Rates of agonism among female primates: a cross-taxon perspective

Disputes over provisioned resources are no more intense between groups than within groups in free-ranging Sapajus libidinosus

Socioecological models predict that disputes between primate groups will be more intense than those within groups, given that the systematic loss of contests over a given resource will restrict the access of all of the members of that group to that resource. Higher levels of aggression are also expected for provisioned resources that have a more lucrative cost:benefit ratio. The levels of aggression in and between two free-ranging tufted capuchin monkey (Sapajus libidinosus) groups in the context of daily provisioning with bananas were evaluated. The aim of a complementary analysis was to identify possible predictors of the frequency of disputes at the site of the provisioned resource. The disputes were recorded using all-events sampling, while the social behaviour of the study groups was recorded by instantaneous scan sampling. The data were analysed using t-test, Mann-Whitney's U, and generalised linear modelling. Between-group disputes were no more intense than within-group events, and did not involve more individuals, or more adult females. The frequency of disputes increased as the number of individuals eating bananas increased. No evidence was found that disputes between groups were any more intense than those within groups. Dominance patterns may have affected these findings, by mediating intergroup disputes. An increase in the number of competitors affected the frequency of disputes at the site of the provisioned resource.

Do food distribution and competitor density affect agonistic behaviour within and between clans in a high fission-fusion species?

According to the ecological model of female social relationships (EMFSR), within-group competition and between-group competition in female-bonded species are shaped by food distribution. Strong between-group contests are expected over large, monopolizable resources and high population density, but not when low-quality food is distributed across large, undefended home ranges. Within-group contests are expected to be more frequent with increasing heterogeneity among feeding sites and with group size. We tested these predictions in female Asian elephants, which show traits associated with infrequent contests-graminivory, high fission-fusion and overlapping home ranges. We examined how food distribution and competitor density affected agonistic interactions within and between female elephant clans (social groupings) in the Kabini grassland, southern India. We found stronger between-clan contest in the grassland than that known from neighbouring forests, and more frequent agonism between females between clans than within clans. Such strong between-clan contest was attributable to the grassland being a food-rich habitat patch, thus supporting the EMFSR. Within-clan agonism was also frequent, but did not increase with food heterogeneity, contradicting the EMFSR. Contrary to recent claims, increasing within-clan agonism with group size suggested ecological constraints on large groups despite high fission-fusion. High population density may explain such frequent contests despite graminivory and fission-fusion.

Patch choice decisions by a fission–fusion forager as a test of the ecological constraints model

Abstract

Chapman et al.’s (Behav Ecol Sociobiol 36:59–70, 1995) ecological constraints model posits that the size and distribution of food patches place restrictions upon foraging group size. Larger groups incur increased travel costs for any given array of patches, and thus, to fulfil individual energetic and nutritional requirements, foragers should adjust group sizes to balance energy obtained against that spent on travelling. Support for this model comes from both comparative and species-specific studies but findings are contradictory, and the utility of the model has been questioned. This study provides a rigorous test, analysing measurements from distinct food patches and individual inter-patch movements, on an appropriately shorter temporal scale. Using data drawn from two social groups of a species characterised by a high degree of fission–fusion dynamics, the chimpanzee (Pan troglodytes), we show that larger parties foraged in larger food patches and for longer durations, and that larger parties were associated with further travel between patches. Overt contest competition over food increased with party size. We found no evidence of distinct sex differences in either party size or travel distances: the predictive power of forager sex was low compared to that of ecological variables. We propose that analysis at the patch level is more appropriate than a daily averaging approach that may smooth out the very variation being investigated. Our findings suggest that, despite certain limitations, Chapman et al.’s (Behav Ecol Sociobiol 36:59–70, 1995) model of ecological constraints remains a useful tool. Ecology does indeed constrain grouping patterns, and the impact of this is not necessarily differentiated by sex.

