Sniffing behavior of semi free-ranging Barbary macaques (Macaca sylvanus).

Olfaction is one of the evolutionarily oldest senses and plays a fundamental role in foraging and social interactions across mammals. In primates, the role of olfaction is now well recognized, but better investigated in strepsirrhine and platyrrhine primates than in catarrhines. We observed the sniffing behavior of semi-free ranging Barbary macaques, Macaca sylvanus, at Affenberg Salem, Germany, to assess how frequently macaques sniff and in which contexts, and how sniffing is affected by sex and age. Focal observations of 24 males and 24 females aged 1-25 years showed that Barbary macaques sniffed, on average, 5.24 times per hour, with more than 80% of sniffs directed at food. Irrespective of the context, younger individuals sniffed more often than older ones. Females' sniffs were more often directed at food than male sniffs, while males sniffed more often than females in a social context. Sniffs at conspecifics occurred primarily in a sexual context, with 70% of social sniffs directed at female anogenital swellings performed by males. Of the observed 176 anogenital inspections, 51 involved sniffing of the swelling. Olfactory inspections were followed by copulation significantly less often than merely visual inspections, suggesting that anogenital odors may play a role in male mating decisions, but the role of olfaction in sexual interactions warrants further investigations. In sum, results show that Barbary macaques routinely use olfaction during feeding, but also in a socio-sexual context, corroborating the relevance of the olfactory sense in the lives of catarrhine primates.

Orangutan males make increased use of social learning opportunities, when resource availability is high.

Humans' colonization of diverse habitats relied on our ancestors' abilities to innovate and share innovations with others. While ecological impacts on innovations are well studied, their effect on social learning remains poorly understood. We examined how food availability affects social learning in migrant orangutan unflanged males, who may learn from local orangutans through peering (i.e., observational social learning). We analyzed 1,384 dyadic associations, including 360 peering events, among 46 wild Sumatran orangutan and 25 Bornean orangutan males, collected over 18 years. Migrants' peering rates significantly increased with higher food availability and time spent in proximity to others. Furthermore, migrants in the more sociable Sumatran population exhibited significantly higher peering rates compared to the Borneans, suggesting intrinsic and/or developmental effects of food availability on social learning. These findings emphasize the importance of investigating ecological effects on social learning on the immediate, developmental, and intrinsic levels for our understanding of cultural evolution.

Agricultural habitat use affects infant survivorship in an endangered macaque species.

Infant survival is a major determinant of individual fitness and constitutes a crucial factor in shaping species' ability to maintain viable populations in changing environments.1 Early adverse conditions, such as maternal loss, social isolation, and ecological hazards, have been associated with reduced rates of infant survivorship in wild primates.2,3,4 Agricultural landscapes increasingly replacing natural forest habitats may additionally threaten the survival of infants through exposure to novel predators,5 human-wildlife conflicts,6,7 or the use of harmful chemicals.8,9 Here, we investigated potential links between agricultural habitat use and high infant mortality in wild southern pig-tailed macaques (Macaca nemestrina) inhabiting a mosaic landscape of rainforest and oil palm plantation in Peninsular Malaysia. Longitudinal data revealed that 57% of all infants born during the study period (2014-2023) died before the age of 1 year, far exceeding mortality rates reported for other wild primates.10,11,12,13,14 Importantly, prolonged time spent in the plantation during infancy decreased the likelihood of infant survival by 3-fold, likely caused by increased exposure to the threats inherent to this environment. Further, mortality risk was elevated for infants born to primiparous mothers and predicted by prolonged maternal interbirth intervals, suggesting potential long-term effects attributed to the uptake and/or accumulation of pesticides in mothers' bodies.15,16,17 Indeed, existing literature reports that pesticides may cross the placental barrier, thus impacting fetal development during pregnancy.18,19,20 Our findings emphasize the importance of minimizing anthropogenic threats to wildlife in agricultural landscapes by establishing environmentally friendly cultivation practices that can sustain wildlife populations in the long term.

Taking identity-by-descent analysis into the wild: Estimating realized relatedness in free-ranging macaques.

