Combined perceptual and chemical analyses show no compelling evidence for ovulatory cycle shifts in women's axillary odour.

Although men's attraction to women's body odour has been suggested to vary over the ovulatory cycle, peaking around the fertile window, we still lack methodologically robust evidence corroborating this effect. Further, the chemical underpinnings of male preference for the odour of ovulating women remain unknown. Here, we combined perceptual and chemical analyses to investigate the axillary odour of naturally cycling women over 10 days, covering the gradual change in fertility across the ovulatory cycle with a focus on fertile days. The fertile state was confirmed by urinary ovulation tests as well as salivary oestradiol and progesterone levels. Men rated the scent of unfamiliar women, resembling a first encounter. We used multivariate analyses to relate variation in both odour ratings and chemical composition to female conception probability, temporal distance to ovulation and ovarian hormone levels. Our results provide no evidence that males prefer the odour of fertile women. Furthermore, the volatile analysis indicated no link between axillary odour composition and current fertility status. Together, our results showed no convincing support for a chemical fertility cue in women's axillary odour, questioning the presence of olfactory fertility information that is recognizable during first encounters in modern humans.

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.

The effect of individual and food characteristics on food retrieval and food sharing in captive Guinea baboons (Papio papio).

Access to food is of major importance to the fitness and survival of every individual, particularly in group-living animals, in which individual characteristics and food distribution can affect food intake. Additionally, several species of primates are known to share food under certain conditions. Such unresisted transfer of food from one individual to another appears to be adaptive, for instance as a tool to maintain and reinforce social bonds. In this study, we aimed to test how food retrieval and food sharing varies depending on the social relationship between individuals, and on the characteristics of the food. In six different test conditions, we provided a captive group of Guinea baboons (Papio papio, N = 23) with multiple food items, differing in quality, quantity, density, monopolizability, and effort required to obtain it. We further used behavioral observations to assess individual relationships and possible variations in grooming exchanges linked to food sharing events. Out of 424 events in which food items were retrieved by the subjects, we detected no instances of active food sharing and only 17 of passive food sharing. The way food was retrieved was affected by individual and food characteristics (i.e., quantity, quality, and monopolizability of food): Males and central individuals (i.e., those connected to many partners, and/or having partners with many connections in the social network) were more likely to retrieve food during test conditions. In particular, events of passive food sharing mostly happened when the quality of food was low, and between individuals belonging to the same community (i.e., having close relationships). No other food characteristics affected the probability to share food, and the occurrence of food sharing had no immediate effect on grooming exchanges. Overall, our findings suggest that food sharing is relatively rare in Guinea baboons unless the food has a low quality and individuals form close social bonds.

Comparative chemical analysis of body odor in great apes.

Olfaction is important across the animal kingdom for transferring information on, for example, species, sex, group membership, or reproductive parameters. Its relevance has been established in primates including humans, yet research on great apes still is fragmentary. Observational evidence indicates that great apes use their sense of smell in various contexts, but the information content of their body odor has not been analyzed. Our aim was therefore to compare the chemical composition of body odor in great ape species, namely Sumatran orangutans (Pongo abelii (Lesson, 1827), one adult male, five adult females, four nonadults), Western lowland gorillas (Gorilla gorilla gorilla (Savage, 1847), one adult male, two adult females, one nonadult), common chimpanzees (Pan troglodytes (Blumenbach, 1775), four adult males, nine adult females, four nonadults), and bonobos (Pan paniscus (Schwarz, 1929), two adult males, four adult females, two nonadults). We collected 195 samples (five per individual) of 39 captive individuals using cotton swabs and analyzed them using gas chromatography mass spectrometry. We compared the sample richness and intensity, similarity of chemical composition, and relative abundance of compounds. Results show that species, age, and potentially sex have an impact on the variance between odor profiles. Richness and intensity varied significantly between species (gorillas having the highest, bonobos the lowest richness and intensity), and with age (both increasing with age). Richness and intensity did not vary between sexes. Odor samples of the same species were more similar to each other than samples of different species. Among all compounds identified some were associated with age (N = 7), sex (N = 6), and species-related (N = 37) variance. Our study contributes to the basic understanding of olfactory communication in hominids by showing that the chemical composition of body odor varies across species and individuals, containing potentially important information for social communication.

Stress behaviour and physiology of developing Arctic barnacle goslings ( Branta leucopsis) is affected by legacy trace contaminants.

