Intergroup encounters in Verreaux's sifakas (Propithecus verreauxi): who fights and why?

When population growth intensifies intergroup competition, female colobus monkeys free-ride less

Intergroup aggression often results in the production of public goods, such as a safe and stable social environment and a home range containing the resources required to survive and reproduce. We investigate temporal variation in intergroup aggression in a growing population of colobus monkeys (Colobus vellerosus) to ask a novel question: "Who stepped-up to produce these public goods when doing so became more difficult?". Both whole-group encounters and male incursions occurred more frequently as the population grew. Males and females were both more likely to participate in whole-group encounters when monopolizable food resources were available, indicating both sexes engaged in food defence. However, only females increasingly did so as the population grew, suggesting that it was females who increasingly produced the public good of home range defence as intergroup competition intensified. Females were also more active in male incursions at high population densities, suggesting they increasingly produced the public good of a safe and stable social environment. This is not to say that males were chronic free-riders when it came to maintaining public goods. Males consistently participated in the majority of intergroup interactions throughout the study period, indicating they may have lacked the capacity to invest more time and effort.

Intergroup encounters in pair-living primates: Comparative analysis and a case study of pair-living and monogamous owl monkeys (Aotus azarae) of Argentina

The function of intergroup encounters (IGEs) may differ substantially among species of different group sizes and social organizations. Research in group-living primates has shown that the behavioral responses during IGEs can vary widely from affiliative to neutral or aggressive interactions; still, little is known about IGEs in pair-living taxa. We conducted a systematic literature review to find relevant studies on the functions of IGEs in pair-living nonhuman primates that could inform analyses of IGE data (n = 242 IGEs, 21 groups and 10 solitary individuals, 1997-2020) from wild owl monkeys, a pair-living, monogamous primate with extensive biparental care. We identified 1315 studies published between 1965 and 2021; only 13 of them (n = 10 species) contained raw data on the number of IGEs. Our review of those studies showed that IGEs are common, but highly variable in their nature and characteristics in pair-living primates. To examine the non-mutually exclusive hypotheses of resource-, and mate defense, and infanticide avoidance we analyzed data from the Owl Monkey Project 27-year long database to build first an a priori model set. To incorporate prior knowledge from the literature review, we conducted our analyses as a consecutive series of binomial logistic regressions. All IGEs including all biologically relevant parameters (N = 156) were codified into three different behavioral categories (Reaction, Agonism, and Physical Aggression). The analysis showed that owl monkeys regularly engaged in IGEs, most of which were agonistic. They showed more reaction when infants were present, but reactions were less physically aggressive when infants and pregnant females were involved. Overall, our results lend more support for the infant and mate defense hypotheses than they do for the resource defense one.

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.

Linking glucocorticoid variations to monthly and daily behavior in a wild endangered neotropical primate

Identifying the factors swaying physiological stress levels in wild animals can help depict how they cope with environmental and social stressors, shedding light on their feeding ecology, behavioral plasticity, and adaptability. Here, we used noninvasive methods to explore the link between glucocorticoid levels and behavior in an endangered neotropical primate facing habitat fragmentation pressure, the black lion tamarin (Leontopithecus chrysopygus). We investigated monthly and day-to-day glucocorticoid variations independently to attempt to disentangle the complex nature of the adrenocortical activity. Between May 2019 to March 2020, we followed two groups of black lion tamarins in two different areas, a continuous forest and a small fragment, and gathered behavioral data (over 95 days in total; 8.6 ± 3.9 days/month) and fecal samples (N_samples  = 468; 4.93 ± 3.5 samples/day) simultaneously. Preliminary analyses enabled us to identify circadian variations linked to the biological rhythm, which were taken into account in subsequent models. Monthly analyses revealed that black lion tamarin fecal glucocorticoid metabolite levels vary according to changes in activity budget associated with the fruit consumption, movement, and resting time of the groups. At a day-to-day level, while intergroup encounters led to increases in fecal glucocorticoid metabolite concentrations, we found that changes in food intake or activity level did not trigger physiological stress responses. These findings suggest that diet and ranging patterns, driven by food availability and distribution, influence physiological stress at a seasonal scale, while acute stressors such as interspecific competition trigger short-term stress responses. Exploring fecal glucocorticoid metabolite variations over different timescales can help uncover the predictive and reactive facets of physiological stress in wild species. Moreover, having a comprehensive understanding of the physiological state of species is a valuable conservation tool for evaluating how they cope in changing environments.

