Collective movements, leadership and consensus costs at reunions in spotted hyaenas

Leadership and the finder's advantage in mantled howler monkeys

Leading collective movements and arriving first at feeding sites may improve food acquisition. Specifically, the first individual to discover and exploit a feeding site may gain a feeding advantage known as the "finder's advantage." The aim of this research was to verify if the probability of leading group movements to feeding sites in mantled howler monkeys (Alouatta palliata) varied by sex and reproductive status, and whether finders had higher foraging success than followers. We studied 18 adult individuals from two groups in La Flor de Catemaco over a year (978 h), and sampled group movements (n = 211) and foraging behavior (n = 215 feeding episodes). Gestating females were leaders and finders of group movements to feeding sites more often than expected but were also replaced in the leading position more frequently than individuals of other sex/reproductive states. Feeding behavior was not influenced by the order of arrival at feeding sites per se, but gestating females had higher food intake rate, bite rate, and feeding time when arriving earlier (i.e., occupying front group positions) than later. Therefore, leadership and the finder's advantage occur in this species and are probably employed by gestating females to maintain their energetic condition.

Disentangling influence over group speed and direction reveals multiple patterns of influence in moving meerkat groups

Animals that travel together in groups must constantly come to consensus about both the direction and speed of movement, often simultaneously. Contributions to collective decisions may vary among group members, yet inferring who has influence over group decisions is challenging, largely due to the multifaceted nature of influence. Here we collected high-resolution GPS data from five habituated meerkat groups in their natural habitat during foraging and developed a method to quantify individual influence over both group direction and speed. We find that individual influence over direction and speed are correlated, but also exhibit substantial variation. Comparing patterns across social statuses reveals that dominant females have higher influence than other individuals over both group direction and speed. Individuals with high influence also tend to spend more time in the front of the group. We discuss our results in light of meerkat life-history and current literature on influence during group movement. Our method provides a general approach which can be applied to disentangle individual influence over group direction and speed in a wide range of species with cohesive movement, emphasizing the importance of integrating multiple lines of inquiry when inferring influence in moving animal groups.

Using optimal foraging theory to infer how groups make collective decisions

Studying animal behavior as collective phenomena is a powerful tool for understanding social processes, including group coordination and decision-making. However, linking individual behavior during group decision-making to the preferences underlying those actions poses a considerable challenge. Optimal foraging theory, and specifically the marginal value theorem (MVT), can provide predictions about individual preferences, against which the behavior of groups can be compared under different models of influence. A major strength of formally linking optimal foraging theory to collective behavior is that it generates predictions that can easily be tested under field conditions. This opens the door to studying group decision-making in a range of species; a necessary step for revealing the ecological drivers and evolutionary consequences of collective decision-making.

The effects of social rank and payoff structure on the evolution of group hunting

Group hunting is common among social carnivores, and mechanisms that promote this behavior are a central topic in evolutionary biology. Increased prey capture success and decreased losses from competitors are often invoked as factors promoting group hunting. However, many animal societies have linear dominance hierarchies where access to critical resources is determined by social rank, and group-hunting rewards are shared unequally. Despite this inequality, animals in such societies cooperate to hunt and defend resources. Game theoretic models predict that rank and relative rewards from group hunting vs. solitary hunting affect which hunting strategies will evolve. These predictions are partially supported by empirical work, but data needed to test these predictions are difficult to obtain in natural systems. We use digital evolution to test how social rank and tolerance by dominants of subordinates feeding while sharing spoils from group hunting influence which hunting strategies evolve in digital organisms. We created a computer-simulated world to reflect social and hunting dynamics of spotted hyenas (Crocuta crocuta). We found that group hunting increased as tolerance increased and as the relative payoff from group hunting increased. Also, top-ranking agents were more likely to group hunt than lower-ranking agents under despotic sharing conditions. These results provide insights into mechanisms that may promote cooperation in animal societies structured by dominance hierarchies.

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'.

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'.

