Abstract Social networks formed via play interactions offer a powerful framework for investigating the social dynamics of animals and the functions of play behavior. Using this framework in mantled howler monkeys ( Alouatta palliata ), we examined whether social play might fulfil some of the affiliative and bonding functions typically addressed to allogrooming. We analyzed play networks in seven groups of mantled howler monkeys (89 individuals), analyzing group and individual-level patterns based on 1774 observation hours collected over approximately 3 years. We examined the relationship between group size and network complexity using edge density (the proportion of connections), modularity (the degree to which the network is subdivided into clusters), and average path lengths (the number of steps required to reach others in the network). We analyzed centrality across age-sex classes using betweenness, closeness, and eigenvector metrics. We tested which centrality measure best predicted play evenness, defined as the degree to which individuals distributed their playtime across partners. We found that network interconnectivity generally declined with group size, though average path lengths were not significantly different from those of random networks. Immatures showed the highest centrality values, while adult females exhibited the highest play evenness. However, centrality measures did not significantly predict play evenness. These patterns suggest age-specific functions of social play: immatures may benefit from frequent play to support development, while adult females may use evenly distributed play to navigate social competition. Our findings support the hypothesis that social play serves similar functions to allogrooming, contributing to social bonding.

ABSTRACT Animals exhibit varying degrees of behavioral tolerance to anthropogenic noise. However, our understanding of how different species tolerate noise, and the specific behavioral consequences of noise exposure remains limited. We hypothesized that mantled howler monkeys ( Alouatta palliata ) show differential tolerance (i.e., likelihood to change their behavior) to different noise traits. We observed 18 adult subjects from two groups at Los Tuxtlas, Mexico, for 606 focal samples, recording 1488 noise events. Assessed noise traits were the number of noise occurrences, sound pressure level ( SPL , measured with a sound meter), noise duration, and proximity to noise. The number of noises and SPL significantly increased the probability of activity change. Behavioral transition analysis revealed non‐random patterns, with noise mostly disrupting rest‐to‐feed sequences while increasing move‐to‐feed transitions. The number of noises had the most consistent effects across behaviors, altering the activity budget for feeding, resting, locomotion, and vigilance. As the number of noises increased, feeding time rose from 9% to 14%, locomotion from 7% to 14%, and vigilance more than tripled from 2% to 7%, while resting declined from 80% to 63%. Increased noise duration resulted in decreased feeding (from 12% to 4%) but increased locomotion (from 7% to 15%). Thus, mantled howler monkeys exhibit a complex, trait‐specific tolerance pattern to noise, showing greater sensitivity to the number of noises and noise intensity while potentially developing higher tolerance to duration and distance. These findings highlight the importance of implementing noise mitigation strategies in areas where wildlife interfaces with humans.

Acoustic correlates of physiological stress in a wild primate.

Abstract Primates exhibit thermal behaviors (use of postures in tree stratum) that help regulate body temperature in response to environmental conditions, reducing the energetic and water costs of thermoregulation. This study examined the relationship between ambient temperature and thermal behavioral strategies during resting periods in mantled howler monkeys (Alouatta palliata) across three sites in Veracruz, Mexico: Agaltepec (AGA; 28.4 ± 4.7 °C), Mirador Pilapa (MP; 28.8 ± 2.1 °C), and Zapoapan (ZAP; 28.9 ± 2.5 °C). We used focal-animal sampling, randomly selecting individuals and recording the time invested in each posture and the tree stratum in which they rested, and the ambient temperature. All statistical analyses were performed independently between sites. The results indicate that, across the three study sites, a consistent pattern of posture adopted within specific tree strata was observed, despite differences in altitude, vegetation, and ambient temperature. We found a strong positive correlation between lower temperatures and the time recorded in the semi-fetal posture in the high strata (Rc>0.7547), and between higher temperatures and the time recorded in the extended posture in the low strata (Rc>0.6803). These results suggest howler monkeys combine posture and vertical positioning to optimize thermoregulation. However, as temperatures rise, the preference for extended postures in lower strata—where temperatures may be cooler—leads to longer rest periods. This behavioral shift could reduce feeding and hydration, potentially increasing the risk of dehydration or heat stress under future climate conditions.