Numerical cognition in black-handed spider monkeys (Ateles geoffroyi)

We assessed two aspects of numerical cognition in a group of nine captive spider monkeys (Ateles geoffroyi). Petri dishes with varying amounts of food were used to assess relative quantity discrimination, and boxes fitted with dotted cards were used to assess discrete number discrimination with equally-sized dots and various-sized dots, respectively. We found that all animals succeeded in all three tasks and, as a group, reached the learning criterion of 70% correct responses within 110 trials in the quantity discrimination task, 160 trials in the numerosity task with equally-sized dots, and 30 trials in the numerosity task with various-sized dots. In all three tasks, the animals displayed a significant correlation between performance in terms of success rate and task difficulty in terms of numerical similarity of the stimuli and thus a ratio effect. The spider monkeys performed clearly better compared to strepsirrhine, catarrhine, and other platyrrhine primates tested previously on both types of numerical cognition tasks and at the same level as chimpanzees, bonobos, and orangutans. Our results support the notion that ecological traits such as a high degree of frugivory and/or social traits such as a high degree of fission-fusion dynamics may underlie between-species differences in cognitive abilities.

Sex and dominance: How to assess and interpret intersexual dominance relationships in mammalian societies

The causes and consequences of being in a particular dominance position have been illuminated in various animal species, and new methods to assess dominance relationships and to describe the structure of dominance hierarchies have been developed in recent years. Most research has focused on same-sex relationships, however, so that intersexual dominance relationships and hierarchies including both sexes have remained much less studied. In particular, different methods continue to be employed to rank males and females along a dominance hierarchy, and sex biases in dominance are still widely regarded as simple byproducts of sexual size dimorphism. However, males and females regularly compete over similar resources when living in the same group, and sexual conflict takes a variety of forms across societies. These processes affect the fitness of both sexes, and are mitigated by intersexual hierarchies. In this study, we draw on data from free-ranging populations of nine species of mammals that vary in the degree to which members of one sex dominate members of the other sex to explore the consequences of using different criteria and procedures for describing intra- and intersexual dominance relationships in these societies. Our analyses confirmed a continuum in patterns of intersexual dominance, from strictly male-dominated species to strictly female-dominated species. All indices of the degree of female dominance were well correlated with each other. The rank order among same-sex individuals was highly correlated between the intra- and intersexual hierarchies, and such correlation was not affected by the degree of female dominance. The relative prevalence of aggression and submission was sensitive to variation in the degree of female dominance across species, with more submissive signals and fewer aggressive acts being used in societies where female dominance prevails. Thus, this study provides important insights and key methodological tools to study intersexual dominance relationships in mammals.

Key individuals catalyse intergroup violence

Intergroup violence is challenging to understand: why do individuals cooperate to harm members of other groups when they themselves may be killed or injured? Despite progress in understanding the evolutionary and proximate mechanisms that underlie violence, we still have little insight into the processes that lead to the emergence of coalitionary aggression. We argue that an overlooked component is the presence of individuals who have a crucial role in initiating violence. In instigating intergroup violence, these key individuals may expect to face lower costs, receive greater benefits, or garner benefits that have a greater value to them than others. Alternatively, key individuals may be motivated by individual traits such as increased boldness, propensity for aggression or exploratory behaviour. Key individuals catalyse the emergence of coalitionary violence through one of several processes including altering the costs and benefits that accrue to others, paying a greater share of the startup costs, signalling privileged knowledge, or providing coordination, among other factors. Here we integrate diverse lines of empirical research from humans and non-human animals demonstrating that inter-individual variation is an important factor in the emergence of intergroup violence. Focusing on the role of key individuals provides new insights into how and why violence emerges. This article is part of the theme issue 'Intergroup conflict across taxa'.

Sex bias in intergroup conflict and collective movements among social mammals: male warriors and female guides

Intergroup conflict is a major evolutionary force shaping animal and human societies. Males and females should, on average, experience different costs and benefits for participating in collective action. Specifically, among mammals, male fitness is generally limited by access to mates whereas females are limited by access to food and safety. Here we analyse sex biases among 72 species of group-living mammals in two contexts: intergroup conflict and collective movements. Our comparative phylogenetic analyses show that the modal mammalian pattern is male-biased participation in intergroup conflict and female-biased leadership in collective movements. However, the probability of male-biased participation in intergroup conflicts decreased and female-biased participation increased with female-biased leadership in movements. Thus, female-biased participation in intergroup conflict only emerged in species with female-biased leadership in collective movements, such as in spotted hyenas and some lemurs. Sex differences are probably attributable to costs and benefits of participating in collective movements (e.g. towards food, water, safety) and intergroup conflict (e.g. access to mates or resources, risk of injury). Our comparative review offers new insights into the factors shaping sex bias in leadership across social mammals and is consistent with the 'male warrior hypothesis' which posits evolved sex differences in human intergroup psychology. This article is part of the theme issue 'Intergroup conflict across taxa'.