Sex-associated and context-dependent leadership in the rock hyrax

In many mammalian species, both sexes may take leadership role, but different traits may play a role in determining variation within species. Here we examine the effect of sex on leadership. We present three complementary datasets derived from a well-studied population of wild rock hyrax (Procavia capensis). The findings demonstrated that male and female rock hyraxes take on different leadership positions, depending on the context. When risk is moderate, more likely to lead are younger resident males, which experience high cortisol and lower testosterone levels. However, during acute predation scenarios, more likely to lead are males with lower centrality status. We suggest that hyrax males exhibit risky behaviors that may reflect their need for self-advertisement. In contrast, leadership among group females is more equally distributed. Females have little to gain from risky actions due to the lack of competition among them, but nonetheless take leadership positions.

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Different traits play a role in hyrax male and female leaders, in different contexts

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On moderate risk, younger resident males with high cortisol and low testosterone lead

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During predation scenario, lower centrality status males lead

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Among group females, leadership is more equally distributed

Coordination during group departures and progressions in the tolerant multi-level society of wild Guinea baboons (Papio papio)

Collective movement of social groups requires coordination between individuals. When cohesion is imperative, consensus must be reached, and specific individuals may exert disproportionate influence during decision-making. Animals living in multi-level societies, however, often split into consistent social subunits during travel, which may impact group coordination processes. We studied collective movement in the socially tolerant multi-level society of Guinea baboons (Papio papio). Using 146 group departures and 100 group progressions from 131 Guinea baboons ranging in Senegal's Niokolo-Koba National Park, we examined individual success at initiating group departures and position within progressions. Two-thirds of attempted departures were initiated by adult males and one third by adult females. Both sexes were equally successful at initiating departures (> 80% of initiations). During group progressions, bachelor males were predominantly found in front, while reproductively active 'primary' males and females were observed with similar frequency across the whole group. The pattern of collective movement in Guinea baboons was more similar to those described for baboons living in uni-level societies than to hamadryas baboons, the only other multi-level baboon species, where males initiate and decide almost all group departures. Social organization alone therefore does not determine which category of individuals influence group coordination.

Coordination and consensus: the role of compromisers in Tibetan macaques

Coordination and consensus in collective behavior have attracted a lot of research interest. Although previous studies have investigated the role of compromisers in group consensus, they provide little insight into why compromisers would allow such social arrangements to persist. In this study, the potential relationship between group movements and conflict management in Tibetan macaques in Anhui province, China, was investigated using hierarchical cluster analyses. Some members with higher social centrality or social rank often formed a front-runner cluster during group movements. They had higher leadership success than individuals outside the front-runner cluster. Other members with lower social centrality or social rank often followed the group movements initiated by the front-runner cluster, and thus formed the compromiser cluster. Compromisers’ proximity relations with front-runners increased with their following scores to front-runners. Compromisers had fewer events of being attacked when they followed group movements initiated by the front-runners. The compromising process made compromisers lose the choice of direction preference, but it could increase their individual safeties. This trade-off suggests that compromisers play a role of decision-maker in coordination and consensus scenarios among social animals.

Kinship dynamics: patterns and consequences of changes in local relatedness

Mounting evidence suggests that patterns of local relatedness can change over time in predictable ways, a process termed kinship dynamics. Kinship dynamics may occur at the level of the population or social group, where the mean relatedness across all members of the population or group changes over time, or at the level of the individual, where an individual's relatedness to its local group changes with age. Kinship dynamics are likely to have fundamental consequences for the evolution of social behaviour and life history because they alter the inclusive fitness payoffs to actions taken at different points in time. For instance, growing evidence suggests that individual kinship dynamics have shaped the evolution of menopause and age-specific patterns of helping and harming. To date, however, the consequences of kinship dynamics for social evolution have not been widely explored. Here we review the patterns of kinship dynamics that can occur in natural populations and highlight how taking a kinship dynamics approach has yielded new insights into behaviour and life-history evolution. We discuss areas where analysing kinship dynamics could provide new insight into social evolution, and we outline some of the challenges in predicting and quantifying kinship dynamics in natural populations.