Biological relatedness is a key consideration in studies of behavior, population structure, and trait evolution. Except for parent-offspring dyads, pedigrees capture relatedness imperfectly. The number and length of DNA segments that are identical-by-descent (IBD) yield the most precise estimates of relatedness. Here, we leverage novel methods for estimating locus-specific IBD from low coverage whole genome resequencing data to demonstrate the feasibility and value of resolving fine-scaled gradients of relatedness in free-living animals. Using primarily 4-6× coverage data from a rhesus macaque (Macaca mulatta) population with available long-term pedigree data, we show that we can call the number and length of IBD segments across the genome with high accuracy even at 0.5× coverage. The resulting estimates demonstrate substantial variation in genetic relatedness within kin classes, leading to overlapping distributions between kin classes. They identify cryptic genetic relatives that are not represented in the pedigree and reveal elevated recombination rates in females relative to males, which allows us to discriminate maternal and paternal kin using genotype data alone. Our findings represent a breakthrough in the ability to understand the predictors and consequences of genetic relatedness in natural populations, contributing to our understanding of a fundamental component of population structure in the wild.

Ten Years of Positive Impact of a Conservation Education Program on Children's Knowledge and Behaviour Toward Crested Macaques (Macaca nigra) in the Greater Tangkoko Area, North Sulawesi, Indonesia.

In areas where primates are threatened, environmental education interventions are a key way to increase the local population's knowledge of their environment and encourage positive attitudes and habits to preserve the environment and wildlife on a local and global scale. This study assesses the impact of Tangkoko Conservation Education (TCE), the Macaca Nigra Project's conservation education programme, running since 2011 in North Sulawesi for school children, teachers, and the local population. TCE's goal is to help people increase their knowledge of their environment and develop more positive habits and behaviours towards their local environment, especially the Critically Endangered and endemic crested macaques (Macaca nigra). We measured the efficacy of TCE's programme for pupils using questionnaires provided one month before and one month after a year-long conservation education programme at school. Pupils' knowledge and behaviour scores increased significantly after their participation in the programme. Their habits score also increased but this increase was not statistically significant. Girls scored significantly higher than boys in terms of positive behaviour towards wildlife. Children participating in the programme more than once seemed to obtain higher scores in their second participation, although the sample size was too small for formal analysis. Despite some limitations, this study demonstrates the positive impact of our programme on young people living in an area where primates are threatened. We hope that this research will inspire similar programmes in Sulawesi and elsewhere by providing methods and activities to help prevent primate extinction. Supplementary Information: The online version contains supplementary material available at 10.1007/s10764-023-00356-9.

Corrigendum: Intra-specific variation in the social behavior of Barbary macaques (Macaca sylvanus).

[This corrects the article DOI: 10.3389/fpsyg.2021.666166.].

A comparison between mobile and stationary gas chromatography-mass spectrometry devices for analysis of complex volatile profiles.

On-site analysis of volatile organic compounds (VOCs) with miniaturized gas chromatography-mass spectrometry (GC-MS) systems is a very rapidly developing field of application. While, on the one hand, major technological advances are improving the availability of these systems on the market, on the other hand, systematic studies to assess the performance of such instruments are still lacking. To fill this gap, we compared three portable GC-MS devices to a state-of-the-art benchtop (stationary) system for analysis of a standard mixture of 18 VOCs. We systematically compared analytical parameters such as the sensitivity and similarity of the signal response pattern and the quality of the obtained mass spectra. We found that the investigated mobile instruments (i) showed different response profiles with a generally lower number of identified analytes. Also, (ii) mass spectral reproducibility (% relative standard deviation (RSD) of the relative abundance of selective fragments) was generally worse in the mobile devices (mean RSD for all targeted fragments ~9.7% vs. ~3.5% in the stationary system). Furthermore, mobile devices (iii) showed a poorer mass spectral similarity to commercial reference library spectra (>20% deviation of fragment ion relative intensity vs. ~10% in the stationary GC-MS), suggesting a less reliable identification of analytes by library search. Indeed, (iv) the performance was better with higher-mass and/or more abundant fragments, which should be considered to improve the results of library searches for substance identification. Finally, (v) the estimation of the signal-to-noise ratio (S/N) in mobile instruments as a measure of sensitivity revealed a significantly lower performance compared to the benchtop lab equipment (with a ratio among medians of ~8 times lower). Overall, our study reveals not only a poor signal-to-noise ratio and poor reproducibility of the data obtained from mobile instruments, but also unfavorable results with respect to a reliable identification of substances when they are applied for complex mixtures of volatiles.

Temporal dynamics and fitness consequences of coalition formation in male primates.