Predation risk fundamentally shapes primate social structure by influencing cooperation, competition, and spatial organization within groups. Although domestic dogs (Canis lupus familiaris) represent one of the world's most impactful invasive species affecting wildlife, their influence on primate social structure remains largely understudied. We examined how dog presence and barks influence interindividual proximity among group members in mantled howler monkeys (Alouatta palliata). We studied two habituated groups (10 and 8 adults, respectively) in Veracruz, Mexico, combining 242h of observational data with experimental playbacks of dog barks at different intensities (40 and 80dB). We calculated proximity indices from instantaneous sampling of interindividual proximity. Howler monkeys were closer immediately following exposure to dogs, partially supporting our hypothesis that interindividual proximity would change in response to dogs. Contrary to predictions, individuals were farther from each other as bark intensity increased and as proximity to dog stimuli decreased. During longer timeframes, individuals were closer when exposed to low-intensity barks compared to periods without dog stimuli. These findings demonstrate that dogs influence howler monkey spatial organization through context-dependent effects that vary with stimulus characteristics and temporal scale. Despite the short-term nature of this study, the consistency between observational and experimental results suggests these patterns are reliable. Therefore, these results contribute to our understanding of how invasive predators affect primate social structure and highlight the importance of managing free-ranging dogs in areas where they overlap with wildlife populations.

Transitional meningioma in a free-ranging mantled howler monkey (Alouatta palliata) from Costa Rica

Background: The Manuel Antonio National Park (MANP) and its surrounding areas in Costa Rica are home to endangered non-human primates (NHPs), including Saimiri oerstedii citrinellus and Cebus imitator, that coexist in close contact with humans and domestic animals. Such interactions may facilitate the transmission of vector-borne zoonotic diseases. This study aimed to investigate the presence of selected vector-transmitted pathogens of zoonotic relevance in NHPs from this region. Methods: Between 2021 and 2022, 40 free-ranging NHPs (19 C. imitator, 19 S. oerstedii citrinellus, 2 Alouatta palliata) were sampled in MANP and surrounding areas. Blood samples were tested for Trypanosoma spp., Plasmodium spp., Mycoplasma spp., and arboviruses including dengue virus (DENV), chikungunya virus (CHIKV), Zika virus (ZIKV), Venezuelan equine encephalitis virus (VEEV), and West Nile virus (WNV). Molecular analyses were conducted using PCR and RT-qPCR protocols, with sequencing for species confirmation. Results: Eight of 40 NHPs (20.0%) tested positive for at least one pathogen. Trypanosoma minasense was confirmed by sequencing in three C. imitator (16.7%). Plasmodium spp. was detected in one C. imitator and one A. palliata (5.0%), though species-level identification was unsuccessful. One C. imitator tested positive for CHIKV (Ct 36.8), representing the first PCR-confirmed case of CHIKV in NHPs in Central America. Two C. imitator were positive for Mycoplasma spp. (11.1%). No co-infections were observed. All animals were negative for DENV, ZIKV, VEEV, and WNV. Conclusions: This study provides the first molecular evidence of CHIKV infection in NHPs in Costa Rica and highlights the circulation of Trypanosoma spp., Plasmodium spp., and Mycoplasma spp. in wild primates from MANP. These findings emphasize the need for a One Health approach to surveillance, particularly in regions where human-wildlife interactions are frequent. Longitudinal and serological studies are recommended to assess the potential reservoir role of NHPs and their implications for public health and conservation.

Howler monkeys are considered high-quality seed dispersers; however, the changes that seeds undergo after passing through their digestive tract, which influence their germination capacity, are still unknown. In this study, we assessed whether the consumption of seeds from three species of the Moraceae family by mantled howler monkeys results in seed coat scarification, and whether this treatment enhances their permeability to water and germination rate. Using seeds from fruits, we induced chemical scarification of their outer coat, and with these, intact seeds and seeds found in droppings, we conducted micromorphological analyses, water permeability tests and germination assays. We observed that the passage of seeds through the digestive tract of mantled howler monkeys did not scarify their outer coat, nor did it enhance their water absorption capacity. There was both a positive and negative effect on the germination of Ficus americana and Ficus insipida seeds, respectively. Chemical scarification of the seeds, although it reduced their coat thickness, did not increase water permeability, significantly decreased their viability, and prevented germination. These results suggest that, in the case of small seeds, scarification is not a relevant treatment for germination and may, in fact, be unfavorable. Therefore, the importance of seed consumption by mantled howler monkeys, and other frugivores, may lie in the removal of pulp in terms of the facilitation of germination. Thus, by moving, releasing and cleaning the seeds, these animals may increase the probability that they will encounter adequate environmental conditions for germination and establishment.