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.

Seasonality impacts collective movements in a wild group-living bird

BACKGROUND

A challenge faced by animals living in groups with stable long-term membership is to effectively coordinate their actions and maintain cohesion. However, as seasonal conditions alter the distribution of resources across a landscape, they can change the priority of group members and require groups to adapt and respond collectively across changing contexts. Little is known about how stable group-living animals collectively modify their movement behaviour in response to environment changes, such as those induced by seasonality. Further, it remains unclear how environment-induced changes in group-level movement behaviours might scale up to affect population-level properties, such as a population's footprint.

METHODS

Here we studied the collective movement of each distinct social group in a population of vulturine guineafowl (Acryllium vulturinum), a largely terrestrial and non-territorial bird. We used high-resolution GPS tracking of group members over 22 months, combined with continuous time movement models, to capture how and where groups moved under varying conditions, driven by seasonality and drought.

RESULTS

Groups used larger areas, travelled longer distances, and moved to new places more often during drier seasons, causing a three-fold increase in the area used at the population level when conditions turned to drought. By contrast, groups used smaller areas with more regular movements during wetter seasons.

CONCLUSIONS

The consistent changes in collective outcomes we observed in response to different environments raise questions about the role of collective behaviour in facilitating, or impeding, the capacity for individuals to respond to novel environmental conditions. As droughts will be occurring more often under climate change, some group living animals may have to respond to them by expressing dramatic shifts in their regular movement patterns. These shifts can have consequences on their ranging behaviours that can scale up to alter the footprints of animal populations.

Reproduction Within a Hierarchical Society from a Female's Perspective

The reproductive biology of many female mammals is affected by their social environment and their interactions with conspecifics. In mammalian societies structured by linear dominance hierarchies, such as that of the spotted hyena (Crocuta crocuta), a female's social rank can have profound effects on both her reproductive success and her longevity. In this species, social rank determines priority of access to food, which is the resource limiting reproduction. Due largely to rank-related variation in access to food, reproduction from the perspective of a female spotted hyena can only be understood in the context of her position in the social hierarchy. In this review, we examine the effects of rank on the various phases of reproduction, from mating to weaning. Summed over many individual reproductive lifespans, the effect of rank at these different reproductive phases leads to dramatic rank-related variation in fitness among females and their lineages. Finally, we ask why females reproduce socially despite these apparent costs of group living to low-ranking females. Gregariousness enhances the fitness of females regardless of their positions in the social hierarchy, and females attempting to survive and reproduce without clanmates lose all their offspring. The positive effects of gregariousness appear to result from having female allies, both kin and non-kin, who cooperate to advertise and defend a shared territory, acquire, and defend food resources, maintain the status quo, and occasionally also to rise in social rank.

Tibetan Macaques with Higher Social Centrality and More Relatives Emit More Frequent Visual Communication in Collective Decision-Making

Simple Summary

It is well known that visual communication plays an important role in collective decision-making. However, there is not much research on the influencing factors of visual signals, especially kinship and social relations. In this study, we not only confirmed the function of visual communication in collective decision-making, but also found the effect of kinship and social relations on visual communication. Tibetan macaques with higher social centrality and more relatives emit more frequent visual communication, providing a reference for further research on decision-making. Understanding the link between communication and decision-making can elucidate the powers of group maintenance in animal societies.

Abstract

Animals on the move often communicate with each other through some specific postures. Previous studies have shown that social interaction plays a role in communication process. However, it is not clear whether the affinity of group members can affect visual communication. We studied a group of free-ranging Tibetan macaques (Macaca thibetana) at Huangshan Mountain, China, and answered whether and how social centrality or relatives matter in visual signals during group movement using Tobit regression modeling. All individuals emitted the signals of back-glances and pauses in collective movement. The emission of two signals decreased with the number of participants increased. The back-glance and pause signals emitted by the participating individuals were stronger as the position moved backward in the group. Sex, age, and rank had no significant influence on back-glance and pause signals. Individuals with higher social centrality would emit more pause signals, but social centrality had no effect on the back-glance signal. Individuals with more relatives in the group had more back-glance signals, but this had no effect on the pause signal. This study verifies that social centrality and the number of relatives have effects on visual signals in Tibetan macaques. We provide insights into the relationship between communication behaviors and group cooperation in social animals.

