The influence of anthropogenic noise on the behavior of male mantled howler monkeys.

Anthropogenic noise is increasingly disturbing natural soundscapes and affecting the physiology, behavior, and fitness of wildlife. However, our knowledge about the impact of anthropogenic noise on wild primates is scant. Here, we assess the effects of anthropogenic noise on the behavior of male mantled howler monkeys (Alouatta palliata). Specifically, we describe the types, rates, and sound pressure level (SPL) of anthropogenic noise that occurs in areas inhabited by mantled howler monkeys and determine if the behavioral responses of males to anthropogenic noise are influenced by noise attributes. For 1 year (1753 h), we characterized anthropogenic noise in the Los Tuxtlas Biosphere Reserve (Veracruz, Mexico) and studied the behavior of males belonging to five groups. Anthropogenic noise was common, diverse, and varied among areas in terms of rate, type, and SPL. Males did not display behavioral responses toward most (60%) anthropogenic noises, but were more likely to respond to certain noise types (e.g., aerial traffic) and toward noise with high SPL. Group identity influenced the likelihood of displaying behavioral responses to noise. The most common behavioral responses were vocalizations and vigilance. Males vocalized in response to noise with high SPL, although this relationship depended on group identity. The effect of the number of noises on vocalizations also varied among groups. Males were more likely to display vigilance toward high SPL and infrequent noise, but, again, these relationships varied among groups. In sum, anthropogenic noise is pervasive in areas inhabited by mantled howler monkeys and influences male behavior. Experience and frequency of exposure may modulate the behavioral responses of male mantled howler monkeys to noise and explain the group differences.

The effect of energetic and psychosocial stressors on glucocorticoids in mantled howler monkeys (Alouatta palliata).

The proximate causes of variation in glucocorticoids (GCs) of many free-ranging primates are still unclear, and in some cases, the available evidence is contradictory. Such is the case of mantled howler monkeys. In the present study, we tested whether variation in GC levels in this species could be predicted by energetic challenges or by psychosocial stressors. We focused on two groups living in Los Tuxtlas (Veracruz, Mexico) that differed in a number of parameters including: group size, habitat size, number of groups, and solitary males within the same habitat. Furthermore, one of the groups experienced changes in composition during our observations. From March to December 2009 we determined food availability in each group's habitat, studied the behavior of all adult individuals (N = 17), including, feeding, time budgets, ranging, and social interactions (N = 426.6 h), and measured weekly GCs in fecal samples (N = 160 individual/weeks) of both females and males. We found that participation in agonistic interactions, which were more frequent in the group that lived in the smaller habitat, was associated with increased weekly GCs, particularly in pregnant and lactating females. During the dry season weekly GCs were also higher in the group that lived in the smaller habitat. Although in this group individuals significantly increased travel time during the dry season, weekly GC levels were unrelated to time-budgets or ranging distances, contrasting with previous findings on mantled howler monkeys' GC response. We found no evidence that weekly variation in GC levels between groups resulted from differences in food availability. Our results indicate that mantled howler monkey GC levels respond to the effects of agonism, reproductive state, and the influence of a seasonal stressor, which may be attributable to anthropogenic disturbance. We conclude that psychosocial stressors affect the GC response of mantled howler monkeys, and that this response is modulated by reproductive state.