Intergroup conflict: origins, dynamics and consequences across taxa

Although uniquely destructive and wasteful, intergroup conflict and warfare are not confined to humans. They are seen across a range of group-living species, from social insects, fishes and birds to mammals, including nonhuman primates. With its unique collection of theory, research and review contributions from biology, anthropology and economics, this theme issue provides novel insights into intergroup conflict across taxa. Here, we introduce and organize this theme issue on the origins and consequences of intergroup conflict. We provide a coherent framework by modelling intergroup conflicts as multi-level games of strategy in which individuals within groups cooperate to compete with (individuals in) other groups for scarce resources, such as territory, food, mating opportunities, power and influence. Within this framework, we identify cross-species mechanisms and consequences of (participating in) intergroup conflict. We conclude by highlighting crosscutting innovations in the study of intergroup conflict set forth by individual contributions. These include, among others, insights on how within-group heterogeneities and leadership relate to group conflict, how intergroup conflict shapes social organization and how climate change and environmental degradation transition intergroup relations from peaceful coexistence to violent conflict.

This article is part of the theme issue ‘Intergroup conflict across taxa’.

Leaders of war: modelling the evolution of conflict among heterogeneous groups

War, in human and animal societies, can be extremely costly but can also offer significant benefits to the victorious group. We might expect groups to go into battle when the potential benefits of victory (V) outweigh the costs of escalated conflict (C); however, V and C are unlikely to be distributed evenly in heterogeneous groups. For example, some leaders who make the decision to go to war may monopolize the benefits at little cost to themselves ('exploitative' leaders). By contrast, other leaders may willingly pay increased costs, above and beyond their share of V ('heroic' leaders). We investigated conflict initiation and conflict participation in an ecological model where single-leader-multiple-follower groups came into conflict over natural resources. We found that small group size, low migration rate and frequent interaction between groups increased intergroup competition and the evolution of 'exploitative' leadership, while converse patterns favoured increased intragroup competition and the emergence of 'heroic' leaders. We also found evidence of an alternative leader/follower 'shared effort' outcome. Parameters that favoured high contributing 'heroic' leaders, and low contributing followers, facilitated transitions to more peaceful outcomes. We outline and discuss the key testable predictions of our model for empiricists studying intergroup conflict in humans and animals. This article is part of the theme issue 'Intergroup conflict across taxa'.

Drivers of gut microbiome variation within and between groups of a wild Malagasy primate

BACKGROUND

Various aspects of sociality can benefit individuals' health. The host social environment and its relative contributions to the host-microbiome relationship have emerged as key topics in microbial research. Yet, understanding the mechanisms that lead to structural variation in the social microbiome, the collective microbial metacommunity of an animal's social network, remains difficult since multiple processes operate simultaneously within and among animal social networks. Here, we examined the potential drivers of the convergence of the gut microbiome on multiple scales among and within seven neighbouring groups of wild Verreaux's sifakas (Propithecus verreauxi) - a folivorous primate of Madagascar.

RESULTS

Over four field seasons, we collected 519 faecal samples of 41 animals and determined gut communities via 16S and 18S rRNA gene amplicon analyses. First, we examined whether group members share more similar gut microbiota and if diet, home range overlap, or habitat similarity drive between-group variation in gut communities, accounting for seasonality. Next, we examined within-group variation in gut microbiota by examining the potential effects of social contact rates, male rank, and maternal relatedness. To explore the host intrinsic effects on the gut community structure, we investigated age, sex, faecal glucocorticoid metabolites, and female reproductive state. We found that group members share more similar gut microbiota and differ in alpha diversity, while none of the environmental predictors explained the patterns of between-group variation. Maternal relatedness played an important role in within-group microbial homogeneity and may also explain why adult group members shared the least similar gut microbiota. Also, dominant males differed in their bacterial composition from their group mates, which might be driven by rank-related differences in physiology and scent-marking behaviours. Links to sex, female reproductive state, or faecal glucocorticoid metabolites were not detected.

CONCLUSIONS

Environmental factors define the general set-up of population-specific gut microbiota, but intrinsic and social factors have a stronger impact on gut microbiome variation in this primate species. Video abstract.

The dynamics of social cohesion in response to simulated intergroup conflict in banded mongooses