Shared decision-making allows subordinates to lead when dominants monopolize resources

The concepts of leadership and dominance are often conflated, with individuals high in the social hierarchy assumed to be decision-makers. Dominants can exclusively benefit from monopolizing food resources and, therefore, induce an intragroup conflict when leading their group to these resources. We demonstrate that shared decision-making reduces such conflicts by studying movement initiations of wild vulturine guineafowl, a species that forms large, stable social groups with a steep dominance hierarchy. When dominant individuals displace subordinates from monopolizable food patches, the excluded subordinates subsequently initiate collective movement. The dominants then abandon the patch to follow the direction of subordinates, contrasting with nonmonopolizable resources where no individuals are excluded, and dominant individuals contribute extensively to group decisions. Our results demonstrate the role of shared decision-making in maintaining the balance of influence within animal societies.

Who’s there? Third parties affect social interactions between spider monkey males

Spider monkeys provide an intriguing opportunity to examine behavioural flexibility in relation to their social environment given their high degree of fission–fusion dynamics and the nature of male–male relationships. These characteristics allow us to examine how flexibility in social interactions is modulated by the perception of risk and uncertainty related to other group members. We investigated whether male–male interactions vary according to partner identity and presence of third parties in wild Geoffroy’s spider monkey (Ateles geoffroyi). We used proportion of approaches followed by an embrace, an indicator of risk and uncertainty, or by grooming, an affiliative behaviour. To confirm the monkeys’ perception of risk or uncertainty we used aggression rates and time spent vigilant. We collected data on eight adult spider monkey males: three of them belonged to one clique and the other five to another clique based on distinct patterns of residence. We found higher proportions of approaches followed by embraces and lower proportions of approaches followed by grooming between males of different cliques than between males of the same clique. In addition, we found higher aggression rates between males from different cliques. The proportions of approaches followed by embraces in the five-male clique were higher when the three-male clique was no longer in the group. The five males were more vigilant when the other three males were present in the group, indicating the monkeys perceived higher risk or uncertainty under these circumstances. We found lower proportions of approaches followed by grooming between two males when there was at least one other male in the subgroup than when there were only the two males. Our results provide evidence for behavioural flexibility in the interactions between spider monkey males as an example of how animals can cope with social challenges by adjusting their behaviour.

Decision-making process during collective movement initiation in golden snub-nosed monkeys (Rhinopithecus roxellana)

Collective decision-making is important for coordination and synchronization of the activities among group-living animals and the mechanisms guiding such procedure involve a great variety of characteristics of behavior and motivation. This study provides some evidence investigating collective movement initiation in a multi-level social band of the golden snub-nosed monkeys (Rhinopithecus roxellana) located in the Mts. Qinling, China. We collect 1223 datum records relevant to decision initiation from six OMUs. The results indicate that collective movement initiation could be divided into two continual but relatively independent processes: decisions on moving direction and movement implementation. In both processes, adult individuals are more likely to initiate the decision-making, while other adults vote on initiator's preference, with a threshold, a supporting number required for a success. Thus, voting behavior and quorum fulfillment contribute to a successful decision-making. Adult individuals play important role in making decisions for moving direction and implementation. For a successful collective movement initiation, the individuals being more central in grooming network initiate decisions more frequently than the others, and attract voters more easily. Furthermore, following the initiation, at least four positive voters are required for a direction decision and at least three positive voters are needed for the decision on movement implementation, which could be considered as the threshold of quorum numbers required for a successful decision. This study has provided some very interesting information and scientific evidence in understanding social structure and behaviors of the nonhuman primates with a social structure very similar to humans'. Thus, some results can directly be referred to the comprehension of human social structure and behavior.