Significance statement

Foraging animals face the ‘more mouths to feed’ problem: as the numbers in a group increase, the group must travel further to find enough food, using up energy. Hence, foragers should adjust numbers to minimise these costs, but tests of this idea have proved inconclusive. We investigated the foraging behaviour of chimpanzees, a species with highly flexible grouping, considering their travel between specific patches of food. We found clear support for this proposition, with larger patches of food hosting larger numbers of foragers, and such groups having to travel further to find food. Although it is often thought that female animals should respond more strongly to foraging costs, we found little evidence of sex differences. Our results show that ecology does indeed constrain grouping patterns, and that the impact is felt equally by males and females.

Where do we stand with the covariation framework in primate societies?

Comparative study of the social systems of macaques has revealed correlated variations between species in multiple social traits such as the asymmetry of dominance relationships, preferential treatment of kin, patterns of aggression and reconciliation, modes of socialization, and access to food resources. Macaques can be classified on a scale of four categories of social styles, ranging from the least to the most tolerant species. This led to the development of the covariation framework, which addresses the constraints responsible for the linkages between social traits, and their consequences on the evolution of primate social systems. Decades of research have provided a wealth of information that supports, complements, expands, or challenges the covariation framework. In this article, I review this body of knowledge and consider covariation in its two aspects, that is, as a pattern and as a hypothesis. I first consider the extent to which social styles can be invariant, the strength of correlations between traits, and the possible extension of the framework to nonhuman primates other than macaques. I then discuss how to formulate hypotheses, identify sources of linkage between traits, make predictions about the effects of social constraints, assess the tolerance dimension of social styles, and consider the breaking of linkages between traits. Whereas socioecological studies aim to understand how adaptation to the ecological environment determines the shape of social systems, the covariation framework is a complementary research program that seeks to unravel the internal processes that restrict or channel change in social behavior.

Competitive gossip: the impact of domain, resource value, resource scarcity and coalitions

Those with better reputations often obtain more resources than those with poorer reputations. Consequently, gossip might be an evolved strategy to compete for valuable and scarce material and social resources. Influenced by models of non-human primate competition, we test the hypotheses that gossip: (i) targets aspects of reputation relevant to the domain in which the competition is occurring, (ii) increases when contested resources are more valuable, and (iii) increases when resources are scarcer. We then test hypotheses derived from informational warfare theory, which proposes that coalitions strategically collect, analyse and disseminate gossip. Specifically, we test whether: (iv) coalitions deter negative gossip, and (v) whether they increase expectations of reputational harm to competitors. Using experimental methods in a Mechanical Turk sample (n = 600), and survey and ego network analysis methods in a sample of California sorority women (n = 74), we found that gossip content is specific to the context of the competition; that more valuable and scarcer resources cause gossip, particularly negative gossip, to intensify; and that allies deter negative gossip and increase expectations of reputational harm to an adversary. These results support social competition theories of gossip. This article is part of the theme issue 'The language of cooperation: reputation and honest signalling'.

Reproductive males are effective at managing conflict in captive Sulawesi crested macaques (Macaca nigra)

Group size, density, and composition significantly influence the expression of agonism and, as demography is frequently manipulated in captivity, natural mechanisms of conflict management may be disrupted. Here, we examine how changes to the social and physical environment of a captive group of Sulawesi crested macaques, Macaca nigra, influenced mechanisms of conflict management. The initial group was comprised of females, juveniles, and castrated males; under a series of management interventions, the group was moved to a new enclosure, 75% of the castrated adult males were removed, 67% of the females were contracepted, a novel, intact male was introduced, and the group was allowed access to an outdoor enclosure. Despite a decreasing trend in aggression following the changes to the social group, we found no significant differences in aggression or intervention behavior in the 5-week periods immediately following changes to the social group, apart from a significant decrease in aggression following the introduction of the novel male. This decrease in the frequency of aggression was still evident 10 weeks after the interventions occurred, and was coupled with a decrease in the intensity of aggression. Moreover, the intensity of aggression as well as the frequency of aggression and intervention were significantly lower under low-density conditions. Our results highlight how management changes to the social and physical environment can influence aggressive behavior, albeit following a period of acclimation. We discuss the relative impacts of social group composition, social density, and individual reproductive status on the management of conflict behavior in a captive setting.

