Biology subjects

van de Waal, E.

Publications and source records attributed to van de Waal, E..

16 recordsLinked to original sources

bioRxiv2026

Quantifying direct and indirect maternal effects on male vervets rank post-dispersal

Early life experiences have important fitness consequences across animals, with maternal environments playing a crucial role in some species survival and reproduction during adulthood. A potential pathway linking maternal behaviour with individual fitness is dominance rank, which in some primates - like cercopithecines - is maternally inherited during the juvenile period and strongly linked to individual social behaviour and reproductive success. While females of these species maintain their rank for life in their natal group, males disperse and must integrate into new dominance hierarchies. In this context, the long-term effects of maternal characteristics on male rank post-dispersal and the potential of males social behaviour in mediating this relationship remains unexplored. Using 16 years of data from a wild vervet monkey population (Chlorocebus pygerythrus), we tested the effects of maternal rank, grooming and proximity on the rank males acquired their first year post-dispersal. In addition, we measured the extent to which the males own social behaviour both pre- and post-dispersal mediate this relationship combining Bayesian social networks with mediation analyses. We did not find support for a mediating effect of males social behaviour on the relationship between maternal characteristics and male rank post-dispersal. Instead, we found that maternal characteristics influence males social behaviour during the juvenile period and that males own social behaviour post-dispersal (grooming out-degree) is related to their rank when they integrate into a new group. In addition, we found that consistency in males social behaviour pre- to post-dispersal is restricted to grooming behaviour, highlighting the different roles of proximity and grooming relationships in vervet societies. Taken together these results reveal very limited long-term effects of maternal characteristics on male behaviour during the heavily underexplored process of dispersal and the flexibility of male vervet social behaviour following a change in social environment.

animal behavior and cognition↗

bioRxiv2026

Between-group conflict participation is associated with lower long-term cortisol in wild vervet monkeys

Male-biased cooperative behaviours that produce public goods (male services) are widespread in primates. Recent behavioural studies suggest that male services may function beyond paternal investment, either as displays of competitive ability or as honest signals eliciting female choice. Because these hypotheses make contrasting predictions about the physiological state of service providers, endocrine measures offer a complementary approach for distinguishing between them. We investigated whether long-term testosterone and cortisol concentrations, measured from hair samples, were associated with four male services in wild vervet monkeys: alarm calling, participation in between-group conflicts (BGCs), leading river crossings, and sentinelling. Testosterone and cortisol concentrations increased with male competitive status, measured as a combination of dominance rank and recent rank trajectory, particularly during the mating period, establishing the endocrine profile associated with competition. In contrast, most male services showed few relationships with hormone concentrations: participation in BGCs was associated with lower cortisol concentrations and, to a lesser extent, lower testosterone concentrations. Because BGC participation is the strongest predictor of male mating frequency in this population, these findings indicate that males contributing to BGCs present an endocrine profile distinct from that associated with competitive state and instead support the prediction that high-quality males, irrespective of their dominance rank, can perform costly cooperative behaviours while maintaining relatively low physiological costs. More broadly, our results provide physiological support for reputation-based partner choice as a mechanism contributing to the maintenance of male services in primates.

physiology↗

bioRxiv2026

Beyond paternal care: career stage and reproductive opportunities shape male services in vervet monkeys

Why male primates invest in costly behaviours producing public goods remains debated, with two leading explanations, paternal care and reputation-based partner choice (RBPC). Using long-term data from four groups of wild vervet monkeys, we tested: (1) whether males show a bias in four protective "male services" (predator alarm calling, participation in between-group conflicts, leading river crossings and sentinelling); (2) which males contribute most; and (3) whether service provision predicts mating success during the mating season. We confirmed a male bias in all services. Consistent with the paternal care hypothesis, contributions were positively associated with past mating success, independently of rank, although potential fathers did not contribute more than non-fathers. Among non-fathers, service provision varied with rank, suggesting that newly immigrated males adjust their behaviour according to competitive state. Crucially, variation in alarm calling and between-group conflicts predicted future mating success, with between-group conflict emerging as the strongest and most consistent predictor of mating success across years and within mating seasons, whereas rank, tenure and social integration added little explanatory power. In contrast, sentinelling and leading river crossings did not reliably translate into mating benefits. Our findings indicate that male services are shaped by multiple selective pressures operating across different male career stages and that some forms of public goods provision function as signals of quality and cooperativeness to females. By directly linking cooperative investment to mating outcomes in a wild primate, this study provides rare empirical support for reputation-based partner choice beyond humans and highlights female choice as a potentially important force in the evolution of cooperation.