Inferring influence and leadership in moving animal groups

Collective decision-making is a daily occurrence in the lives of many group-living animals, and can have critical consequences for the fitness of individuals. Understanding how decisions are reached, including who has influence and the mechanisms by which information and preferences are integrated, has posed a fundamental challenge. Here, we provide a methodological framework for studying influence and leadership in groups. We propose that individuals have influence if their actions result in some behavioural change among their group-mates, and are leaders if they consistently influence others. We highlight three components of influence (influence instances, total influence and consistency of influence), which can be assessed at two levels (individual-to-individual and individual-to-group). We then review different methods, ranging from individual positioning within groups to information-theoretic approaches, by which influence has been operationally defined in empirical studies, as well as how such observations can be aggregated to give insight into the underlying decision-making process. We focus on the domain of collective movement, with a particular emphasis on methods that have recently been, or are being, developed to take advantage of simultaneous tracking data. We aim to provide a resource bringing together methodological tools currently available for studying leadership in moving animal groups, as well as to discuss the limitations of current methodologies and suggest productive avenues for future research.This article is part of the theme issue 'Collective movement ecology'.

Selection for Collective Aggressiveness Favors Social Susceptibility in Social Spiders

Particularly socially influential individuals are present in many groups [1-8], but it is unclear whether their emergence is determined by their social influence versus the social susceptibility of others [9]. The social spider Stegodyphus dumicola shows regional variation in apparent leader-follower dynamics. We use this variation to evaluate the relative contributions of leader social influence versus follower social susceptibility in driving this social order. Using chimeric colonies that combine potential leaders and followers, we discover that leader-follower dynamics emerge from the site-specific social susceptibility of followers. We further show that the presence of leaders increases colony survival in environments where leader-follower dynamics occur. Thus, leadership is driven by the "social susceptibility" of the population majority, rather than the social influence of key group members.

Variation among free-living spotted hyenas in three personality traits

Inter-individual differences in behaviour, termed ‘animal personality’, are often consistent over time and across contexts and can be significantly related to fitness. However, most studies of animal personality are conducted in the laboratory or involve experimental protocols. Here, we used longitudinal scan and all-occurrence data collected under naturalistic conditions over 21 years to study boldness, aggressiveness, and sociability in a wild population of spotted hyenas (Crocuta crocuta). We found significant variation among individual hyenas in all three traits; however, whereas both sexes exhibited consistent inter-individual differences in aggressiveness and sociability, males were less consistent than females in their boldness. Heritability and maternal effects were small but significant for both boldness and sociability. A large proportion of the variation in aggressiveness could be attributed to genetic and maternal effects, supporting previous research linking hormone exposure in utero to aggressive behaviour later in life. All three traits were correlated with at least one measure of fitness. Particularly aggressive females were more successful in raising their offspring to reproductive maturity than less aggressive females. Interestingly, the reproductive benefits of sociability depended on social rank; for low-ranking hyenas, gaining feeding tolerance via sociability may significantly enhance reproductive success. Both boldness and sociability were linked to survival, but these traits affected longevity in different ways; highly social hyenas lived longer than those that were less social, but selection on boldness was stabilizing, favoring hyenas with intermediate boldness values that balance the benefits of risky behaviour against risks of injury and death.

Leadership in Mammalian Societies: Emergence, Distribution, Power, and Payoff

Leadership is an active area of research in both the biological and social sciences. This review provides a transdisciplinary synthesis of biological and social-science views of leadership from an evolutionary perspective, and examines patterns of leadership in a set of small-scale human and non-human mammalian societies. We review empirical and theoretical work on leadership in four domains: movement, food acquisition, within-group conflict mediation, and between-group interactions. We categorize patterns of variation in leadership in five dimensions: distribution (across individuals), emergence (achieved versus inherited), power, relative payoff to leadership, and generality (across domains). We find that human leadership exhibits commonalities with and differences from the broader mammalian pattern, raising interesting theoretical and empirical issues.