Prenatal Androgen Effects as a Proximate Mechanism Underpinning Variation in Social Behavior Among Female Nonhuman Primates

While the role of ecological factors in shaping primate social systems has been a central focus for decades, less attention has been given to phylogenetic relationships and the potential role of underlying proximate mechanisms. This study aimed to investigate the relationship between one such proximate mechanism, prenatal androgen effects (PAEs), and aspects of social behavior in female nonhuman primates using the 2D:4D ratio as a proxy for PAEs and phylogenetically controlled methods. In general, female 2D:4D ratios were highest in monogamous species (low inferred PAEs) and lowest in polygynandrous and polygynous species (high inferred PAEs). 2D:4D ratios also varied with the form of polygyny/polygynandry, potentially with regard to the need for competitive over cooperative behaviors and the intensity of female reproductive competition. Species characterized by female dominance had lower 2D:4D ratios than species characterized by male dominance or codominance. There were no significant relationships between 2D:4D ratio and either degree of frugivory or group size. Relationships between 2D:4D ratios and the directional consistency index and 2D:4D ratios and rates of female–female agonism were also nonsignificant although sample sizes for both of these variables were small. Female social relationships are a manifestation of complex competitive and cooperative behaviors and the results suggest that PAEs may act as a proximate mechanism underlying the expression of certain aspects of behavior in female primates in ways that are adaptive to their social system.

The costs of competition: injury patterns in 2 Asian colobine monkeys

Aggression rarely escalates to physical conflict because doing so puts individuals at risk of injury. Escalation only pays off when the potential benefits outweigh the potential costs, that is, when resources critical to fitness are at stake. Here, we investigated the occurrence of injury in 2 Asian colobine species: Nepal gray langurs (Semnopithecus schistaceus) and Phayre’s leaf monkeys (Trachypithecus phayrei crepusculus). In both species, younger individuals are higher-ranking and might have greater incentive to fight. However, Nepal gray langurs have a strict breeding season, which may magnify male mating competition, and Phayre’s leaf monkeys, unlike Nepal gray langurs, have female-biased dispersal, which may increase female injury risk during subadulthood. Using long-term data on observed injuries (Nepal gray langurs: n = 208; Phayre’s leaf monkeys: n = 225), we modeled the monthly occurrence of injury (Y/N) and found that males received more injuries than females in both species. Also, subadults generally experienced frequent injury, as young individuals likely face challenges when competing for group membership and/or establishing rank. In Nepal gray langurs, males received 3 times more injuries during the mating season, suggesting strong competition for mates during this period, and females experienced more injuries before conception, suggesting competition to meet the nutritional requirements for reproduction. Unexpectedly, females in smaller groups received more injuries in Nepal gray langurs. Overall, these results indicate that injuries are most likely when fighting may aid in establishing group membership, achieving high rank, and reproducing. Future research should investigate the influence of injuries on fitness outcomes.

Quadratic relationships between group size and foraging efficiency in a herbivorous primate

The effect of feeding competition on foraging efficiency is an important link between ecological factors and the social organization of gregarious species. We examined the effects of group size on daily travel distances, activity budgets, and energy intake of mountain gorillas in Rwanda. We measured daily travel distances of five groups, activity budgets of 79 gorillas in nine groups, and energy intake data for 23 adult females in three groups over a 16-month period. Travel distances and the proportion of time spent traveling increased with size for most groups, which would be expected if their foraging efficiency is limited by intragroup feeding competition. However, travel distances and times decreased for the largest group, which also had higher energy intake rates than intermediate sized groups. The improved foraging efficiency of the largest group may be explained by advantages in intergroup contest competition. The largest group had much lower home range overlap than the other study groups which may be due to groups avoiding one another as a result of male mating competition. Collectively, our results indicate that intermediate sized groups had the lowest foraging efficiency and provide a new twist on the growing evidence of non-linear relationships between group size and foraging efficiency in primates.

