call “nasty neighbor effect”. In addition to these neighbor-stranger discrimination abilities, some species are also capable of recognizing neighbors individually, allowing them to avoid risky encounters based on identity and past experiences, minimizing the probability of losing an encounter. In this study we tested if black-and-gold howler monkey (Alouatta caraya) males can recognize neighbors individually and investigated the nature of long-term relationships under the dear enemy/nasty neighbors hypotheses. We conducted 36 playback experiments on four dominant males in “El Cachapé” reserve, in Argentina. We exposed each male to three different treatments, consisting of roars from: 1- Neighbors from the area of home range overlap, 2- Misplaced neighbors from the opposite side to the area of home range overlap, and 3- Strangers, quantifying eight response variables during each experiment. Our results showed that dominant males recognize neighbors individually (by roaring longer in response to misplaced neighbors), clearly reacting more aggressively to neighbors who violate mutual agreements (like home range boundaries). Also, dominant males displayed a longer roar duration and closer approach to the sound source when hearing roars from strangers, supporting the hypothesis that neighbors are dear enemies in this species. Our results show that neighbor vocal recognition is key to understanding the configuration of areas of collective use and navigation decision in primates and that strangers exert the major threat to group stability in howler monkeys.