Intergroup conflict is widespread in nature and is proposed to have strong impacts on the evolution of social behavior. The conflict-cohesion hypothesis predicts that exposure to intergroup conflict should lead to increased social cohesion to improve group success or resilience in future conflicts. There is evidence to support this prediction from studies of affiliative responses to outgroup threats in some animal societies. However, most of these studies have focused on behavioral changes over short time periods (minutes and hours after exposure to an outgroup), and hence very little is known about the dynamics and durability of responses to intergroup conflict over the longer term. We investigated this question by simulating intergroup encounters in wild banded mongooses (Mungos mungo) and measuring social behavior before, during, and after these encounters over a 5-day period. We also ran control trials with non-threatening stimuli. Banded mongooses reacted immediately to intrusion stimuli by vocalizing, grouping together, and advancing on the stimulus. In the first 5 min after simulated intrusions, we saw an elevation in grooming levels, but in the hour after exposure grooming rates declined sharply, contrary to our expectation. In the two subsequent days, grooming rates remained at this depressed rate. In control trials, the initial increase in grooming was not seen, but grooming declined compared to the longer-term time periods. Grooming changed across time, but not in the same pattern as during intrusions, suggesting that intrusions had an impact above and beyond that of the experimental setup. The dynamics of grooming responses were short lived and more complex than we initially expected. We suggest this unexpected result may be linked to the frequency of aggressive intergroup encounters in this system. As control and experimental trials were run at different times of year, future work would be needed to confirm that these relative patterns are replicable. Our results indicate short-lived impacts of outgroup threat on measures of social cohesion in this species, but cannot confirm longer-term changes.

An Evolutionary Explanation for the Female Leadership Paradox

Social influence is distributed unequally between males and females in many mammalian societies. In human societies, gender inequality is particularly evident in access to leadership positions. Understanding why women historically and cross-culturally have tended to be under-represented as leaders within human groups and organizations represents a paradox because we lack evidence that women leaders consistently perform worse than men. We also know that women exercise overt influence in collective group-decisions within small-scale human societies, and that female leadership is pervasive in particular contexts across non-human mammalian societies. Here, we offer a transdisciplinary perspective on this female leadership paradox. Synthesis of social science and biological literatures suggests that females and males, on average, differ in why and how they compete for access to political leadership in mixed-gender groups. These differences are influenced by sexual selection and are moderated by socioecological variation across development and, particularly in human societies, by culturally transmitted norms and institutions. The interplay of these forces contributes to the emergence of female leaders within and across species. Furthermore, females may regularly exercise influence on group decisions in less conspicuous ways and different domains than males, and these underappreciated forms of leadership require more study. We offer a comprehensive framework for studying inequality between females and males in access to leadership positions, and we discuss the implications of this approach for understanding the female leadership paradox and for redressing gender inequality in leadership in humans.

Life on the edge: behavioural and physiological responses of Verreaux's sifakas (Propithecus verreauxi) to forest edges

Forest edges change micro-environmental conditions, thereby affecting the ecology of many forest-dwelling species. Understanding such edge effects is particularly important for Malagasy primates because many of them live in highly fragmented forests today. The aim of our study was to assess the influence of forest edge effects on activity budgets, feeding ecology, and stress hormone output (measured as faecal glucocorticoid metabolite - fGCM - levels) in wild Verreaux's sifakas (Propithecus verreauxi), a group living, arboreal lemur. We observed five habituated groups: three living in the forest interior and two at an established forest edge. There was no difference in average daily temperatures between edge and interior habitats; however, within the edge site, the average daily temperature incrementally increased over 450 m from the forest edge towards the interior forest of the edge habitat, and the population density was lower at the edge site. Activity budgets differed between groups living in the two microhabitats, with individuals living near the edge spending more time travelling and less time feeding. Groups living near the edge also tended to have smaller home ranges and core areas than groups in the forest interior. In addition, edge groups had elevated average fGCM concentrations, and birth rates were lower for females living in the edge habitat. Combined with lower levels of fruit consumption at the edge, these results suggest that nutritional stress might be a limiting factor for Verreaux's sifakas when living near a forest edge. Hence, Verreaux's sifakas appear to be sensitive to microhabitat characteristics linked to forest edges; a result with implications for the conservation of this critically endangered lemurid species.

Variation in aggression rates and urinary cortisol levels indicates intergroup competition in wild bonobos

Intergroup competition is a widespread phenomenon across taxa and groups typically compete over access to limited resources, such as food and mates. Such competition may be quantified by changes in individuals' behavioral and physiological status in response to intergroup encounters (IGEs). Bonobos, one of our closest living relatives, are often regarded as xenophilic and exhibit high tolerance towards out-group individuals. This tolerance between groups may still be accompanied by intergroup competition over resources. We hereby compared variation in aggression rates and urinary cortisol levels of bonobos during and outside contexts of IGEs in the Kokolopori Bonobo Reserve and investigated whether food and mate availability influenced males' and females' aggression and cortisol levels, when controlling for dominance rank and the number of individuals present. We found that although females had higher aggression rates and urinary cortisol levels during than outside contexts of IGEs, these increases were not related to food availability or changes in between-group dynamics when maximally tumescent females were present, rather than absent. Furthermore, males showed higher aggression rates and urinary cortisol levels during than outside contexts of IGEs. However, males' responses during IGEs were not related to the presence of maximally tumescent females and food availability. Taken together, while competition intensified during seemingly tolerant IGEs in bonobos, such competition was unrelated to short-term changes in food and mate availability. Despite physical and physiological costs of aggression, bonobos associate with out-group individuals frequently and for extended periods. This suggests potential benefits of bonobo intergroup associations.

