Odile Petit

Decision-making processes: The case of collective movements

Besides focusing on the adaptive significance of collective movements, it is crucial to study the mechanisms and dynamics of decision-making processes at the individual level underlying the higher-scale collective movements. It is now commonly admitted that collective decisions emerge from interactions between individuals, but how individual decisions are taken, i.e. how far they are modulated by the behaviour of other group members, is an under-investigated question. Classically, collective movements are viewed as the outcome of one individuals initiation (the leader) for departure, by which all or some of the other group members abide. Individuals assuming leadership have often been considered to hold a specific social status. This hierarchical or centralized control model has been challenged by recent theoretical and experimental findings, suggesting that leadership can be more distributed. Moreover, self-organized processes can account for collective movements in many different species, even in those that are characterized by high cognitive complexity. In this review, we point out that decision-making for moving collectively can be reached by a combination of different rules, i.e. individualized (based on inter-individual differences in physiology, energetic state, social status, etc.) and self-organized (based on simple response) ones for any species, context and group size.

How Can Social Network Analysis Improve the Study of Primate Behavior

When living in a group, individuals have to make trade‐offs, and compromise, in order to balance the advantages and disadvantages of group life. Strategies that enable individuals to achieve this typically affect inter‐individual interactions resulting in nonrandom associations. Studying the patterns of this assortativity using social network analyses can allow us to explore how individual behavior influences what happens at the group, or population level. Understanding the consequences of these interactions at multiple scales may allow us to better understand the fitness implications for individuals. Social network analyses offer the tools to achieve this. This special issue aims to highlight the benefits of social network analysis for the study of primate behaviour, assessing its suitability for analyzing individual social characteristics as well as group/population patterns. In this introduction to the special issue, we first introduce social network theory, then demonstrate with examples how social networks can influence individual and collective behaviors, and finally conclude with some outstanding questions for future primatological research. Am. J. Primatol. 73:703–719, 2011.

Organization of Group Members at Departure Is Driven by Social Structure in Macaca

Researchers have often explained order of progression of group members during joint movement in terms of the influence of ecological pressures but rarely that of social constraints. We studied the order of joining by group members to a movement in semifree-ranging macaques with contrasting social systems: 1 group of Tonkean macaques (Macaca tonkeana) and 1 group of rhesus macaques (M. mulatta). We used network metrics to understand roles and associations among individuals. The way the macaques joined a movement reflected the social differences between the species in terms of dominance and kinship. Old and dominant male rhesus macaques were more often at the front of the movement, contrary to the Tonkean macaques, which exhibited no specific order. Moreover, rhesus macaques preferred to join high-ranking or related individuals, whereas Tonkean macaques based associations during joining mostly on sexual relationships with a subgroup of peripheral males.

A comparative study of conflict resolution in macaques: insights into the nature of trait covariation

Conflict resolution is an essential component of primate sociality that has been studied extensively within primate social groups, but few studies have examined how conflict resolution behaviours covary at evolutionary scales. We assembled a standardized data set on social interactions of sexually mature females to analyse patterns of conflict resolution in 15 groups of nine macaque species. Between-group comparisons revealed no significant difference in nonkin reconciliation levels between groups of the same species, whereas substantial interspecific differences were found. We tested for associations between four behavioural traits involving reconciliation and dominance asymmetry that play a central role in primate social systems. Regression analyses using group values indicated that these traits (conciliatory tendencies, proportions of explicit reconciliatory contacts, kin bias and levels of counteraggression) exist as an integrated suite of characters. We found strong phylogenetic signal in most traits, which further indicates that they evolved during the adaptive radiation of macaques. Using the method of independent contrasts, relationships between conciliatory tendencies and proportions of explicit reconciliatory contacts and between kin bias and levels of counteraggression remained consistent after controlling for phylogeny. This reveals that evolutionary change in one trait leads to correlated changes in other traits. Collectively, these results show how comparative studies of detailed behavioural interactions can be used to elucidate primate socioecology.

Shared or unshared consensus decision in macaques

Members of a social group have to make collective decisions in order to synchronise their activities. In a shared consensus decision, all group members can take part in the decision whereas in an unshared consensus decision, one individual, usually a dominant member of the group, takes the decision for the rest of the group. It has been suggested that the type of decision-making of a species could be influenced by its social style. To investigate this further, we studied collective movements in two species with opposed social systems, the Tonkean macaque (Macaca tonkeana) and the rhesus macaque (Macaca mulatta). From our results, it appears that the decision to move is the result of the choices and actions of several individuals in both groups. However, this consensus decision involved nearly all group members in Tonkean macaques whereas dominant and old individuals took a prominent role in rhesus macaques. Thus, we suggest that Tonkean macaques display equally shared consensus decisions to move, whereas in the same context rhesus macaque exhibit partially shared consensus decisions. Such a difference in making a collective decision might be linked to the different social systems of the two studied species.