animal behavior and cognition↗

bioRxiv2026

Who takes the risk to forage? Testing consistent inter-individual behavioural variation in wild vervet monkeys (Chlorocebus pygerythrus)

Risk-taking behaviour permeates daily decision-making across many taxa and has significant impact on fitness outcomes. Previous research finds that individuals display consistent tendencies in their propensity to take risks, often labelled as a "Boldness-Shyness" personality trait. In this experimental study, we investigated repeatability in risk-taking behaviour in 241 wild vervet monkeys by measuring their responses to capture threat (via human-initiated darting procedure) and predation threat (via two predator models). We further examined the influence of socio-demographic, group identity and hormonal factors on risk-taking behaviour. We found that multiple behavioural responses, including the likelihood of approaching a food source under threat, were consistent across contexts. Risk-taking behaviour was influenced by age, sex, dominance rank, and hormonal profiles: juveniles, males, and higher-ranking individuals were more likely to approach food under (perceived) predation risk. Additionally, we observed significant among-group differences, suggesting that individuals within the same social group exhibit similar risk sensitivities, potentially due to social facilitation or shared environmental effects. The findings contribute to our understanding of animal personality and the ecological and social drivers underlying variation in risk-related behaviours.

animal behavior and cognition↗

bioRxiv2026

Which senses do wild vervet monkeys (Chlorocebus pygerythrus) use for evaluating potential food items?

Traditionally, primates have been considered primarily visual animals. However, studies across a variety of taxa suggest that, in the context of food evaluation, the reliance on this sense might be more nuanced that previously thought, with dietary specialization and food item properties leading to differences in sensory prioritization. We performed a field-based study assessing the use of sensory cues during food evaluation as well as food-related behaviours such as muzzle contact in two mixed-sex groups of wild vervet monkeys including three age classes over a period of five months (nmonkeys = 44). Using a total of 18868 food evaluation observations collected over 44 hours of focal follows, we found that vervets mainly relied on their sense of vision when evaluating food (96.8% of all instances). Sensory usage varied according to food category and sex differences were only observed in the use of smell for a subset of these. Juveniles initiated muzzle contact and used tactile inspection more often than adults whereas females received muzzle contact more often than males. In addition, the low rejection rates suggest that most food items were familiar to the vervets regardless of age and sex. These findings are in line with optimal foraging theory according to which the food evaluation process should be adapted to the familiarity of food items and allows individuals to maximize their intake of energy and critical nutrients, while minimizing the time and effort in food evaluation.

animal behavior and cognition↗

bioRxiv2026

Classifier architecture and data preprocessing jointly shape accelerometer-based behavioural inference

O_LIAlthough the use of accelerometer-based behavioural classification to quantify animal activity budgets is gaining widespread traction, the interactions between key preprocessing decisions and modern classification algorithms remain poorly understood. Moreover, classification pipelines are commonly assessed using global performance metrics, despite increasing evidence that such metrics poorly reflect behaviour-specific patterns and ecological reliability. C_LIO_LIUsing a free-ranging primate (Chlorocebus pygerythrus) as a case study, we benchmarked how temporal segmentation (burst length), collar orientation correction, and model architecture jointly shape behavioural inference. We compared nine supervised algorithms spanning classical machine learning, feature-based deep learning including a tabular foundation model (TabPFN), and state of the art time-series architectures (HydraMultiROCKET). Beyond conventional metrics, performance was further evaluated using ecological validation against independent focal observations to assess model stability and biological plausibility. C_LIO_LIModel architecture exerted the strongest influence on classification outcomes. Modern deep-learning approaches substantially outperformed classical models, doubling recall for rare behaviours (e.g., grooming, self-scratching) without compromising precision. In contrast, burst length and collar orientation correction had little effect on global metrics but produced substantial, behaviour-specific trade-offs. Shorter bursts improved the detection of rare events by increasing training instances, while orientation correction suppressed dataset-specific artifacts at the cost of degrading common behaviours. Crucially, models with similar global and behaviour-level validation metrics produced divergent predictions when applied outside the annotated context. C_LIO_LIOur findings reveal that global metrics are insufficient for optimizing behavioural inference in complex wild systems. We demonstrate that modern deep-learning architectures, such as the ROCKET family, provide a robust, accessible baseline that handles class imbalance more effectively than traditional methods. We propose that reliable inference requires behaviour-aware evaluation frameworks that integrate ecological validation, and advocate for ensemble or hierarchical strategies to leverage the complementary strengths of different preprocessing and modelling configurations. C_LI

animal behavior and cognition↗

bioRxiv2026

Mothers letting go: postnatal maternal investment shapes sex-specific social development in wild vervet monkeys