Howler monkeys are well-known for producing long calls ("roars") as a means of long-distance communication. The ecological resource defense hypothesis posits that animals roar to defend food resources; thus monkeys should modify roaring behavior across seasons and between sites with varying food resource abundance. We investigated mantled howler monkey roaring behavior between the wet and dry seasons at a large, continuous lowland wet forest (La Selva Research Station) in northeastern Costa Rica as well as between La Selva and a nearby small forest fragment (La Suerte Biological Research Station, LSBRS). We quantified a proxy of resource abundance at each site by measuring tree diameter at breast height (DBH) and canopy cover and used all-occurrences sampling to collect data on roaring behavior. We calculated number of roars per minute, length of roar bouts, and roar bouts per hour. Number of roars per minute was significantly higher at La Selva during the wet season than the dry season. With respect to comparisons across sites, both canopy cover and DBH were significantly higher at La Selva. Correspondingly, the length of roar bouts and the number of roars per minute were significantly higher at La Selva. Roar bouts per hour, however, were significantly higher at LSBRS, which may be explained by the very high population density at LSBRS. The results for our comparison demonstrate that some features of howler monkey roaring behavior vary along with ecological conditions and that other non-ecological factors may also drive elements of roaring.

ABSTRACTTropical dry forests are among the most threatened tropical ecosystems due to ongoing agricultural expansion and development. Despite this, small forest patches persist within fragmented landscapes, though their conservation value remains poorly understood. To evaluate the role of one such patch in supporting wildlife, we deployed camera traps across three zones (Dense Forest, Grassland, and Sparse Forest) within the Murren Reserve, a small dry forest patch on the southern coastline of Guanacaste, Costa Rica. Camera traps detected 16 vertebrate species, with opportunistic encounters adding 16 more. The most detected species included the white‐nosed coati (Nasua narica), raccoon (Procyon lotor), and white‐tailed deer (Odocoileus virginianus). Additionally, we recorded nationally endangered species, including the ocelot (Leopardus pardalis), puma (Puma concolor), and mantled howler monkey (Alouatta palliata), as well as nationally threatened species such as the spotted skunk (Spilogale angustifrons) and white‐throated magpie‐jay (Calocitta formosa). Extrapolated rarefaction curves suggest that further sampling could improve species richness estimates for the Dense Forest and Grassland. Although we did not detect a difference in species composition among zones (ANOSIM, R = 0.14, p = 0.26), a generalized linear mixed model indicated that zone explained 17% of the variation in species richness. These findings highlight the role that small dry forest patches play in conserving both threatened species and broader vertebrate communities. We also provide recommendations for future research to address current data gaps and improve long‐term monitoring in fragmented landscapes.

Mantled howler monkeys Alouatta palliata display a daily behavior pattern, but the time they rest can vary with the use of thermal behaviors to prevent endogenous heat production, maintain body temperature, and reduce the need to consume water or energy. The primary objective of this exploratory study was to determine the effects of fluctuations in temperature and relative humidity and behavioral thermoregula¬tion (body postures and tree stratum selection) on the resting time of A. palliata introduced in Agaltepec Island, Veracruz, Mexico. Focal animal sampling was carried out between October 2018 and August 2019, collecting 384 hours of fieldwork (observation/data recording) with 628 thermal behavioral records. We used a generalized linear mixed-effects model (GLMM) to observe the possible influence of the weather variables and behavioral thermoregulation on the rest time. The results showed differences in the temperature recorded in each posture and tree strata but according to the statistical models, the adoption of postures better explained (percentage of variance: 48.21 %) the time A. palliata rests. At higher ambient temperatures and lower humidities, the monkeys adopted an ex¬tended posture in the lower tree stratum and increased their resting time. These warm conditions likely pose a significant thermoregulatory challenge for howler monkeys, promoting the use of specific thermal behaviors that may affect resting times and potentially time allocated to other behaviors, such as locomotion and feeding. These changes in their behavior could compromise their survival if the temperature continues to rise and the relative humidity decreases due to the effect of climate change. Key words: Temperature, Thermoregulation, Body postures, Arboreal stratum, Daily behavior