Coalition formation is one of the most striking forms of cooperation found in animals. Yet, there is substantial variation between taxa regarding the mechanisms by which coalitions can result in fitness consequences. Here, we investigate the influence of coalitions on dominance rank trajectories and subsequently on reproductive success in wild male crested macaques (Macaca nigra) at Tangkoko Nature Reserve (Sulawesi, Indonesia). We observed 128 coalition events involving 28 males and tested how a variety of coalition properties and factors related to the social environment influenced future male rank. We further used genetic paternity analysis of 19 infants conceived during the study to assess male reproductive success. Our results show that males participating in coalitions achieved higher-than-expected future ranks, while coalition targets had lower-than-expected future ranks. Additionally, all-up coalitions had stronger effects on rank than all-down and bridging coalitions, and these were modulated by the relative strength of coalition partners versus targets. Finally, higher ranking males were more likely to sire infants than lower ranking males. These results provide important insights regarding the mechanisms underlying coalition formation and support the idea that one major path by which coalitions can affect fitness is through influencing male dominance trajectories.

Towards an understanding of multimodal traits of female reproduction in chimpanzees.

Although primates have long been regarded as microsmatic, recent studies indicate that olfaction is an important sensory mode of primate communication, e.g., in the context of reproduction. However, large gaps remain in our understanding of primate olfactory traits, especially in the great apes. Female chimpanzees (Pan troglodytes) possess an exaggerated sexual swelling, which is an imprecise signal of fertility that is thought to serve to confuse paternity. However, some high-ranking males that copulate most frequently on the days when females are most fertile seem to have more precise information on the timing of ovulation, which suggests the existence of an olfactory fertility trait. In order to examine, and provide evidence for, fertility-related chemical information in female chimpanzees, we used gas chromatography-mass spectrometry to analyze the chemical composition of female body odor collected across the menstrual cycle during various stages of sexual swelling (97 samples of six females). The chemical composition was significantly affected by swelling stage, and eight substances were detected that were strongly related to the latter. The existence of an additional, olfactory, fertility trait may help males to fine-tune their sexual behavior or allow females to strengthen concealment of the exact timing of ovulation, and needs to be further investigated in follow-up studies. The results of our study provide much-needed evidence for the existence of an olfactory cue related to reproduction in chimpanzees, and form a basis for future studies on the interplay between visual and olfactory information on female fertility.

Maternal stress, child behavior and the promotive role of older siblings.

BACKGROUND: In the first years of their lives, children develop the cognitive, social and emotional skills that will provide the foundations for their lifelong health and achievements. To increase their life prospects and reduce the long-term effects of early aversive conditions, it is therefore crucial to understand the risk factors that negatively affect child development and the factors that are instead beneficial. In this study, we tested (i) the effects of different social and environmental stressors on maternal stress levels, (ii) the dynamic relationship between maternal stress and child behavior problems during development, and (iii) the potential promotive (i.e. main) or protective (i.e. buffering) effect of siblings on child behavior problems during development. METHODS: We used longitudinal data from 373 mother-child pairs (188 daughters, 185 sons) from pregnancy until 10 years of age. We assessed maternal stress and child behavior problems (internalizing and externalizing) with validated questionnaires, and then used linear mixed models, generalized linear mixed models and longitudinal cross-lagged models to analyze the data. RESULTS: Our results showed that higher maternal stress levels were predicted by socio-environmental stressors (i.e. the lack of sufficient social areas in the neighborhood). Moreover, prenatal maternal stress reliably predicted the occurrence of behavior problems during childhood. Finally, the presence of older siblings had a promotive function, by reducing the likelihood that children developed externalizing problems. CONCLUSIONS: Overall, our results confirm the negative effects that maternal stress during pregnancy may have on the offspring, and suggest an important main effect of older siblings in promoting a positive child development.

Intra-specific Variation in the Social Behavior of Barbary macaques (Macaca sylvanus).