Sex differences in behaviour and life-history trajectories are widespread across species, yet the mechanisms through which mothers shape these differences remain poorly understood. Classic theories emphasize sex-biased allocation at birth or differential energetic investment, but how maternal effects might operate instead through postnatal investment remains understudied. Using long-term demographic and behavioural data from female-philopatric vervet monkeys (Chlorocebus pygerythrus), we examined how maternal age and dominance rank influence offspring sex ratios at birth, survival to adulthood, maternal investment, and offspring social integration in both sons and daughters. Maternal age, but not rank, influenced offspring sex ratios, with older females producing more daughters. Maternal rank was positively associated with daughters survival and social engagement, with estimated effects consistently stronger in daughters than in sons. While both sexes were highly vulnerable to maternal loss, post-hoc trends suggested a potentially steeper effect on sons. Sons received more maternal proximity (under some maternal conditions) and maternal grooming, whereas daughters seemed to gain earlier and greater engagement with other group members and appeared to derive indirect advantages from maternal rank through social exposure. Together, these findings indicate that maternal investment in this species differs in form rather than in magnitude, primarily through postnatal developmental pathways rather than biased allocation at birth. By demonstrating how maternal age and social status shape divergent early-life trajectories, our study highlights the role of early social environments in generating sex-specific life histories. HighlightsO_LISex differences in life-history trajectories can arise through postnatal social development, not only sex allocation at birth; C_LIO_LIIn wild female-philopatric vervets, maternal age (not rank) predicts offspring sex ratios, with older females producing more daughters; C_LIO_LIMaternal rank shows stronger associations with daughters survival and social engagement, while sons were potentially more dependent on maternal presence; C_LIO_LIMaternal investment differs in form rather than magnitude, consistent with role-specific developmental preparation. C_LI

evolutionary biology↗

bioRxiv2025

Leadership under risk: male vervet monkey's roles in group progression across high-risk terrain

The evolution of cooperative behaviour among unrelated individuals is still incompletely understood. One example is the reported service provisioning by males in primate societies, which is difficult to explain by traditional reciprocity-based models. In this study, we investigated group progressions and sex roles in wild vervet monkeys (Chlorocebus pygerythrus) to test two hypotheses for the observed male bias in cooperative and protective behaviours: paternal care versus reputation-based partner choice. Over more than two years, we recorded the order and timing of individual progressions in five neighbouring groups of vervet monkeys across two terrain types differing in risk (rivers and roads). Males, particularly dominant males, were more likely to lead than females or juveniles, and their probability of leading increased in high-risk terrain. When leading, males often crossed alone, thereby assuming higher predation risk, whereas when not leading, males usually crossed within subgroups. Dominant males who had no offspring yet were most likely to lead during risky progressions. Males who did not lead did not preferentially cross with infants based on their paternal status, refuting the paternal care hypothesis. Instead, male leadership in risky contexts appears to represent a cooperative service. While some of these findings are consistent with the reputation-based partner choice model, we found no clear evidence that such services translated into direct mating advantages. Overall, our results highlight the role of dominance and reproductive status in shaping male service provisioning and underscore its potential adaptable function in primate social organisation.

evolutionary biology↗

bioRxiv2025

Uncovering variation in social tolerance among wild vervet monkeys through a novel co-feeding paradigm

Social tolerance, defined as the probability of individuals maintaining proximity with minimal aggression, is essential for the functioning of social groups. This study investigates social tolerance in wild vervet monkeys (Chlorocebus pygerythrus), with a focus on co-feeding tolerance. Previous research has highlighted that social tolerance varies both between and within species, influenced by stable group-specific and temporally variable factors. However, it remains unclear how these differences manifest in wild primates, particularly among vervet monkeys. To address this, we employed a novel experimental co-feeding paradigm adapted for field conditions, presenting a fake grass carpet with a fixed density of corn and a plot area proportional to group size to measure group-level co-feeding tolerance. The study was conducted with two habituated groups of wild vervet monkeys at the iNkawu Vervet Project in South Africa. Results revealed significant group differences in co-feeding tolerance despite similarities in the groups demographics and environments. The presence of mothers with infants was associated with higher levels of co-feeding tolerance, though this effect differed by group. These findings highlight the need for further research to elucidate the factors driving group-specific social tolerance levels and the mechanisms behind these differences.