The role of intragroup agonism in parent-offspring relationships and natal dispersal in monogamous owl monkeys (Aotus azarae) of Argentina

Agonistic behaviors are common in many group-living taxa and may serve a variety of functions, ranging from regulating conflicts over reproduction to defending food resources. However, high rates of agonism are not expected to occur among close relatives or individuals in established mating relationships, which are characteristics of monogamous groups. To contribute to our understanding of agonism within socially monogamous groups, we collected behavioral and demographic data from Azara's owl monkeys (Aotus azarae) in the Gran Chaco of Argentina over 14 years. We examined factors related to age, sex, kinship, and behavioral context to evaluate predictions of the hypotheses that agonism functions to regulate dispersal and that it mediates competition for food and/or mates. Intragroup agonism was relatively rare: the group rate was approximately one event every three and a half hours. Rates of agonism were generally similar for both sexes, but there were marked differences among age categories. Agonism performed by adults was more frequently directed at subadults than at younger offspring. In contrast, agonistic interactions involving infants were very rare. Among interactions between adults and subadults, adults were much more frequently the actors than the recipients, suggesting that agonism from adults may influence natal dispersal of subadults. Agonistic events were most frequent during foraging, but also occurred more frequently than expected during bouts of social behavior. Overall, our results suggest that agonism in owl monkeys serves as a mechanism for regulating dispersal, and also likely plays a role in mediating mating and feeding competition.

Sex roles and adult sex ratios: insights from mammalian biology and consequences for primate behaviour

Theoretical models and empirical studies in various taxa have identified important links between variation in sex roles and the number of adult males and females (adult sex ratio (ASR)) in a population. In this review, I examine these relationships in non-human primates. Because most existing theoretical models of the evolution of sex roles focus on the evolutionary origins of sex-biased behaviour, they offer only a general scaffold for predicting variation in sex roles among and within species. I argue that studies examining sex role variation at these more specific levels need to take social organization into account to identify meaningful levels for the measurement of ASR and to account for the fact that ASR and sex roles mutually influence each other. Moreover, taxon-specific life-history traits can constrain sex role flexibility and impact the operational sex ratio (OSR) by specifying the minimum length of female time outs from reproduction. Using examples from the primate literature, I highlight practical problems in estimating ASR and OSR. I then argue that interspecific variation in the occurrence of indirect forms of paternal care might indeed be linked to variation in ASR. Some studies also indicate that female aggression and bonding, as well as components of inter-sexual relationships, are sensitive to variation in ASR. Thus, links between primate sex roles and sex ratios merit further study, and such studies could prompt the development of more specific theoretical models that make realistic assumptions about taxon-specific life history and social organization.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

What cortisol can tell us about the costs of sociality and reproduction among free-ranging rhesus macaque females on Cayo Santiago

Research with the rhesus macaque population on Cayo Santiago can provide a unique perspective on the costs of sociality and reproduction in primates. Because the Cayo macaques live in unusually large groups and in a predator-free environment, in which their artificial food source lacks seasonal variation in abundance or quality, these monkeys constitute a semi-experimental study of the costs and benefits of group living. Here we review several long- and short-term studies that have focused on female life history and stress physiology. Long-term demographic data have shown that rhesus macaque females of middle- and low-ranking matrilines have lower adult survival probabilities than females of high-ranking matrilines. Costs of reproductive effort are also evident: adult females were more likely to die during the birth than during the mating season and they experienced higher cortisol levels when lactating. Lower-ranking females, in particular, experienced greater relative increase in cortisol production during lactation, in comparison to middle- and high-ranking females. Older high-ranking females had lower plasma cortisol levels than younger ones but cortisol levels were similarly high among young and old middle- and low-ranking females. Higher plasma cortisol levels and/or fecal glucocorticoid concentrations are associated with higher plasma concentrations of some proinflammatory cytokines. High cortisol, in turn, may be associated with chronic inflammation, and perhaps also with immunosuppression. In sum, the studies reviewed here provide multiple lines of evidence that sociality and reproductive effort impose measurable costs on female rhesus macaques. In line with socio-ecological theory, female dominance rank consistently emerges as an important modulator of variation in female life histories and physiology. The Cayo Santiago macaques are therefore a valuable model for elucidating the mechanisms by which within-group competition and reproduction impact health and survival in nonhuman primates and in humans.