Non-human primates show an impressive behavioral diversity, both across and within species. However, the factors explaining intra-specific behavioral variation across groups and individuals are yet understudied. Here, we aimed to assess how group size and living conditions (i.e., captive, semi-free-ranging, wild) are linked to behavioral variation in 5 groups of Barbary macaques (N=137 individuals). In each group, we collected observational data on the time individuals spent in social interactions and on the group dominance style, along with experimental data on social tolerance over food and neophobia. Our results showed that differences in group size predicted differences in the time spent in social interactions, with smaller groups spending a higher proportion of time in close spatial proximity, but a lower proportion of time grooming. Moreover, group size predicted variation in dominance style, with smaller groups being more despotic. Social tolerance was affected by both group size and living conditions, being higher in smaller groups and in groups living in less natural conditions. Finally, individual characteristics also explained variation in social tolerance and neophobia, with socially integrated individuals having higher access to food sources, and higher-ranking ones being more neophobic. Overall, our results support the view that intra-specific variation is a crucial aspect in primate social behavior and call for more comparative studies to better understand the sources of within-species variation.

Neophobia in 10 ungulate species-a comparative approach.

ABSTRACT: Neophobia (the fearful reaction to novel stimuli or situations) has a crucial effect on individual fitness and can vary within and across species. However, the factors predicting this variation are still unclear. In this study, we assessed whether individual characteristics (rank, social integration, sex) and species socio-ecological characteristics (dietary breadth, group size, domestication) predicted variation in neophobia. For this purpose, we conducted behavioral observations and experimental tests on 78 captive individuals belonging to 10 different ungulate species-an ideal taxon to study inter-specific variation in neophobia given their variety in socio-ecological characteristics. Individuals were tested in their social groups by providing them with familiar food, half of which had been positioned close to a novel object. We monitored the individual latency to approach and eat food and the proportion of time spent in its proximity. Using a phylogenetic approach and social network analyses, we showed that across ungulate species neophobia was higher in socially more integrated individuals, as compared to less integrated ones. In contrast, rank and sex did not predict inter-individual differences in neophobia. Moreover, species differed in their levels of neophobia, with Barbary sheep being on average less neophobic than all the other study species. As group size in Barbary sheep was larger than in all the other study species, these results support the hypothesis that larger group size predicts lower levels of neophobia, and confirm ungulates as a highly promising taxon to study animal behavior and cognition with a comparative perspective. SIGNIFICANCE STATEMENT: In several species, individuals may respond fearfully to novel stimuli, therefore reducing the risks they may face. However, it is yet unclear if certain individuals or species respond more fearfully to novelty. Here, we provided food to 78 individual ungulates with different characteristics (e.g., sex, rank, social integration, group size, domestication, dietary breadth) in different controlled conditions (e.g., when food was close to novel or to familiar objects). Across species, we found that socially integrated individuals responded more fearfully in all species. Moreover, being in larger groups decreased the probability of fearfully responding to novelty. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00265-021-03041-0.

Oil palm cultivation critically affects sociality in a threatened Malaysian primate.

Human-induced habitat alterations globally threaten animal populations, often evoking complex behavioural responses in wildlife. This may be particularly dramatic when negatively affecting social behaviour, which fundamentally determines individual fitness and offspring survival in group-living animals. Here, we provide first evidence for significant behavioural modifications in sociality of southern pig-tailed macaques visiting Malaysian oil palm plantations in search of food despite elevated predation risk. Specifically, we found critical reductions of key positive social interactions but higher rates of aggression in the plantation interior compared to the plantation edge (i.e. plantation areas bordering the forest) and the forest. At the plantation edge, affiliation even increased compared to the forest, while central positions in the macaques' social network structure shifted from high-ranking adult females and immatures to low-ranking individuals. Further, plantations also affected mother-infant relationships, with macaque mothers being more protective in the open plantation environment. We suggest that although primates can temporarily persist in human-altered habitats, their ability to permanently adapt requires the presence of close-by forest and comes with a trade-off in sociality, potentially hampering individual fitness and infant survival. Studies like ours remain critical for understanding species' adaptability to anthropogenic landscapes, which may ultimately contribute to facilitating their coexistence with humans and preserving biodiversity.

Female fertile phase synchrony, and male mating and reproductive skew, in the crested macaque.

High social status is the primary determinant of reproductive success among group-living male mammals. Primates living in multimale-multifemale groups show the greatest variation in the strength of this link, with marked variation in reproductive skew by male dominance among species, dependent on the degree of female fertile phase synchrony, and the number of competing males. Here, we present data on two groups of wild crested macaques (Macaca nigra), living in the Tangkoko Reserve, Sulawesi, Indonesia. We investigated male monopolization of fertile females in 31 cycles of 19 females, and genetic paternity of 14 offspring conceived during the study period. We show that female fertile phase synchrony was low, that females had few mating partners in their fertile phase, and that dominant males monopolized a high proportion of consortships and matings, resulting in marked and steep mating and reproductive skew. We conclude that female cycle asynchrony provides the opportunity for strong direct male-male competition in crested macaques, resulting in monopolization of females by dominant males, consistent with their marked sexual dimorphism. Our study provides a test of the underlying factors that determine the relative occurrence and strength of different mechanisms of sexual selection, and the phenotypes that evolve as a result.

