“Dear Enemies”, “Nasty Neighbors”, and the Strength in Numbers: Exploring the Behavioral and Hormonal Responses of Mantled Howler Monkey Males to Simulated Intruder Loud Calls

Abstract

Loud calls are critical for primate intergroup communication, influencing resource defense, territorial boundaries, and conflict management. This study examined the interplay of familiarity and numeric odds in shaping the behavioral and hormonal responses of male mantled howler monkeys (Alouatta palliata) to simulated intruder calls. Using playback experiments, we tested whether responses were consistent with “dear enemy” dynamics, which predict stronger responses to unfamiliar intruders, or “nasty neighbor” dynamics, which predict stronger responses to familiar intruders. Additionally, we assessed the influence of numeric odds, hypothesizing stronger responses under favorable conditions. Behavioral responses, including vocalization and approach latencies, were primarily influenced by numeric odds, with shorter latencies observed under favorable odds. Vocal duration, however, was longer when intruders were familiar, consistent with “nasty neighbor” dynamics. Hormonal responses, measured via fecal glucocorticoid and testosterone metabolites, increased in response to unfamiliar intruders, supporting “dear enemy” predictions. Interaction effects showed that familiarity modulated approach duration and latency when numeric odds were unfavorable. These findings reveal the distinct roles of numeric odds and familiarity in shaping immediate and prolonged responses to intruders. Numeric odds influenced engagement decisions, while familiarity was related to stress-related hormonal changes and vocal communication strategies. This study contributes to the understanding of intergroup dynamics by clarifying the role of competitive asymmetries and social relationships in conflict management. Future work incorporating multimodal cues and ecological variability will further elucidate the adaptive significance of these responses.