Network-level consequences of outgroup threats in banded mongooses: Grooming and aggression between the sexes

Animal groups are heterogeneous assemblages of individuals with differing fitness interests, which may lead to internal conflict over investment in group territorial defence. Differences between individuals may lead to different behavioural responses to intergroup conflict, particularly between the sexes. These potential impacts have been little studied. We used social network analysis to investigate the impact of simulated intergroup conflicts on social relationships in groups of wild banded mongooses Mungos mungo, in which intergroup fights are more costly for males than females. We predicted that social cohesion (specifically male-to-male and female-to-male grooming) would increase after conflict, and aggression would decrease, to minimize conflict between the sexes. Simulated intergroup conflicts were performed by exposing banded mongoose groups to scents, 'war cry' playbacks, and live intruders from a rival group. All grooming and aggression interactions between individuals were recorded, and grooming and aggression social networks were created for the 2 days preceding a simulated intergroup conflict (pre-conflict network) and the 2 days after (post-conflict network). We found no evidence of an increase in social cohesion after simulated conflicts, measured as grooming eigenvector centrality. Male-to-male, male-to-female and female-to-male grooming strength decreased after simulated intrusions compared to female-to-female grooming strength. However, male-female aggression decreased in intrusion trials compared to other interaction types, consistent with the hypothesis that intergroup encounters reduce the level of intragroup conflict between males and females. Males were more affected socially by intergroup encounters than females, which may be because they are investing in defence rather than internal relationships. Focusing on individual relationship changes, using social network analysis, can reveal changes in the directionality of behaviour in response to intergroup encounters, and highlight how individual responses to conflict may scale up to affect social networks and, potentially, group performance. This study highlights the importance of studying both group-level behaviours and individual relationships to more fully understand responses to intergroup encounters.

Interactions with conspecific outsiders as drivers of cognitive evolution

The social intelligence hypothesis (SIH) posits that within-group interactions drive cognitive evolution, but it has received equivocal support. We argue the SIH overlooks a major component of social life: interactions with conspecific outsiders. Competition for vital resources means conspecific outsiders present myriad threats and opportunities in all animal taxa across the social spectrum (from individuals to groups). We detail cognitive challenges generated by conspecific outsiders, arguing these select for ‘Napoleonic’ intelligence; explain potential influences on the SIH; and highlight important considerations when empirically testing these ideas. Including interactions with conspecific outsiders may substantially improve our understanding of cognitive evolution.

The social intelligence hypothesis predicts that social organisms tend to be more intelligent because within-group interactions drive cognitive evolution. Here, authors propose that conspecific outsiders can be just as important in selecting for sophisticated cognitive adaptations.

Dynamics and determinants of glucocorticoid metabolite concentrations in wild Verreaux's sifakas

Glucocorticoids have wide-ranging effects on animals' behaviour, but many of these effects remain poorly understood because numerous confounding factors have often been neglected in previous studies. Here, we present data from a 2-year study of 7 groups of wild Verreaux's sifakas (Propithecus verreauxi), in which we examined concentrations of faecal glucocorticoid metabolites (fGCMs, n = 2350 samples) simultaneously in relation to ambient temperatures, food intake, rank, reproduction, adult sex ratios, social interactions, vigilance and self-scratching. Multi-variate analyses revealed that fGCM concentrations were positively correlated with increases in daily temperature fluctuations and tended to decrease with increasing fruit intake. fGCM concentrations increased when males were sexually mature and began to disperse, and dominant males had higher fGCM concentrations than subordinate males. In contrast to males, older females showed a non-significant trend to have lower fGCM levels, potentially reflecting differences in male and female life-history strategies. Reproducing females had the highest fGCM concentrations during late gestation and had higher fGCM levels than non-reproducing females, except during early lactation. Variation in fGCM concentrations was not associated with variation in social interactions, adult sex ratios, vigilance and self-scratching. Altogether, we show that measures of glucocorticoid output constitute appropriate tools for studying energetic burdens of ecological and reproductive challenges. However, they seem to be insufficient indicators for immediate endocrinological responses to social and nonsocial behaviours that are not directly linked to energy metabolism.