Preliminary Evidence of Maternal Care Styles in Mantled Howler Monkeys (Alouatta palliata)

Genetic diversity is of prime importance for biological diversity and for the long-term conservation and survival of species and populations. The conservation of hybrid individuals originating from natural hybridization is still controversial; nearly every hybrid system is different and general conservation rules are not effective. The black howler monkey (Alouatta pigra) is and endangered primate endemic to the Yucatán peninsula. It hybridizes with the mantled howler, Alouatta palliata, in a narrow contact zone located in southern Mexico. We evaluated the genetic diversity and structure patterns in natural populations of A. pigra across its distribution range in Mexico (16 localities), including the hybrid populations, and estimated the overall time of divergence of the hybrid individuals. We successfully genotyped 12 microsatellite loci and obtained a 587 bp cytochrome-b fragment alignment for 222 and 107 individuals, respectively. We identified 33 hybrids, three A. palliata and 186 non-admixed A. pigra. Our results showed the highest diversification for A. pigra in Mexico occurring at ~1.4 My, while identifying a more recent diversification event including A. palliata and hybrids (~0.56 My), supporting different diversification events along these primates’ evolutionary history. Also, divergence of hybrid individuals is not a contemporary process in the evolutionary history of these sister species, suggesting they came into contact at the zone of sympatry at least some ~0.8 My ago. Hybrids exhibited higher mitochondrial diversity in comparison with non-admixed individuals; notably, hybrids with A. pigra mitochondria exhibited both higher diversity values and number of unique haplotypes than those with A. palliata mitochondria. Comparatively, hybrid nuclear genetic diversity was higher compared with A. palliata but not with A. pigra. Regarding the A. pigra populations, we found moderate diversity values, being highest in the most conserved and continuous habitat. We also found significant genetic differentiation (five genetic clusters) and limited gene flow between them, and potential inbreeding signals, which render A. pigra prone to drift effects and increased extinction risk. Our study provides novel information regarding the historical and contemporary genetic patterns of black howler monkey and hybrid populations in Mexico, which is key for developing strategies for conserving and ensuring the survival of viable primate populations.

La Alouatta palliata, conocida comúnmente como el mono aullador, esta especie hábitat en los bosques húmedos tropicales y secos, presentando mayor registro en la región costera del Ecuador. Sin embargo, su hábitat está siendo cada vez más amenazado por la deforestación, la expansión agrícola y la urbanización, lo que ha provocado la fragmentación de los ecosistemas en los que vive. Teniendo como objetivo determinar la distribución potencial del hábitat de Alouatta palliata en Ecuador, identificando las áreas clave para su conservación mediante herramientas como MaxEnt. Para la obtención los datos de presencia de la especie se utilizó la plataforma Global Biodiversity Information Facility (GBIF), en cuanto al procedimiento de lo datos y mapas fue por medio los sistemas de información geográfica ArcGIS. En cuanto al modelo distribución potencial y análisis de la información se realizó en MaxEnt. De este modo se identificó las áreas más adecuadas para la especie Alouatta palliatta en las provincias del Ecuador como son Esmeraldas, Guayas, Manabí, Santa Elena y Azuay. En conclusión, este análisis de la distribución potencial es crucial para su identificación de áreas prioritarias para la especie en donde se podrá desarrollar estrategias de conservación que aseguren la factibilidad de la especie en el futuro.

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.