Natural populations are persistently exposed to environmental pollution, which may adversely impact animal physiology and behaviour and even compromise survival. Responding appropriately to any stressor ultimately might tip the scales for survival, as mistimed behaviour and inadequate physiological responses may be detrimental. Yet effects of legacy contamination on immediate physiological and behavioural stress coping abilities during acute stress are virtually unknown. Here, we assessed these effects in barnacle goslings ( Branta leucopsis) at a historical coal mine site in the Arctic. For three weeks we led human-imprinted goslings, collected from nests in unpolluted areas, to feed in an abandoned coal mining area, where they were exposed to trace metals. As control we led their siblings to feed on clean grounds. After submitting both groups to three well-established stress tests (group isolation, individual isolation, on-back restraint), control goslings behaved calmer and excreted lower levels of corticosterone metabolites. Thus, legacy contamination may decisively change stress physiology and behaviour in long-lived vertebrates exposed at a young age.

Comparing the sniffing behavior of great apes.

The importance of smell in humans is well established but we know little about it in regard to our closest relatives, the great apes, as systematic studies on their olfactory behavior are still lacking. Olfaction has long been considered to be of lesser importance in hominids given their relatively smaller olfactory bulbs, fewer functional olfactory receptor genes than other species and absence of a functional vomeronasal organ. Therefore, the aim of this study was to evaluate the use of olfaction in hominids. In particular, we observed sniffing behavior in captive groups of four species (Sumatran orangutans, Pongo abelii; Western lowland gorillas, Gorilla gorilla gorilla; Western chimpanzees, Pan troglodytes verus; bonobos, Pan paniscus) and evaluated in which contexts sniffing was used. Our results show that all investigated species frequently used the sense of smell, and that the sniffing frequency varied with species, sex, age, and context. Most sniffing events were observed in gorillas in comparison to the three other species. Sniffing frequencies were also influenced by sex, with males sniffing slightly more often than females. Furthermore, our results revealed an effect of age, with younger individuals sniffing more often than older individuals. All species mainly sniffed in the non-social context (i.e., toward food and other environmental items) rather than in the social context (i.e., at conspecifics), suggesting that the evaluation of the environment and the nutritional value of food items is of major importance to all great ape species investigated here. In contrast to the other species and female chimpanzees, however, male chimpanzees most often used olfaction to inspect their conspecifics. Together, our study suggests that olfaction is likely to be more important in great apes than previously appreciated.

Current Challenges in Plant Eco-Metabolomics.

The relatively new research discipline of Eco-Metabolomics is the application of metabolomics techniques to ecology with the aim to characterise biochemical interactions of organisms across different spatial and temporal scales. Metabolomics is an untargeted biochemical approach to measure many thousands of metabolites in different species, including plants and animals. Changes in metabolite concentrations can provide mechanistic evidence for biochemical processes that are relevant at ecological scales. These include physiological, phenotypic and morphological responses of plants and communities to environmental changes and also interactions with other organisms. Traditionally, research in biochemistry and ecology comes from two different directions and is performed at distinct spatiotemporal scales. Biochemical studies most often focus on intrinsic processes in individuals at physiological and cellular scales. Generally, they take a bottom-up approach scaling up cellular processes from spatiotemporally fine to coarser scales. Ecological studies usually focus on extrinsic processes acting upon organisms at population and community scales and typically study top-down and bottom-up processes in combination. Eco-Metabolomics is a transdisciplinary research discipline that links biochemistry and ecology and connects the distinct spatiotemporal scales. In this review, we focus on approaches to study chemical and biochemical interactions of plants at various ecological levels, mainly plant⁻organismal interactions, and discuss related examples from other domains. We present recent developments and highlight advancements in Eco-Metabolomics over the last decade from various angles. We further address the five key challenges: (1) complex experimental designs and large variation of metabolite profiles; (2) feature extraction; (3) metabolite identification; (4) statistical analyses; and (5) bioinformatics software tools and workflows. The presented solutions to these challenges will advance connecting the distinct spatiotemporal scales and bridging biochemistry and ecology.

The importance of the altricial - precocial spectrum for social complexity in mammals and birds - a review.

Various types of long-term stable relationships that individuals uphold, including cooperation and competition between group members, define social complexity in vertebrates. Numerous life history, physiological and cognitive traits have been shown to affect, or to be affected by, such social relationships. As such, differences in developmental modes, i.e. the 'altricial-precocial' spectrum, may play an important role in understanding the interspecific variation in occurrence of social interactions, but to what extent this is the case is unclear because the role of the developmental mode has not been studied directly in across-species studies of sociality. In other words, although there are studies on the effects of developmental mode on brain size, on the effects of brain size on cognition, and on the effects of cognition on social complexity, there are no studies directly investigating the link between developmental mode and social complexity. This is surprising because developmental differences play a significant role in the evolution of, for example, brain size, which is in turn considered an essential building block with respect to social complexity. Here, we compiled an overview of studies on various aspects of the complexity of social systems in altricial and precocial mammals and birds. Although systematic studies are scarce and do not allow for a quantitative comparison, we show that several forms of social relationships and cognitive abilities occur in species along the entire developmental spectrum. Based on the existing evidence it seems that differences in developmental modes play a minor role in whether or not individuals or species are able to meet the cognitive capabilities and requirements for maintaining complex social relationships. Given the scarcity of comparative studies and potential subtle differences, however, we suggest that future studies should consider developmental differences to determine whether our finding is general or whether some of the vast variation in social complexity across species can be explained by developmental mode. This would allow a more detailed assessment of the relative importance of developmental mode in the evolution of vertebrate social systems.