The Role of Bioacoustic Signals in Koala Sexual Selection: Insights from Seasonal Patterns of Associations Revealed with GPS-Proximity Units

Despite being a charismatic and well-known species, the social system of the koala (Phascolarctos cinereus--the only extant member of the family Phascolarctidae) is poorly known and much of the koala's sociality and mating behaviors remain un-quantified. We evaluated these using proximity logging-GPS enabled tracking collars on wild koalas and discuss their implications for the mating system of this species. The frequency and duration of male-female encounters increased during the breeding season, with male-male encounters quite uncommon, suggesting little direct mating competition. By comparison, female-female interactions were very common across both seasons. Body mass of males was not correlated with their interactions with females during the breeding season, although male size is associated with a variety of acoustic parameters indicating individuality. We hypothesise that vocal advertising reduces the likelihood of male-male encounters in the breeding season while increasing the rate of male-female encounters. We suggest that male mating-season bellows function to reduce physical confrontations with other males allowing them to space themselves apart, while, at the same time, attracting females. We conclude that indirect male-male competition, female mate choice, and possibly female competition, mediate sexual selection in koalas.

Feeding behavior and aggression in wild Siberut macaques (Macaca siberu) living under low predation risk

Investigating which factors influence feeding competition is crucial for our understanding of the diversity of social relationships. Socio-ecological models differ in their predictions whether predation risk directly influences feeding competition and which factors exactly predict contest competition. We investigated feeding competition in Siberut macaques (Macaca siberu), a species endemic to Siberut Island (West Sumatra, Indonesia). Siberut macaques experience low predation risk, as major predators (felids, raptors) are absent. They are therefore appropriate subjects to test the prediction that low predation risk reduces feeding competition. To estimate contest potential, we quantified size, spatial distribution and density of food plants, and the availability of alternative resources. We recorded behavior in food patches using a modified focal tree method. Food patches, sorted by decreasing average feeding group size, included large trees (40% of focal plant observations), lianas/strangler (16%), medium trees (9%), small (palm) trees (20%), and rattan (15%). Most food patches were clumped but occurred at low densities relative to the area of average group spread. Thus, availability of alternative food patches was low. Although food patch characteristics indicate high contest potential, the observed aggression rate (0.13 bouts between adults/h) was low relative to other primates. Average feeding group size was small relative to total group size, and feeding group size matched crown volume. Perceived predation risk was low, based on spatial and feeding behavior of juveniles. Together, these results suggest that predation risk may influence feeding competition. Social and temporal factors (patch feeding time), but not ecological factors (fruit abundance in patch and forest, alternative resources) predicted aggression frequency in food patches. Overall, comparative data are still relatively scarce, and researchers should collect more data on group spread, sub-grouping, perceived predation risk, and aggression in food patches before we can draw final conclusions about the role of predation risk for feeding competition.

Understanding stable bi-female grouping in gibbons: feeding competition and reproductive success

INTRODUCTION

Species of the order Primates are highly gregarious with most species living in permanent heterosexual social groups. According to theory in socioecology maximum social group size is limited by rates of intra-group feeding competition and associated increases in travel costs. Unlike other hylobatids, which are predominantly pair living, cao vit gibbons (Nomascus nasutus), and two other species of crested gibbon (Nomascus spp.) living in northern seasonal forest, regularly exhibit larger bi-female groups. To better understand the ability of northern gibbons to live in stable bi-female groups, we examined food distribution, feeding competition and reproductive success over a period of six years in a small cao vit gibbon population at Bangliang, Guangxi, China.