Lactating Coquerel's sifaka (Propithecus coquereli) exhibit reduced stress responses in comparison to males and nonlactating females

Cortisol is a glucocorticoid hormone that plays a principal role in metabolic function and stress responses in wild primates. Stressors are ubiquitous in environments and elicit a variety of physiological and behavioral responses. While stress responses are adaptive in the short-term, they can have negative effects when experienced over longer durations. As a physiological stressor, the process of lactation is an energetically expensive activity for mammals. Milk production increases water loss and increased hydration demands are amplified in mammalian species inhabiting xeric habitats, including lemur species living in northwestern Madagascar-the region for this research work. Here, sifakas give birth during the dry season (May-October) and wean infants during the subsequent wet season (November-April). The author collected fecal samples during the 24 weeks following infant births in 10 groups of Coquerel's sifaka in Ankarafantsika Park, Madagascar. The author analyzed the samples by comparing the first 12-week time block to the second 12-week time block, which corresponded to the dry and the beginning of the wet seasons, respectively. Analyses were based on 375 samples collected over two birth seasons (2010 and 2011). A linear mixed model determined the relationships between reproductive class and temporal cortisol variation. The three reproductive classes had significantly different cortisol concentrations. Lactating females had lower cortisol than adult males and nonlactating females in all weeks postnatal. Males had significantly higher cortisol in weeks 13-24 relative to weeks 1-12. Examining seasonal changes in cortisol concentrations demonstrates how lemurs respond physiologically to the energetic constraints of lactation during the critical life history stage of infant development.

Against all odds: Numerical assessment by tufted capuchin monkeys

When competitors are able to assess the asymmetry in the resource holding potential before interacting, individuals or groups should avoid interacting with stronger opponents, thereby avoiding the energy costs and risk of injury associated with aggressive intergroup encounters. Thus, escalated aggression is expected only between closely matched competitors. Among Argentine tufted capuchin monkeys (Sapajus nigritus), intergroup dominance is decided by the asymmetry in male group size. Using playback experiments, I simulated intergroup encounters with neighboring groups, manipulating both the apparent numerical asymmetry and the resource context. During experimental trials, I recorded the approach behavior of the focal individual, as well as changes in neighbor density and individual travel speed following the presentation of the playback stimulus, to assess whether individual willingness to participate in resource defense was affected by the probability of winning the encounter. In spite of the competitive disadvantage, neither males nor females showed a decreased probability of approach when the numerical odds strongly favored the opposing group. Instead decisions regarding whether to participate appear to be driven primarily by the resource context. Nevertheless, changes in individual behavior during approaches suggest that tufted capuchin monkeys are sensitive to the relative odds. Individuals accelerated less when approaching a larger group, although no changes in neighbor density were apparent. The absence of an effect of the numerical asymmetry on willingness to approach the playback speaker suggests that subordinate groups benefit from engaging in intergroup aggression with larger neighbors, despite the high probability of losing. These encounters may serve to assess the current subjective resource valuation of the neighboring group or limit territorial expansion by large groups by decreasing the marginal value of home range exclusivity. Because these encounters are riskier, however, individuals appear to alter their approaches, becoming more tentative as the numerical odds increasingly favor the opposing group.

Effect of Group Size and Individual Characteristics on Intergroup Encounters in Primates

Intergroup encounters are common in nonhuman primates and can vary from affiliative to aggressive. We extracted data from the literature to test five different hypotheses: 1) where there are group size differences between opposing groups, whether the larger group is more likely to win an intergroup encounter than the smaller group; 2) whether the likelihood of a group engaging in aggressive intergroup encounters increases with group size; and 3–5) whether dominant, older individuals, and/or males are more likely to participate aggressively in intergroup encounters than subordinate, younger individuals and/or females. Our data set comprised 52 studies on 31 primate species (3 lemur species, 5 New World monkeys, 19 Old World monkeys, and 4 apes). We found that the larger group is more likely to win an encounter against a smaller group than vice versa. We found no significant relationship between group size and propensity to be aggressive during intergroup encounters. We found weak/no support for the effect of age, dominance rank, and sex on the frequency of aggression displayed toward outgroup individuals during intergroup encounters. Species- and population-specific differences in inter- and intragroup competition and in the degree of the unequal distribution of resources across group members may explain why age, dominance rank, and sex are not strong predictors of aggression during intergroup encounters.

Male monkeys use punishment and coercion to de-escalate costly intergroup fights

In numerous social species, males direct aggression towards female group members during intergroup fights, and this behaviour is commonly thought to function as mate guarding, even though males often target non-receptive females. In studying intergroup fights in a wild population of vervet monkeys, we found that male intragroup aggression was primarily directed towards individuals who had either just finished exhibiting, or were currently attempting to instigate intergroup aggression. Targeted females were less likely to instigate intergroup aggression in the future, indicating that male intragroup aggression functioned as coercion (when directed towards those who were currently trying to instigate a fight) and punishment (when directed towards those who had recently fought). These manipulative tactics effectively prevented intergroup encounters from escalating into fights and often de-escalated ongoing conflicts. Males who were likely sires were those most likely to use punishment/coercion, particularly when they were wounded, and, therefore, less able to protect vulnerable offspring should a risky intergroup fight erupt. This work, along with our previous finding that females use punishment and rewards to recruit males into participating in intergroup fights, highlights the inherent conflict of interest that exists between the sexes, as well as the role that social incentives can play in resolving this conflict. Furthermore, unlike other studies which have found punishment to be used asymmetrically between partners, these works represent a novel example of reciprocal punishment in a non-human animal.

