Collective Movements And Decision Making In Tibetan Macaques(Macaca Thibetana)

Animals'collective behavior is the adaptation to their habitat, which is beneficial to information sharing, mutual cooperation and strength enhancement. Accordingly, animals can reach the goals of finding foods, group defense, and reproduction by collective behavior. However, the social rank, reproductive state, individual interests and needs among group members are usually different. These differences could lead to the dispersion and instability of animals'society and weaken the cohesion of the population, so that it may have adverse effects on the growth, development and reproduction for animals. Obviously, the basis of the success to social animals is individuals must coordinate and cooperate each other. Otherwise, the advantages of group living would not exist, and even cause some individuals facing threats to their survival. Among long term evolution, animals have formed a series of behavioral mechanisms to maintain the stability of society.Collective movement is one kind of common behavior in daily life among group-living animals. It refers to the process that majority of group members move to the same direction with uniform actions. Generally, collective movements are initiated by one or more individuals and followed by other group members. During the movements, group members need to work together to achieve consensus decision. Therefore, studying on collective movements is an important way to explore collective decision-making in animals.During collective movements, the first individual to depart is the initiator, then the individuals behind the initiator are followers. Apparently, different initiators have different results about movement initiations, namely, this reflects individual leadership of initiators. Studying on leadership is the premise of learning collective decision making, and meanwhile, the joining decision of followers is the guarantee to achieve consensus decision. Till now, studies about relationship between collective movements and decision making have only focused on group movements made by a few participants rather than including the majority of group members.From August 2012 to May 2013, we studied one group of wild Tibetan macaques (Macaco, thibetana) at Mt. Huangshan of Anhui province. Our study observed monkeys' group movements from the provisioning area to the forest, their biological (e.g., sex, age) and social (e.g., social rank and affiliation) attributes by some behavioral sampling methods (i.e., the all occurrence sampling method, the focal sampling method and continuously recording method). Based on these behavioral data, we analyzed the influencing factors of consensus decision making in Tibetan macaques. Our study also discussed the relationship between collective movements and decision making, and we expected to solve the following issues:1) the leadership type of collective movements in Tibetan macaques,2) the influencing factors of monkeys'collective movements, and 3) the joining rules of collective movements in macaques. The results are as follows:First, we found the leadership type of collective movements in Tibetan macaques. All adult members could successfully initiate group movements, but there was a significant difference on the frequencies of successful initiations among different individuals (Kruskal-Wallis test:H= 20.549, df= 11, P= 0.038). There was no correlation between age (Spearman rank correlation:rs= 0.299, N= 12, P= 0.345)/social rank (Spearman rank correlation:rs= 0.049, N= 12, P= 0.879) and successful initiations, respectively. Also, there was no significant difference on the frequencies of successful initiations between adult males and females (Mann-Whitney U test:Z=-0.515, P= 0.607). However, the frequencies of successful initiations were positively correlated to the duration of social grooming (Spearman rank correlation:rs= 0.707, N= 12, P= 0.010), including grooming given (Spearman rank correlation:rs= 0.643, N= 12, P= 0.024) and grooming received (Spearman rank correlation:rs= 0.587, N=12,P= 0.045). We consider Tibetan macaques had distributed leadership during the initiation of collective movements and suggest that there was a close connection between affiliated behavior and individual leadership.Second, we revealed the social factors of collective movements. We analyzed the relationship between eigenvector centrality coefficients and consensus decision making by social network analysis. Results showed that the social centrality based on the dyadic proximity was positively correlated with the frequency of successful initiations (Spearman rank correlation:rs= 0.704, N= 12, P= 0.011), but also to the success ratio of initiations (Spearman rank correlation:rs= 0.757, N= 12, P= 0.004). Meanwhile, the social centrality based on triadic social interactions (e.g., bridging behavior) was positively correlated to the successful initiations (Spearman rank correlation:rs= 0.952, N= 8, P= 0.001) and with the success ratio of initiations (Spearman rank correlation:rs = 0.857, N= 12, P= 0.003). Our results indicate that the individuals who were more affiliated in the group, they also had more successful movements initiations (i.e., the frequency and success ratio of initiations). We propose using the affiliation-leadership model to explain the influencing factors of consensus decision making. Social affiliation played a crucial role during the process of decision making in Tibetan macaques.Third, we evaluated the joining rules of collective movements in Tibetan macaques. The joining order of individuals was positively correlated with the eigenvector centrality coefficient (Spearman rank correlation:rs= 0.695, N= 12, P-0.003). The result indicates that during the joining process of collective movements, the earlier the individuals joined the process, the more important role they would play. When the number of early joiners (individuals who participated in group movements during the first five minutes after initiations) reached from three to six, the probability of collective movements was from 40%-85%. When the number of early joiners reached or exceeded seven (the half of adult members in the group), collective movements always happened. Specifically, our results showed that there was no significant difference on eigenvector centrality coefficients between adult male and female early joiners (t= 0.738, df= 10, P = 0.477). Early joiners'age was not correlated with their social centralities (Spearman rank correlation:rs=-0.174, N= 12, P= 0.589). However, there was a positive correlation between social rank and social centrality (males:Spearman rank correlation: rs= 0.800, N= 4,P= 0.048; females:Spearman rank correlation:rs= 0.655, N=S,P= 0.035). Moreover, the social centrality based on proximity relations was also positively correlated to the centrality based on joining networks (Spearman rank correlation:rs= 0.614, N= 12, P-0.034). The results showed that Tibetan macaques used two joining rules during collective movements.1) Selective mimetism rule:before the number of early joiners reached the threshold of collective movements, group members decided whether to join the movements or not according to social rank or social affiliation of early joiners. Higher ranking or more affiliated early joiners could attract more followers.2) Quorum rules:once the number of early joiners reached the threshold, other group members always participated in collective movements.In our study, we used the knowledge of ecology, behavioral science, sociology and statistics. The main innovations of this study are as follows:1) We first revealed the type of decision making during collective movements in Tibetan macaques (i.e., consensus decision making).2) We used the affiliation-leadership model to explain the influencing factors of individual leadership during collective movements. This model not only existed in dyadic affiliated interactions (e.g., proximity, social grooming), but also occurred in triadic affiliated associations (e.g., bridging behavior).3) We found selective mimetism and quorum coexisted in animal society. This result was first reported in nonhuman primate species with small group and complicated social relationship.Our study illustrated the behavioral characteristics and the type of decision making in Tibetan macaques. We revealed the behavior mechanism and adaptive evolution of decision making. Our research results could provide scientific basis for the protection and management of social animals, also the results may provide a useful reference for understanding human decision-making and improving the management of human society. As we know, in modern society, people often make some consensus decisions to achieve coordination. From the diplomatic agreement among countries, the election of leaders in nations to planning programs in companies, all of these processes need to have the mechanism of consensus decision-making. How to achieve the orderly and efficient of decision-making? How to play the overall function of society through consensus? How to reflect the majority of members' interest? All of these questions are also our concerns when we participate in social decision-making.