Search

International audience; Research on collective movements has often focused on the sociodemographic parameters explaining the success of some individuals as leaders or initiators of collective movements. Several of these studies have shown the influence of social structure, through kinship and affiliative relationships, on the organization of collective movements. However, these studies have been conducted on semi-freeranging groups of macaques that were not faced with a natural environment and its constraints. In the socially intolerant rhesus macaque (Macaca mulatta) the success of an initiator correlates with its hierarchical rank, the most dominant individuals being the most successful. We investigated the collective movements of another socially intolerant macaque species, Japanese macaques, in the wild, to assess whether the social structure was still a determinant factor under natural conditions. In line with previous studies of macaques, we found that social structure drove the organization of collective movements. More dominant individuals initiated more collective movements. However, dominance did not affect the success of an initiation, i.e., the number of individuals joining. In addition, kinship strongly constrained the associations observed in females during collective movements. These results reflect the social structure of Japanese macaques, in which strong power asymmetry and kinship relationships constrain the majority of interactions between individuals within the group. Moreover, these results are similar to those observed in semi-free-ranging rhesus macaques and support the hypothesis of an effect of social determinants on collective movements of primates even under natural conditions.

International audience; Group membersmust decide collectively when and where to go despite their different nutrient requirements. One mechanism underlying consensus decisions is the proposition by one individual to move. The individual frequently initiating movements is often named a "leader", and this individual may be the most dominant, the oldest or may have the greatest physiological needs. However, high-ranking individuals are often those with the highest body mass, and thus the highest nutrient requirements. In this study, we use a state-dependent model to assess the importance of dominance and nutrient requirements to the initiation of group movements, testing different combinations of effects of these interacting factors. We first show using a theoretical approach that whatever the correlation between the dominance hierarchy and body mass, all decision-making systems are viable (or stable). However, in most cases, one individual does become the leader, and nutrient requirements appear to be more important for determining which individual will emerge as the leader than pre-existing individual characteristics such as their dominance rank. We obtained the same result when we compared the simulated distributions of initiations to the observed distributions in three macaque species. Results of our comparison of three macaque groups suggest that we can predict which group member will be the leader simply by knowing its body mass and its reproductive state. Understanding the link between dominance, needs and leadership may be a key to understanding consensus decisions and collective motion in animal groups, and might provide insight into the management of animal groups and their conservation.

International audience; In wildlife populations, group-living is thought to increase the probability of parasite transmission because contact rates increase at high host densities. Physical contact, such as social grooming, is an important component of group structure, but it can also increase the risk of exposure to infection for individuals because it provides a mechanism for transmission of potentially pathogenic organisms. Living in groups can also create variation in susceptibility to infection among individuals because circulating levels of immunosuppressive hormones like glucocorticoids often depend on an individual's position within the group's social structure. Yet, little is known about the relative roles of socially mediated exposure versus susceptibility in parasite transmission among free-living animal groups. To address this issue, we investigate the relationship between host dominance hierarchy and nematode parasite transmission among females in a wild group of Japanese macaques (Macaca fuscata yakui). We use social network analysis to describe each individual female's position within the grooming network in relation to dominance rank and relative levels of infection. Our results suggest that the number of directly-transmitted parasite species infecting each female, and the relative amount of transmission stages that one of these species sheds in faeces, both increase with dominance rank. Female centrality within the network, which shows positive associations with dominance hierarchy, is also positively associated with infection by certain parasite species, suggesting that the measured rank-bias in transmission may reflect variation in exposure rather than susceptibility. This is supported by the lack of a clear relationship between rank and faecal cortisol, as an indicator of stress, in a subset of these females. Thus, socially mediated exposure appears to be important for direct transmission of nematode parasites, lending support to the idea that a classical fitness trade-off inherent to living in groups can exist.

International audience; 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 individualswere 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 kinrelated 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.

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 it's 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. © 2011 Wiley-Liss, Inc.

International audience; Many animal species live as a group and must therefore move as such. Several authors have suggested that the mechanisms underlying collective movements in primate species appear to rely on complex cognitive skills, given their high level of cognitive abilities. However, recent studies have highlighted the fact that complex patterns do not necessarily imply complex mechanisms. We used a modeling approach to investigate the patterns of collective movement in a semi-freeranging group of brown lemurs. We recorded via digital video cameras the order and joining latencies of the 11 individuals of the group during the departure time of spontaneous group movements. We then assessed whether mimetic mechanisms or the existence of a leader were underlying conditions for the joining process by testing 5 computer models relying respectively on 5 hypotheses: the independence of individuals, an anonymous mimetism, a mimetism according to kinship, a mimetism according to affiliation, and eventually the existence of a leader. We found that departure latencies, associations, and the order of individuals at departure time could all be explained by the mimetism according to affiliation model. Thus, an individual's decision to join the collective movement or not depended on the decision taken by its preferred social partners. These results show the importance of social parameters in primate decision making and that the high cohesion displayed by the group members might not be constrained merely by ecological factors such as predation or foraging consideration

Social Network Analysis is now a valuable tool to study social complexity in many animal species, including primates. However, this framework has rarely been used to implement quantitative data on the social structure of a group within computer models. Such approaches allow the investigation of how social organization constrains other traits and also how these traits can impact the social organization in return. In this commentary, we discuss the powerful potential of social network modeling as a way to study group scale phenomena in primates. We describe the advantages of using such a method and we focus on the specificity of this approach in primates, given the particularities of their social networks compared with those of other taxa. We also give practical considerations and a list of examples as for the choice of parameters that can be used to implement the social layer within the models.