Mantled howler monkeys (Alouatta palliata) modify activity and spatial cohesion in response to seasonality

Diversity of the gut microbiota has proven to be related with host physiology, health and behavior, influencing host ecology and evolution. Gut microbial community relationships often recapitulate primate phylogeny, suggesting phylosymbiotic associations. Howler monkeys (Alouatta) have been a model for the study of host-gut microbiota relationships, showing the influence of different host related and environmental factors. Differences in life-history traits and feeding behavior with other atelids, like spider monkeys, may reveal distinct patterns of bacterial gut communities, yet few wild populations have been studied; likewise, gut microbiota studies of hybrid populations are mostly lacking. We analyzed diversity and abundance patterns of the gut microbiota of wild populations of the three Mexican primates Ateles geoffroyi, Alouatta palliata and A. pigra from different regions across its distribution in the country, including sympatric localities and the Alouatta hybrid zone. Interspecific differences in gut microbial diversity were higher than intraspecific differences, concordant with phylosymbiosis. Ateles harbored the more differentiated diversity with a major presence of rare taxa, while differences were less strong between Alouatta species. Hybrids had a microbial diversity in-between their parental species, yet also showing unique microbe taxa. Genetic distances between Alouatta individuals correlated positively with their gut microbial dissimilarities. Results show that interspecific and intraspecific overall diversity, abundance and composition patterns are affected by environment, geographic distribution and host genetics. Our study provides the first comprehensive study of gut microbiota of the three Mexican primates and hybrid populations.

The golden-mantled howler monkey (Alouatta palliata palliata) is an endangered primate experiencing acute population declines for reasons not fully identified. Health and disease monitoring are critical factors for understanding animal and ecosystem health, yet limited research has been achieved in this species due to difficulties in successful immobilization and sample collection. The purpose of this research is to investigate the efficacy of combined ketamine and dexmedetomidine in providing safe field anesthesia for golden-mantled howler monkeys (A. palliata palliata) to perform physical examinations and biological data collection. Eighteen free-ranging golden-mantled howler monkeys from the Marino las Baulas National Park in Costa Rica received a remote intramuscular injection of a ketamine-dexmedetomidine combination. Anesthetic timing intervals, cardiorespiratory parameters, and venous blood gases were evaluated. Successful anesthesia was induced with mean doses of ketamine 8.65 mg/kg (±0.9) and dexmedetomidine 0.043 mg/kg (±0). Recoveries after reversal administration were rapid and smooth, with all animals successfully reintroduced to their colonies within 49 min (mean 38 min) of reversal administration. Ketamine-dexmedetomidine combination is effective at inducing the appropriate anesthetic plane required for field work in this species and should be considered for insitu capture of free-ranging mantled howler monkeys.

In most primate species that live in social groups, non-mothers frequently interact with infants leading to significant impacts on maternal care strategies and both infant socialization and survivorship. In this study, we focused on infant handling by non-mother individuals in mantled howler monkeys (Alouatta palliata), a species for which very little information is available on this topic. Using focal-animal sampling for 743 h, we observed 218 instances of interest and 232 handling events involving 14 infants (i.e., 1 to 12 months of age) living in four groups in southern Mexico. Interest and handling occurred at similar rates, 0.3 events per hour. Most handling was positive, involving behaviors such as making contact and friendly touching, whereas aggressive behaviors, such as harassing and kidnapping, were infrequent. Adult females, especially when lactating, were the primary handlers, contrasting with findings in other primate species where juveniles or non-lactating females are usually more involved. Handling began within the first week of life and was most frequent during the first 3 months. Mothers rarely intervened in handling, doing so primarily during infrequent kidnapping events. These findings contribute to our understanding of primate social behavior, particularly in species with low social activity levels, and provide a basis for future research on the implications of infant handling for group dynamics and infant development.

We investigated the prevalence of arthropod-borne viral diseases in a population of free-ranging mantled howler monkeys (Alouatta palliata) in Costa Rica in 1998. Blood samples were opportunistically collected from monkeys anesthetized for another study. Serology was performed on 64 individuals to assess exposure of this population to vesicular stomatitis virus, equine encephalitis viruses, Mayaro virus, St. Louis encephalitis virus, yellow fever virus, and dengue virus. The New Jersey serotype of vesicular stomatitis (VSV-NJ) was the only pathogen for which the population tested positive (44% [28/64]). This is the first report of antibodies against VSV-NJ in nonhuman primates in Costa Rica.