Correlates of hormonal modulation in mantled howler monkey males, Alouatta palliata

"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

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.

Linking energy availability, movement and sociality in a wild primate (Papio ursinus)

Proximate mechanisms of 'social ageing', i.e. shifts in social activity and narrowing of social networks, are understudied. It is proposed that energetic deficiencies (which are often seen in older individuals) may restrict movement and, in turn, sociality, but empirical tests of these intermediary mechanisms are lacking. Here, we study wild chacma baboons (Papio ursinus), combining measures of faecal triiodothyronine (fT3), a non-invasive proxy for energy availability, high-resolution GPS data (movement and social proximity) and accelerometry (social grooming durations). Higher (individual mean-centred) fT3 was associated with increased residency time (i.e. remaining in the same area longer), which, in turn, was positively related to social opportunities (i.e. close physical proximity). Individuals with more frequent social opportunities received more grooming, whereas for grooming given, fT3 moderated this effect, suggesting an energetic cost of giving grooming. While our results support the spirit of the energetic deficiencies hypothesis, the directionality of the relationship between energy availability and movement is unexpected and suggests that lower-energy individuals may use strategies to reduce the costs of intermittent locomotion. Thus, future work should consider whether age-related declines in sociality may be a by-product of a strategy to conserve energy.This article is part of the discussion meeting issue 'Understanding age and society using natural populations'.

Mantled howler monkey males assess their rivals through formant spacing of long-distance calls

Formant frequency spacing of long-distance vocalizations is allometrically related to body size and could represent an honest signal of fighting potential. There is, however, only limited evidence that primates use formant spacing to assess the competitive potential of rivals during interactions with extragroup males, a risky context. We hypothesized that if formant spacing of long-distance calls is inversely related to the fighting potential of male mantled howler monkeys (Alouatta palliata), then males should: (1) be more likely and (2) faster to display vocal responses to calling rivals; (3) be more likely and (4) faster to approach calling rivals; and have higher fecal (5) glucocorticoid and (6) testosterone metabolite concentrations in response to rivals calling at intermediate and high formant spacing than to those with low formant spacing. We studied the behavioral responses of 11 adult males to playback experiments of long-distance calls from unknown individuals with low (i.e., emulating large individuals), intermediate, and high (i.e., small individuals) formant spacing (n = 36 experiments). We assayed fecal glucocorticoid and testosterone metabolite concentrations (n = 174). Playbacks always elicited vocal responses, but males responded quicker to intermediate than to low formant spacing playbacks. Low formant spacing calls were less likely to elicit approaches whereas high formant spacing calls resulted in quicker approaches. Males showed stronger hormonal responses to low than to both intermediate and high formant spacing calls. It is possible that males do not escalate conflicts with rivals with low formant spacing calls if these are perceived as large, and against whom winning probabilities should decrease and confrontation costs increase; but are willing to escalate conflicts with rivals of high formant spacing. Formant spacing may therefore be an important signal for rival assessment in this species.

Noise intensity modulates the responses of mantled howler monkeys to anthropophony

Anthropogenic noise is a major global pollutant but its effects on primates are poorly understood, limiting our ability to develop mitigation actions that favor their welfare and conservation. In this study, we used an experimental approach to determine the impact of variation in noise intensity on mantled howler monkeys (Alouatta palliata). We conducted the study at Los Tuxtlas (México), where we studied the physiological stress (proxied via fecal glucocorticoid metabolites, fGCM) and behavioral responses of 16 males. We played back chainsaw noise at two intensities (40 and 80 dB) and used days in which groups were not exposed to noise as matched controls. With increased noise intensity fGCM increased, vigilance and vocalizations were longer, and vigilance, vocalizations, and flight occurred quicker. Physiological and behavioral responses occurred even after low-intensity noise playbacks (i.e., 40 dB). Therefore, noise intensity is a significant factor explaining the responses of mantled howler monkeys to anthropogenic noise. These results imply that management actions aimed at eradicating anthropogenic noise are required for the conservation and welfare of mantled howler monkeys at Los Tuxtlas.