Sampling the Body Odor of Primates: Cotton Swabs Sample Semivolatiles Rather Than Volatiles.

We assessed the suitability of a frequently used sampling method employing cotton swabs for collecting animal body odor for gas chromatography-mass spectrometry (GC-MS) analysis of volatile organic compounds (VOCs). Our method validation showed that both sampling material and sampling protocols affect the outcome of the analyses. Thus, among the tested protocols swabs of pure viscose baked before use and extracted with hexane had the least blank interferences in GC-MS analysis. Most critical for the recovery of VOCs was the handling time: the significant recovery losses of volatiles experienced with this sampling procedure suggest that a rapid processing of such samples is required. In a second part, we used swab sampling to sample the body odor of rhesus macaques (Macaca mulatta), which lack scent glands. First results after GC-MS analysis of the samples collected from these nonhuman primates emphasize that proper analytical performance is an indispensable prerequisite for successful automated data evaluation of the complex GC-MS profiles. Moreover, the retention times and the nature of the identified chemical compounds in our samples suggest that the use of swabs is generally more appropriate for collecting semivolatile rather than VOCs.

Influence of visual information on sniffing behavior in a routinely trichromatic primate.

Most catarrhine primates are considered to be strongly visually oriented, obtaining information about conspecifics and their environment from a diversity of visual cues. Other sensory modalities may provide information that is redundant and/or complimentary to visual cues. When cues from multiple sensory modalities are available, these may reinforce or suppress each other, as shown in several taxa ranging from insects to humans. Here, we tested how the presence and ambiguity of visual information affect the use of olfactory cues when exploring food and non-food items in semi-free-ranging Barbary macaques at Affenberg Salem, Germany. We presented monkeys with pipes containing food (peanuts, popcorn), non-food (stones, feces), or no items in transparent or opaque containers and assessed whether animals looked, sniffed, and/or grabbed into the pipes depending on the visibility of the contents (experiment 1). Visual information had no robust effect on sniffing probability, but monkeys were more likely to sniff before any other form of inspection if the can was opaque than if it was transparent. Both visual and olfactory information affected, whether or not monkeys attempted to retrieve the items from the pipes, whereby monkeys showed an overall decrease in the propensity to grab after sniffing. Furthermore, we manipulated the visual appearance of familiar food items (popcorn) with food colorant (experiment 2), which resulted in substantially increased olfactory inspections compared to unmanipulated popcorn. Taken together, reliance on the olfactory sense was modulated by the available visual information, emphasizing the interplay between different sensory modalities for obtaining information about the environment.

Long-term memory of hierarchical relationships in free-living greylag geese.

Animals may memorise spatial and social information for many months and even years. Here, we investigated long-term memory of hierarchically ordered relationships, where the position of a reward depended on the relationship of a stimulus relative to other stimuli in the hierarchy. Seventeen greylag geese (Anser anser) had been trained on discriminations between successive pairs of five or seven implicitly ordered colours, where the higher ranking colour in each pair was rewarded. Geese were re-tested on the task 2, 6 and 12 months after learning the dyadic colour relationships. They chose the correct colour above chance at all three points in time, whereby performance was better in colour pairs at the beginning or end of the colour series. Nonetheless, they also performed above chance on internal colour pairs, which is indicative of long-term memory for quantitative differences in associative strength and/or for relational information. There were no indications for a decline in performance over time, indicating that geese may remember dyadic relationships for at least 6 months and probably well over 1 year. Furthermore, performance in the memory task was unrelated to the individuals' sex and their performance while initially learning the dyadic colour relationships. We discuss possible functions of this long-term memory in the social domain.

Grey parrots use inferential reasoning based on acoustic cues alone.

Our ability to make logical inferences is considered as one of the cornerstones of human intelligence, fuelling investigations of reasoning abilities in non-human animals. Yet, the evidence to date is equivocal, with apes as the prime candidates to possess these skills. For instance, in a two-choice task, apes can identify the location of hidden food if it is indicated by a rattling noise caused by the shaking of a baited container. More importantly, they also use the absence of noise during the shaking of the empty container to infer that this container is not baited. However, since the inaugural report of apes solving this task, to the best of our knowledge, no comparable evidence could be found in any other tested species such as monkeys and dogs. Here, we report the first successful and instantaneous solution of the shaking task through logical inference by a non-ape species, the African grey parrot. Surprisingly, the performance of the birds was sensitive to the shaking movement: they were successful with containers shaken horizontally, but not with vertical shaking resembling parrot head-bobbing. Thus, grey parrots seem to possess ape-like cross-modal reasoning skills, but their reliance on these abilities is influenced by low-level interferences.