Space partitioning in wild, non-territorial mountain gorillas: the impact of food and neighbours

In territorial species, the distribution of neighbours and food abundance play a crucial role in space use patterns but less is known about how and when neighbours use shared areas in non-territorial species. We investigated space partitioning in 10 groups of wild, non-territorial mountain gorillas (Gorilla beringei beringei). Using location data, we examined factors influencing daily movement decisions and calculated the per cent overlap of annual kernel home ranges and core areas among neighbours. We found that the probability that a group chose an area was positively influenced by both food availability and the previous use of that area by the group. Additionally, groups reduced their overall utilization of areas previously used by neighbouring groups. Lastly, groups used their core areas more exclusively than their home ranges. In sum, our results show that both foraging needs and avoidance of competition with neighbours determined the gorillas' daily movement decisions, which presumably lead to largely mutually exclusive core areas. Our research suggests that non-territorial species actively avoid neighbours to maintain core area exclusivity. Together, these findings contribute to our understanding of the costs and benefits of non-territoriality.

Costs and benefits of group living in primates: an energetic perspective

Group size is a fundamental component of sociality, and has important consequences for an individual's fitness as well as the collective and cooperative behaviours of the group as a whole. This review focuses on how the costs and benefits of group living vary in female primates as a function of group size, with a particular emphasis on how competition within and between groups affects an individual's energetic balance. Because the repercussions of chronic energetic stress can lower an animal's fitness, identifying the predictors of energetic stress has important implications for understanding variation in survivorship and reproductive success within and between populations. Notably, we extend previous literature on this topic by discussing three physiological measures of energetic balance-glucocorticoids, c-peptides and thyroid hormones. Because these hormones can provide clear signals of metabolic states and processes, they present an important complement to field studies of spatial and temporal changes in food availability. We anticipate that their further application will play a crucial role in elucidating the adaptive significance of group size in different social and ecological contexts.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Patterns of seasonality and group membership characterize the gut microbiota in a longitudinal study of wild Verreaux's sifakas (Propithecus verreauxi)

The intestinal microbiota plays a major role in host development, metabolism, and health. To date, few longitudinal studies have investigated the causes and consequences of microbiota variation in wildlife, although such studies provide a comparative context for interpreting the adaptive significance of findings from studies on humans or captive animals. Here, we investigate the impact of seasonality, diet, group membership, sex, age, and reproductive state on gut microbiota composition in a wild population of group-living, frugi-folivorous primates, Verreaux's sifakas (Propithecus verreauxi). We repeatedly sampled 32 individually recognizable animals from eight adjacent groups over the course of two different climatic seasons. We used high-throughput sequencing of the 16S rRNA gene to determine the microbiota composition of 187 fecal samples. We demonstrate a clear pattern of seasonal variation in the intestinal microbiota, especially affecting the Firmicutes-Bacteroidetes ratio, which may be driven by seasonal differences in diet. The relative abundances of certain polysaccharide-fermenting taxa, for example, Lachnospiraceae, were correlated with fruit and fiber consumption. Additionally, group membership influenced microbiota composition independent of season, but further studies are needed to determine whether this pattern is driven by group divergences in diet, social contacts, or genetic factors. In accordance with findings in other wild mammals and primates with seasonally fluctuating food availability, we demonstrate seasonal variation in the microbiota of wild Verreaux's sifakas, which may be driven by food availability. This study adds to mounting evidence that variation in the intestinal microbiota may play an important role in the ability of primates to cope with seasonal variation in food availability.

Group augmentation, collective action, and territorial boundary patrols by male chimpanzees

How can collective action evolve when individuals benefit from cooperation regardless of whether they pay its participation costs? According to one influential perspective, collective action problems are common, especially when groups are large, but may be solved when individuals who have more to gain from the collective good or can produce it at low costs provide it to others as a byproduct. Several results from a 20-y study of one of the most striking examples of collective action in nonhuman animals, territorial boundary patrolling by male chimpanzees, are consistent with these ideas. Individuals were more likely to patrol when (i) they had more to gain because they had many offspring in the group; (ii) they incurred relatively low costs because of their high dominance rank and superior physical condition; and (iii) the group size was relatively small. However, several other findings were better explained by group augmentation theory, which proposes that individuals should bear the short-term costs of collective action even when they have little to gain immediately if such action leads to increases in group size and long-term increases in reproductive success. In support of this theory, (i) individual patrolling effort was higher and less variable than participation in intergroup aggression in other primate species; (ii) males often patrolled when they had no offspring or maternal relatives in the group; and (iii) the aggregate patrolling effort of the group did not decrease with group size. We propose that group augmentation theory deserves more consideration in research on collective action.