RESULTS

In general, we found weak evidences for within-group contest or scramble competition in our two study groups, which we attribute to high spatial and temporal heterogeneity in the distribution of their important food species. Nevertheless, the larger of the two groups studied increased feeding time and group spread during lean periods, factors that may limit cao vit gibbon group size to a maximum of two breeding females. Relative to tropical pair-living gibbons, there is no evidence that cao vit gibbons travel farther or spend more time feeding, but they did consume more leaves and buds and less fruit and figs. Despite their highly folivorous diet, the average inter-birth interval is comparable to tropical gibbon populations, and the survival rate of infants and juveniles in our study groups is high.

CONCLUSION

Cao vit gibbons do not suffer obvious costs in terms of feeding competition and reproductive success by living in bi-female groups, but within-group feeding competition may determine the upper the limit of cao vit gibbon group size to a maximum of two breeding females. These findings contribute to a growing body of evidence that bi-female grouping can be a stable grouping pattern of gibbons in certain habitats and further emphasize the flexibility of gibbon social organization.

Home range overlap as a driver of intelligence in primates

Various socioecological factors have been suggested to influence cognitive capacity in primates, including challenges associated with foraging and dealing with the complexities of social life. Alexander [Alexander, 1989]. Evolution of the human psyche. In: Mellars P, Stringer C, editors. The human revolution: Behavioural and biological perspectives on the origins of modern humans. Princeton: Princeton University Press. p 455-513] proposed an integrative model for the evolution of human cognitive abilities and complex sociality that incorporates competition among coalitions of conspecifics (inter-group conflict) as a major selective pressure. However, one of the premises of this model, i.e., that when confronted with inter-group conflict selection should favor enhanced cognition, has remained empirically untested. Using a comparative approach on species data, I aimed to test the prediction that primate species (n = 104) that face greater inter-group conflict have higher cognitive abilities (indexed by endocranial volume). The degree of inter-group conflict/complexity was approximated via the variable home range overlap among groups. I found a significant relationship between home range overlap and endocranial volume, even after controlling for other predictor variables and covariates such as group size and body mass. I conclude that brain size evolution cannot be attributed exclusively to social factors such as group size, but likely reflects a variety of social and ecological determinants including inter-group conflict which poses cognitive demands on monitoring both the wider social milieu as well as spatial attributes of the habitat.

Facial width-to-height ratio relates to alpha status and assertive personality in capuchin monkeys

Social dominance hierarchies play a pivotal role in shaping the behaviour of many species, and sex differences within these hierarchies often exist. To date, however, few physical markers of dominance have been identified. Such markers would be valuable in terms of understanding the etiology of dominant behaviour and changes in social hierarchies over time. Animals may also use such traits to evaluate the potential dominance of others relative to themselves (i.e. a physical "cue"). Facial width-to-height ratio (fWHR), for example, has been suggested as a cue to dominance in humans, with links to both dominant behaviour and the perception of dominance in other individuals. Whether this association is present in non-human animals is currently not known. Therefore, here we examine within-species links between fWHR and dominant behaviour in 64 brown capuchin monkeys (Sapajus spp.) aged between 2 and 40 years. fWHR was positively associated with alpha status and with a dimensional rating of assertive personality in both males and females. Moreover, fWHR showed significant sexual dimorphism in adults but not juveniles, suggesting a developmental change may occur during puberty. In a sub-sample, sex differences were mediated by weight, suggesting fWHR dimorphism does not exceed what would be expected by differences in body weight. This is the first report of an association between face shape and behaviour in a non-human species. Results are discussed in terms of the role that face-behaviour associations might play within capuchin societies, and the possible selective forces that might have led to the evolution of fWHR-dominance associations in humans.