animal behavior and cognition↗

bioRxiv2025

Making friends in an asymmetric game: the establishment of male-female grooming exchanges in vervet monkeys

An important challenge for individuals of group-living species is to build cooperative relationships with new partners. One famous strategy for doing so is "raise-the-stakes", proposing low initial investments that increase if the partner matches these investments in a series of exchanges. Contrarily, the "all-in" strategy predicts high initial investment that may be maintained, but downregulated if not fully matched. We tested these predictions on grooming exchanges between wild male-female vervet monkeys, a species with regular male dispersal. Contrary to model predictions, we found uneven initial investment between novel partners that reached near reciprocity after approximately six months. To identify which sex altered investment, we examined grooming durations and frequencies of novel and established partners. Females showed consistently high initial investment and gradually reduced grooming over the first six months. Males did not alter grooming duration but had higher grooming frequency after a year of residency. Eventually, grooming exchanges nearly evened out. Female behaviour aligns more closely with the "all-in" strategy, which suggests competition among female groups over dispersing males. Male behaviour partly fits the "raise-the-stakes" hypothesis. The study highlights the complexity of real-life cooperation, emphasizing the need to explicitly incorporate life history parameters to understand cooperative strategies.

evolutionary biology↗

bioRxiv2024

When monkeys meet an ANYmal robot in the wild

Animal-robot interaction studies have been of increasing interest in research, but most of these studies have involved robots interacting with insects, birds, and frogs in laboratory settings. To date, only two studies used non-human primates and no behavioral study has tested the social integration of a robot in a group of wild primates. To fill this gap, we studied the interactions between the quadruped ANYmal robot and a group of 37 wild vervet monkeys in South Africa. The ANYmal robot is a remote-controlled sheep-sized robot with an open box of food on its back. We gradually introduced the robot to the monkeys following five different steps over 6 days for a total exposition time of about 10h. The monkeys habituated to the robot very quickly with six individuals eating the food in the robots box from the second day. A few individuals, mostly juveniles, emitted alarm calls towards the robot. In total, seven individuals, high rankers, spent time near the robot and 21 monkeys approached the robot from a greater distance. High rankers displayed significantly more vigilant and self-centered behaviors and they, with females and juveniles, ate more food in the robots box compared to low rankers, males, and adults conversely. This study offers exciting perspectives on the phenomena of social acceptance of machines in mammalian societies and the automation of field data collection.

animal behavior and cognition↗

bioRxiv2024

Anti-snake behavior and snake discrimination in vervet monkeys

Anti-predator behavior allows to investigate how animals classify potential threats in their environment and which cognitive mechanisms might be involved in risk assessment. Snakes are common predators for many primate species, yet most of our knowledge on primate anti-snake behavior stems from predator model experiments. Few studies have investigated natural predator-prey interactions. Here, we combine an observational study on anti-snake behavior in free-ranging vervet monkeys, Chlorocebus pygerythrus, with an experimental study of snake classification, to achieve an integrated understanding of the monkeys responses to and classification of snakes. Over 25 months, we gathered data from over 200 individuals in five groups and recorded natural encounters between vervet monkeys and five different species of snakes. We assessed whether the time that monkeys spent inspecting snakes from close by varied with the encountered species. We further examined whether age and sex classes differed in their propensity to inspect snakes or alarm call. Pythons were associated with higher average inspection durations and were more likely to elicit inspection responses. While adult males were less likely to approach and inspect snakes, we found no effect of age or sex on alarm calling probability. Alarm calling appeared to be restricted to individuals in close proximity to snakes, suggesting that recruitment of nearby group members is an essential function of this call type. In the experiments, we tested whether vervet monkeys discriminated snake models by length and/or diameter, but found no effect of model size on the intensity of anti-snake behavior. While the experimental data showed the same trends as data from natural encounters, responses towards model pythons were much stronger than responses towards real pythons. These results point to a potential sampling bias in predator model experiments that needs to be taken into account when assessing data from experiments alone.