Dynamic vs. static social networks in models of parasite transmission: predicting Cryptosporidium spread in wild lemurs

Social networks provide an established tool to implement heterogeneous contact structures in epidemiological models. Dynamic temporal changes in contact structure and ranging behaviour of wildlife may impact disease dynamics. A consensus has yet to emerge, however, concerning the conditions in which network dynamics impact model outcomes, as compared to static approximations that average contact rates over longer time periods. Furthermore, as many pathogens can be transmitted both environmentally and via close contact, it is important to investigate the relative influence of both transmission routes in real-world populations. Here, we use empirically derived networks from a population of wild primates, Verreaux's sifakas (Propithecus verreauxi), and simulated networks to investigate pathogen spread in dynamic vs. static social networks. First, we constructed a susceptible-exposed-infected-recovered model of Cryptosporidium spread in wild Verreaux's sifakas. We incorporated social and environmental transmission routes and parameterized the model for two different climatic seasons. Second, we used simulated networks and greater variation in epidemiological parameters to investigate the conditions in which dynamic networks produce larger outbreak sizes than static networks. We found that average outbreak size of Cryptosporidium infections in sifakas was larger when the disease was introduced in the dry season than in the wet season, driven by an increase in home range overlap towards the end of the dry season. Regardless of season, dynamic networks always produced larger average outbreak sizes than static networks. Larger outbreaks in dynamic models based on simulated networks occurred especially when the probability of transmission and recovery were low. Variation in tie strength in the dynamic networks also had a major impact on outbreak size, while network modularity had a weaker influence than epidemiological parameters that determine transmission and recovery. Our study adds to emerging evidence that dynamic networks can change predictions of disease dynamics, especially if the disease shows low transmissibility and a long infectious period, and when environmental conditions lead to enhanced between-group contact after an infectious agent has been introduced.

Sexually dimorphic aggression indicates male gray wolves specialize in pack defense against conspecific groups

Aggression directed at conspecific groups is common among gregarious, territorial species, and for some species such as gray wolves (Canis lupus) intraspecific strife is the leading cause of natural mortality. Each individual in a group likely has different measures of the costs and benefits associated with a group task, such as an aggressive attack on another group, which can alter motivation and behavior. We observed 292 inter-pack aggressive interactions in Yellowstone National Park between 1 April 1995 and 1 April 2011 (>5300days of observation) in order to determine the role of both sexes, and the influence of pack, age, and other traits on aggression. We recorded the behaviors and characteristics of all individuals present during the interactions (n=534 individuals) and which individuals participated in each step (i.e. chase, attack, kill, flight) of the interaction. Overall, all wolves were more likely to chase rivals if they outnumbered their opponent, suggesting packs accurately assess their opponent's size during encounters and individuals adjust their behavior based on relative pack size. Males were more likely than females to chase rival packs and gray-colored wolves were more aggressive than black-colored wolves. Male wolves and gray-colored wolves also recorded higher cortisol levels than females and black-colored wolves, indicating hormonal support for more intense aggressive behavior. Further, we found a positive correlation between male age and probability of chasing, while age-specific participation for females remained constant. Chasing behavior was influenced by the sex of lone intruders, with males more likely to chase male intruders. This difference in behavior suggests male and female wolves may have different strategies and motivations during inter-pack aggressive interactions related to gray wolf mating systems. A division of labor between pack members concerning resource and territory defense suggests selection for specific traits related to aggression is an adaptive response to intense competition between groups of conspecifics.

The role of the residence-effect on the outcome of intergroup encounters in Verreaux's sifakas

Intergroup competition has an important impact on the survival and fitness of individuals in group-living species. However, factors influencing the probability of winning an encounter are not fully understood. We studied the influence of numerical advantage and location of the encounter on the chances of winning in eight neighboring groups of Verreaux’s sifakas (Propithecus verreauxi), in Kirindy Forest, western Madagascar. Intergroup encounters were inferred from spatial data collected via GPS loggers over a period of two years. Location, i.e., the proximity to the respective core area, rather than the numerical advantage of a group in a given encounter, influenced the probability of winning. Accordingly, the high value that resident groups attribute to exclusive and intensively used areas increased their motivation in defending these locations against intruders. Moreover, losers used the encounter area less often than winners within a month after the encounter, suggesting that losing also entails long-term costs. Thus, our results suggest that in gregarious animals the particular circumstances of each encounter, such as the location, can outweigh group characteristics and predict the chances of winning an intergroup encounter.