Occupancy of wild southern pig-tailed macaques in intact and degraded forests in Peninsular Malaysia.

Deforestation is a major threat to terrestrial tropical ecosystems, particularly in Southeast Asia where human activities have dramatic consequences for the survival of many species. However, responses of species to anthropogenic impact are highly variable. In order to establish effective conservation strategies, it is critical to determine a species' ability to persist in degraded habitats. Here, we used camera trapping data to provide the first insights into the temporal and spatial distribution of southern pig-tailed macaques (Macaca nemestrina, listed as 'Vulnerable' by the IUCN) across intact and degraded forest habitats in Peninsular Malaysia, with a particular focus on the effects of clear-cutting and selective logging on macaque occupancy. Specifically, we found a 10% decline in macaque site occupancy in the highly degraded Pasoh Forest Reserve from 2013 to 2017. This may be strongly linked to the macaques' sensitivity to intensive disturbance through clear-cutting, which significantly increased the probability that M. nemestrina became locally extinct at a previously occupied site. However, we found no clear relationship between moderate disturbance, i.e., selective logging, and the macaques' local extinction probability or site occupancy in the Pasoh Forest Reserve and Belum-Temengor Forest Complex. Further, an identical age and sex structure of macaques in selectively logged and completely undisturbed habitat types within the Belum-Temengor Forest Complex indicated that the macaques did not show increased mortality or declining birth rates when exposed to selective logging. Overall, this suggests that low to moderately disturbed forests may still constitute valuable habitats that support viable populations of M. nemestrina, and thus need to be protected against further degradation. Our results emphasize the significance of population monitoring through camera trapping for understanding the ability of threatened species to cope with anthropogenic disturbance. This can inform species management plans and facilitate the development of effective conservation measures to protect biodiversity.

Dominance style only partially predicts differences in neophobia and social tolerance over food in four macaque species.

Primates live in complex social systems with social structures ranging from more to less despotic. In less despotic species, dominance might impose fewer constraints on social choices, tolerance is greater than in despotic species and subordinates may have little need to include novel food items in the diet (i.e. neophilia), as contest food competition is lower and resources more equally distributed across group members. Here, we used macaques as a model to assess whether different dominance styles predict differences in neophilia and social tolerance over food. We provided familiar and novel food to 4 groups of wild macaques (N = 131) with different dominance styles (Macaca fuscata, M. fascicularis, M. sylvanus, M. maura). Our study revealed inter- and intra-specific differences in individuals' access to food, which only partially reflected the dominance styles of the study subjects. Contrary to our prediction, social tolerance over food was higher in more despotic species than in less despotic species. Individuals with a higher dominance rank and being better socially integrated (i.e. higher Eigenvector centrality) were more likely to retrieve food in all species, regardless of their dominance style. Partially in line with our predictions, less integrated individuals more likely overcame neophobia (as compared to more integrated ones), but only in species with more tolerance over food. Our study suggests that individual characteristics (e.g. social integration or personality) other than dominance rank may have a stronger effect on an individual's access to resources.

Higher early life mortality with lower infant body mass in a free-ranging primate.

Traits that reflect the amount of energy allocated to offspring by mothers, such as infant body mass, are predicted to have long-lasting effects on offspring fitness. In very long-lived species, such as anthropoid primates, where long-lasting and obligate parental care is required for successful recruitment of offspring, there are few studies on the fitness implications of low body mass among infants. Using body mass data collected from 253 free-ranging rhesus macaque Macaca mulatta infants on Cayo Santiago, Puerto Rico, we examined if lower infant body mass predicts lower chance of survival through to reproductive maturation (4th year of life). We also used data on inter-birth intervals and suckling behaviours to determine whether the duration of maternal care was adjusted to infant body mass. Rhesus macaque infants experienced on average 5% reduced hazard of death for an increase in body mass of 0.1 SD (~100 g) above the mean within their age-sex class. The positive association between body mass and early life survival was most pronounced in the 1st year of life. Infant body mass tended to be lower if mothers were young or old, but the link between infant body mass and early life survival remained after controlling for maternal age. This finding suggests that maternal effects on early life survival such as maternal age may act through their influence on infant body mass. Mothers of heavier infants were less likely to be delayed in subsequent reproduction, but the estimated association slightly overlapped with zero. The timing of the last week of suckling did not differ by infant body mass. Using infant body mass data that has been rarely available from free-ranging primates, our study provides comparative evidence to strengthen the existing body of literature on the fitness implications of variation in infant body mass.