Physiological implications of pair-bond status in greylag geese.

In group-living vertebrates, reliable social allies play a decisive role in dealing with stressors. For example, support by social allies is known to dampen glucocorticoid responses. It remains unknown, however, how social embedding affects the sympatho-adrenergic axis as indicated by heart rate (HR) in non-human animals. We studied the relationships between HR, pair-bond status and distance from the pair-partner in twenty-five free-ranging greylag geese (Anser anser) in a natural social environment. In three individuals, we investigated HR responses following partner loss. Overall, we found a context- and sex-dependent difference in HR between paired and unpaired individuals, paired males having a lower HR during agonistic encounters, and unpaired females having a lower HR during resting. Also, in paired females HR increased with increasing distance from the partner. Our data suggest that HR is modulated by pair-bond status in greylag geese in a context- and sex-dependent manner, which may be representative for social vertebrates in general. Despite the low sample size, the present study indicates that proper social embedding may optimize an individual's physiological investment in the social domain. This reduces individual energy expenditure and may benefit health and reproductive success.

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.

Early development of gaze following into distant space in juvenile Greylag geese (Anser anser).

Visual co-orientation with another's gaze direction (gaze following) may provide important information about the location of food, social interactions or predators. Gaze following has been shown in a variety of mammals, but only in few bird species, and has not been tested in precocial birds at all. It has been suggested that gaze following is an anti-predator behaviour, and in Common ravens (Corvus corax) and rooks (C. frugilegus), it emerges shortly after fledging, at a time when young birds leave the predator-safe nest. However, if gaze following is adaptive, the developmental pattern should differ between altricial and precocial birds. Greylag geese (Anser anser) are highly social birds with a precocial development. Goslings move and feed independently within 24 h post-hatching, and they are highly vulnerable to aerial predators. We therefore predicted that greylag geese are capable of gaze following and that they develop this skill already pre-fledging. We experimentally tested 19 hand-raised greylag goslings for their ability to follow a conspecific's gaze when they were between 10 days and 6 weeks old. In line with our predictions, first responses were already detectable in 10-day-old goslings. Our results therefore not only demonstrate that greylag geese follow the gaze of conspecifics into distant space, but that they also develop this ability much earlier than altricial birds.

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.

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.

Benefits of family reunions: social support in secondary greylag goose families.

Social interactions are among the most potent stressors. However, social allies may diminish stress, increase success in agonistic encounters and ease access to resources. We studied the role of social support as a major mechanism for individual stress management in families of free-ranging greylag geese (Anser anser). Greylag geese are long-term monogamous, live in a female-bonded social system, and fledged offspring stay with their parents until the next breeding season ('primary families'). Should parents then fail to fledge young, subadults might rejoin them in summer after molt is completed ('secondary families'). We have previously shown that primary greylag goose families reap benefits from active social support in agonistic encounters, and also excrete lower levels of immuno-reactive corticosterone metabolites (CORT, 'passive social support'). Here we investigated how far active and passive social support continues in secondary goose families. Although we found that active support in agonistic encounters was almost absent in secondary families, subadult male geese won an increased number of agonistic encounters due to the mere presence of their secondary family. Particularly adult and subadult females benefited from passive social support through decreased CORT, whereas males did not. Decrease in the hormonal stress response during challenging situations, induced by social allies, may help the females' long-term energy management, thereby improving the odds for successful future reproduction. We discuss whether joining a secondary family may be an alternative tactic for young geese towards optimizing their start into a complex social life.

Chemical cues of female fertility states in a non-human primate.

An increasing number of studies suggest that olfaction is important for communication throughout the order of primates. Callitrichids, in particular, have well-developed olfactory systems and use anogenital glands to produce scent marks. Behavioural studies have shown that male common marmosets (Callithrix jacchus) distinguish between odours from the peri-ovulatory and luteal phase of females. However, large gaps remain in understanding the chemical underpinnings of olfactory cues. To investigate whether chemical cues vary with female fertility and reproductive quality, our study combined behavioural bioassays with chemical analyses of the anogenital odours of female common marmosets using gas chromatography-mass spectrometry. We found that cycle states, age and parity have an impact on chemical profiles and further identified affected chemical substances. Our results confirm and expand on previous behavioural evidence for cues of fertility. Our results indicate that cycle-related substances likely act as chemical cues. Males could use such olfactory fertility cues to optimize their mating effort and thereby increase their paternity certainty. This certainty could enhance paternal care for their infants. The results of our study open a promising avenue to find the metabolic pathways from which chemical cues of fertility arise and to unravel their importance during primate evolution in future comparative studies.

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.