Josefien A. Tankink

Lethal wildlife management operations are typically evaluated through their direct effects on target species, yet their disturbance footprint may extend to non-target animals sharing the same landscape. We opportunistically used a large-scale helicopter-assisted culling event targeting ungulates at Mawana Game Reserve, South Africa, as a natural experiment to examine how such an operation affected the behaviour and space use of non-target vervet monkeys ( Chlorocebus pygerythrus ). Using GPS and tri-axial accelerometry from 21 collared individuals across seven habituated social groups, we quantified behavioural and spatial responses around the culling event, including changes in activity budgets, circadian activity patterns, short-term walking and running responses, daily space use, and habitat composition. Vervets showed fear-consistent responses that differed among behaviours, social groups, and temporal scales. Foraging declined at the population level, with the strongest reduction occurring in the morning, when helicopter and shooting activity were the most intense. Inactivity increased during culling and remained elevated afterwards, suggesting both disturbance-related change and seasonal background effects, while social behaviour showed a transient increase consistent with a possible social-buffering response. Walking and running showed little consistent change in daily averages across the culling period, but finer-scale analyses revealed short-lived deviations concentrated in specific groups and time windows. Spatially, groups remained within existing ranges but showed divergent responses, from contraction and increased use of dense vegetation to greater movement in more open areas, highlighting how population-level inference can obscure context-dependent responses when heterogeneity among groups, individuals, or habitats is not explicitly considered. Together, our findings show that lethal management can generate multiscale, context-dependent responses in non-target wildlife, and that overlooking these effects may lead to systematic underestimation of their ecological consequences.

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.

The evolution of cooperative behaviour among unrelated individuals is still incompletely understood. One example is the provision of costly services by males in primate societies, which is difficult to explain by traditional reciprocity‐based models. Here, we investigated group progressions in wild vervet monkeys ( Chlorocebus pygerythrus ) to test two main hypotheses for male‐biased positioning in potentially risky contexts: paternal care and reputation‐based partner choice. Over more than 2 years, we recorded the order and timing of individual crossings in five neighbouring groups of vervet monkeys across two terrain types differing in presumed risk (rivers and dirt roads). Adult males, particularly dominant males, were more likely to occupy leading positions than females or juveniles, and this tendency was stronger during river crossings. When leading, males often crossed alone, whereas non‐leading males usually crossed within subgroups. Leadership was most pronounced in dominant males without likely offspring. Males who did not lead did not preferentially cross with infants based on their paternal status, and higher leading rates did not predict subsequent mating success or short‐term changes in dominance rank. These findings provide no support for paternal care or direct mating benefits. Instead, male leadership in these contexts likely reflects a combination of ecological conditions, and dominance‐ and paternity‐related differences in behaviour. We conclude that male‐biased positioning in group movement is context‐dependent and may arise from multiple, potentially overlapping processes rather than a single cooperative function. Our results highlight the role of dominance and reproductive status in shaping male service provisioning and underscore its potential flexible function in primate social organisation.

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.

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 6 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 6 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.

Summary: 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. Highlights: Sex differences in life-history trajectories can arise through postnatal social development, not only sex allocation at birth; In wild female-philopatric vervets, maternal age (not rank) predicts offspring sex ratios, with older females producing more daughters; Maternal rank shows stronger associations with daughters’ survival and social engagement, while sons were potentially more dependent on maternal presence; Maternal investment differs in form rather than magnitude, consistent with role-specific developmental preparation.

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.

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.