animal behavior and cognition↗

bioRxiv2023

Alarm barks of male vervet monkeys as displays of male quality

Research on the vocal behaviour of non-human primates is often motivated by a desire to understand the origins of semantic communication, which led to a partial separation of this research from ecological-evolutionary approaches. To bridge this gap, we returned to the textbook example of semantic communication in animals, the vervet monkey, Chlorocebus pygerythrus, alarm call system, and investigated whether male alarm barks fulfil a dual function of alarming and indicating male quality. Barks are loud calls, produced by adult males, in response to large carnivores. However, since barks occasionally occur in agonistic interactions, we investigated whether barks may also indicate male quality. We recorded natural barking events over 23 months, sampling individual male participation from 45 individuals in six free-ranging groups at the Mawana Game Reserve, KwaZulu-Natal, South Africa. We hypothesized that barking frequency is under intra-sexual selection and predicted that barking frequency would increase with male rank and the degree of male-male competition. We found that the highest-ranking males were more likely to produce barks than lower-ranking males and that the number of daily barking events increased during the mating season. We advocate studying primate communication in its evolutionary context to achieve a comprehensive understanding of call "meaning".

animal behavior and cognition↗

bioRxiv2023

Social dynamics of vervet monkeys are dependent upon group identity.

Traditions are widespread across the animal realm. Here, we investigated inter-group variability of social dynamics in wild vervet monkeys (Chlorocebus pygerythrus). We analysed 84 704 social behavioural interactions involving 247 individuals collected over nine years in three neighbouring groups of wild vervet monkeys. We found that, in one group - Ankhase - individuals had a higher propensity to be affiliative (i.e., sociality) and grooming interactions were more reciprocal. Despite yearly fluctuations in sociality, differences between groups remained stable over time. Moreover, our statistical model predictions confirmed that these findings were maintained for similar sex ratios, age distributions and group sizes. Strikingly, our results suggested that dispersing males adapted their sociality to the sociality of the group they integrated with. As a whole, our study sheds light on the existence of stable social dynamics dependent upon group identity in wild vervet monkeys and suggests that at least part of this variability is socially mediated. Graphical Abstract HighlightsO_LIThe sociality of vervet monkeys groups differs consistently across a nine years study despite similar genetic and ecological environments. C_LIO_LIDispersing males adapt their sociality according to the group they integrate with. C_LIO_LIIn the most social group, grooming interactions were more reciprocal. C_LI

animal behavior and cognition↗

bioRxiv2021

Role of immigrants and muzzle contacts in the uptake of a novel food by wild vervet monkeys

The entry into and uptake of information in social groups is critical for behavioural adaptation by long-lived species in rapidly changing environments. We exposed five groups of wild vervet monkeys to a novel food to investigate innovation of processing and consuming it. We report that immigrant males innovated in two groups, and an infant innovated in one group. In two other groups, immigrant males imported the innovation from their previous groups. We compared uptake between groups with respect to the initial innovator to examine the extent to which dispersing males could introduce an innovation into groups. Uptake of the novel food was faster in groups where immigrant males ate first rather than the infant. Younger individuals were more likely overall, and faster, to subsequently acquire the novel food. We also investigated the role of muzzle contact behaviour in information seeking around the novel food. Muzzle contacts decreased in frequency over repeated exposures to the novel food. Muzzle contacts were initiated the most by naive individuals, high rankers and juveniles; and were targeted most towards knowledgeable individuals and high rankers, and the least towards infants. We highlight the potential importance of dispersers in rapidly exploiting novel resources among populations.

animal behavior and cognition↗

bioRxiv2020

Processing of novel food reveal payoff and rank-biased social learning in a wild primate

Social learning - learning from others - is the basis for behavioural traditions. Different social learning strategies (SLS), where individuals biasedly learn behaviours based on their content or who demonstrates them, may increase an individuals fitness and generate behavioural traditions. While SLS have been mostly studied in isolation, their interaction and the interplay between individual and social learning is less understood. We performed a field-based open diffusion experiment in a wild primate. We provided two groups of vervet monkeys with a novel food, unshelled peanuts, and documented how three different peanut opening techniques spread within the groups. We analysed data using hierarchical Bayesian dynamic learning models that explore the integration of multiple SLS with individual learning. We i) report evidence of social learning compared to strictly individual learning, ii) show that vervets preferentially socially learn the technique that yields the highest observed payoff and iii) also bias attention toward individuals of higher rank. This shows that behavioural preferences can arise when individuals integrate social information about the efficiency of a behaviour alongside cues related to the rank of a demonstrator. When these preferences converge to the same behaviour in a group, they may result in stable behavioural traditions.

animal behavior and cognition↗