Innovation in wild Barbary macaques (Macaca sylvanus).

Innovation is the ability to solve novel problems or find novel solutions to familiar problems, and it is known to affect fitness in both human and non-human animals. In primates, innovation has been mostly studied in captivity, although differences in living conditions may affect individuals' ability to innovate. Here, we tested innovation in a wild group of Barbary macaques (Macaca sylvanus). In four different conditions, we presented the group with several identical foraging boxes containing food. To understand which individual characteristics and behavioural strategies best predicted innovation rate, we measured the identity of the individuals manipulating the boxes and retrieving the food, and their behaviour during the task. Our results showed that success in the novel task was mainly affected by the experimental contingencies and the behavioural strategies used during the task. Individuals were more successful in the 1-step conditions, if they participated in more trials, showed little latency to approach the boxes and mainly manipulated functional parts of the box. In contrast, we found no effect of inhibition, social facilitation and individual characteristics like sex, age, rank, centrality, neophobia and reaction to humans, on the individuals' ability to innovate.

Challenges of fast sampling of volatiles for thermal desorption gas chromatography - mass spectrometry.

Fast active sampling of volatile organic compounds (VOCs) under field conditions still is a great challenge especially when the exposure time to the source of emissions is a restricting factor. Hence, to identify ideal conditions for such applications, we systematically compared fast active sampling of VOCs collected on two common adsorbents under two regimes: first, very low gas volumes (from 300 mL) sampled at nominal flow rate and, second, sampling at the maximal applicable flow rate (0.5 L/min) before loss of sorbent material was experienced. For XAD-2 and Tenax TA, efficient sorbents for on-site VOC-sampling followed by thermal desorption GC-MS, significant differences in the signal response of volatile compounds were related not only to the varied experimental factors alone, but also to their interactions and to compound volatility. In the first regime, volatiles (∼0.004-3.13 mM) from Tenax TA gave the highest signal response only above 800 mL sampled gas volume while at low concentrations (∼0.004-0.12 mM), satisfactory recovery from XAD-2 required longer analyte-sorbent interaction. For the second regime, the relative recovery was severely impaired down to 73 ±â€¯23%, n = 56 for Tenax TA and 72 ±â€¯17%, n = 56 for XAD-2 at intermediate concentration, and 79 ±â€¯11%, n = 84 for Tenax TA at high concentration compared to the relative recovery at standard flow rate. Neither Tenax TA nor XAD-2 provided a 100% total recovery (calculated using breakthrough values) for any of the evaluated compounds. Finally, two-way and three-way interactions identified in a multi-variable model, explained not only the dependence of the signal response on different experimental variables, but also their complex interplay affecting the recovery of the VOCs. In conclusion, we show for the first time that XAD-2, a material only recently introduced for the adsorption of volatiles from the gas phase, competes well with the standard material Tenax TA under conditions of fast sampling. Due to the similar absolute recovery with Tenax TA even at low concentration and with regard to the better detection limits, we consider XAD-2 the better choice for fast sampling of VOCs, particularly with low sample volumes at regular flow. For fast sampling with high flow rate, however, both sorbents might be selected only if the corresponding recovery loss can be accepted for the study.

Macaques can contribute to greener practices in oil palm plantations when used as biological pest control.

Conversion of tropical forests into oil palm plantations reduces the habitats of many species, including primates, and frequently leads to human-wildlife conflicts. Contrary to the widespread belief that macaques foraging in the forest-oil palm matrix are detrimental crop pests, we show that the impact of macaques on oil palm yield is minor. More importantly, our data suggest that wild macaques have the potential to act as biological pest control by feeding on plantation rats, the major pest for oil palm crops, with each macaque group estimated to reduce rat populations by about 3,000 individuals per year (mitigating annual losses of 112 USD per hectare). If used for rodent control in place of the conventional method of poison, macaques could provide an important ecosystem service and enhance palm oil sustainability.