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.

Dog barks influence the physiological stress and behavior of a wild primate.

Non-lethal impacts of dogs on primates have seldom been assessed. We used an experimental approach to determine if mantled howler monkeys (Alouatta palliata) perceive dog barks as an aversive stimulus and thus display physiological and behavioral responses toward simulated barks. For one year (1754 h of observations) we studied 16 adult males belonging to five groups in Los Tuxtlas (Mexico), and recorded the occurrence of naturally occurring dog barks, their sound pressure level (SPL), and the behavioral responses of howler monkeys to barks. We then exposed males to bark playbacks at two SPL treatments, 40 and 80 dB in a total of 50 experiments. We assayed glucocorticoid metabolite concentrations in fecal samples (fGCM) as a marker of the physiological stress response of males. We also recorded the duration of vigilance, vocalizations, and flight in relation to playbacks. Naturally occurring barks were frequent and usually elicited behavioral responses by males. fGCM concentrations increased after bark playbacks and with stimuli intensity. Time spent vigilant increased following playbacks independently of stimuli intensity but both vocalizations and flight were linked to stimuli intensity: vocalizations were the longest after barks played-back at 80 dB, but males spent more time fleeing in response to 40 dB bark playbacks. These results provide evidence that dog barks are pervasive in the habitat of mantled howler monkeys living at Los Tuxtlas and disturb males, both physiologically and behaviorally. Although the potential costs of physiological and behavioral responses could not be determined, there is sufficient evidence to assume that they do have negative impacts on individuals. Therefore, our study provides avenues for future research on dog-wildlife interactions and valuable information for the design of conservation actions aimed at mitigating the impact of dogs on mantled howler monkeys.