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

Abstract

Many animal species respond less aggressively to calls emitted by neighbors in comparison with strangers, an asymmetryknown as the “dear enemy” effect. The adaptive significance of having “dear enemies” would be to minimize defensivecosts towards less-threatening individuals (like neighbors). The opposite situation, in which known neighbors becomeuntrustworthy, representing an even greater menace than strangers, is call “nasty neighbor effect”. In addition to theseneighbor-stranger discrimination abilities, some species are also capable of recognizing neighbors individually, allowingthem 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 individuallyand investigated the nature of long-term relationships under the dear enemy/nasty neighbors hypotheses. We conducted36 playback experiments on four dominant males in “El Cachapé” reserve, in Argentina. We exposed each male to threedifferent treatments, consisting of roars from: 1- Neighbors from the area of home range overlap, 2- Misplaced neighborsfrom the opposite side to the area of home range overlap, and 3- Strangers, quantifying eight response variables duringeach experiment. Our results showed that dominant males recognize neighbors individually (by roaring longer in responseto misplaced neighbors), clearly reacting more aggressively to neighbors who violate mutual agreements (like home rangeboundaries). 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 thatneighbor vocal recognition is key to understanding the configuration of areas of collective use and navigation decision inprimates and that strangers exert the major threat to group stability in howler monkeys.