A Comparative Study of Aggression and Conciliation in Three Cercopithecine Monkeys (Macaca Fuscata, Macaca Nigra, Papio Papio)

Patterns of aggression and reconciliation were studied in three captive groups of monkeys belonging to different species: Japanese macaques (Macaca fuscata), crested macaques (M. nigra) and Guinea baboons (Papio papio). Consistent differences were found comparing same dyads of individuals in the three groups. Aggression was often followed by retaliation and reconciliation in the group of crested macaques, such responses occurred less frequently in the group of Japanese macaques, more variable results were found in the group of Guinea baboons. Aggressive manual contacts occurred more frequently in crested macaques than in the other two groups. Rates of biting did not differ consistently among groups but bites could induce bleeding in the group of Japanese macaques. The use of peaceful interventions in conflicts was common in crested macaques, rare in Guinea baboons and unobserved in Japanese macaques. Data from other studies indicate that the contrasts found between groups could be due to interspecific variation. The present results suggest that the covariation between conciliation rates, degree of symmetry in conflicts and level of intensity in aggression may stem from phylogenetic constraints.

A comparative network analysis of social style in macaques

In group-living species, individuals gain significant advantages from establishing an extensive network of social relationships. This results in complex organizations that are difficult to quantify in a comprehensive manner. In this respect, network analyses are an ideal means to pinpoint the overall properties of social structures, and the place of each individual within these structures. We used network measurements to investigate cross-species variations in the social style of macaques, and studied 12 groups from four species. Two species (Macaca mulatta, Macaca fuscata) were characterized by a relatively weak social tolerance, a steep gradient of dominance and a strong preference for kin. The other two species (Macaca nigra, Macaca tonkeana) were known to display higher levels of tolerance, relaxed dominance and low kinship bias. We used a centrality index based on eigenvector centrality to show that in a comparison of intolerant and tolerant species, top-ranking individuals were more central than other group members in the former species than the latter. We also found that networks had higher modularity in intolerant species, indicating that kin-related partners interacted more frequently in subgroups of these species than in those of tolerant species. Consistently, the matrix of body contacts was more strongly correlated with the kinship matrix in intolerant species. This study demonstrates the efficiency of network methodology in detecting fine and overall contrasts in social structures, and also reveals novel dimensions in the social style of macaques.

Collective decision-making in white-faced capuchin monkeys

In group-living animals, collective movements are a widespread phenomenon and occur through consensus decision. When one animal proposes a direction for group movement, the others decide to follow or not and hence take part in the decision-making process. This paper examines the temporal spread of individual responses after the departure of a first individual (the initiator) in a semi-free ranging group of white-faced capuchins (Cebus capucinus). We analysed 294 start attempts, 111 succeeding and 183 failing. Using a modelling approach, we have demonstrated that consensus decision-making for group movements is based on two complementary phenomena in this species: firstly, the joining together of group members thanks to a mimetic process; and secondly, a modulation of this phenomenon through the propensity of the initiator to give up (i.e. cancellation rate). This cancellation rate seems to be directly dependent upon the number of followers: the greater this number is, the lower the cancellation rate is seen to be. The coupling between joining and cancellation rates leads to a quorum: when three individuals join the initiator, the group collectively moves. If the initiator abandons the movement, this influences the joining behaviour of the other group members, which in return influences the initiators behaviour. This study demonstrates the synergy between the initiators behaviour and the self-organized mechanisms underlying group movements.

Group movement decisions in capuchin monkeys : the utility of an experimental study and a mathematical model to explore the relationship between individual and collective behaviours

In primate groups, collective movements are typically described as processes dependent on leadership mechanisms. However, in some species, decision-making includes negotiations and distributed leadership. These facts suggest that simple underlying processes may explain certain decision mechanisms during collective movements. To study such processes, we have designed experiments on white-faced capuchin monkeys (Cebus capucinus) during which we provoked collective movements involving a binary choice. These experiments enabled us to analyse the spatial decisions of individuals in the group. We found that the underlying process includes anonymous mimetism, which means that each individual may influence all members of the group. To support this result, we created a mathematical model issued from our experimental data. A totally anonymous model does not fit perfectly with our experimental distribution. A more individualised model, which takes into account the specific behaviour of social peripheral individuals, revealed the validity of the mimetism hypothesis. Even though white-faced capuchins have complex cognitive abilities, a coexistence of anonymous and social mechanisms appears to influence their choice of direction during collective movements. The present approach may offer vital insights into the relationships between individual behaviours and their emergent collective acts.

Selective mimetism at departure in collective movements of Macaca tonkeana: an experimental and theoretical approach

In primates, authors have reported the specific organization of individuals during collective movements. Some authors have suggested that intentional mechanisms underlie this particular organization because primates have high cognitive abilities that can allow them to use this kind of behaviour. However, mechanisms underlying the emergence of complex systems are not necessarily complex and can be based on local rules. We investigated the joining processes observed during collective movements in one semifree-ranging group of Tonkean macaques, using an individualized agent-based model. The complex patterns observed, such as departure latencies, associations and order of individuals at departure of a collective movement, could be explained using a rule based on affiliative relationships. The decision an individual took to join the movement depended on the departure of its strongly affiliated individuals. Thus even in primates, complex collective behaviour may emerge from interactions between individuals following local behavioural rules.