Collective Movement in the Tibetan Macaques (Macaca thibetana): Early Joiners Write the Rule of the Game

Abstract

Collective behavior has recently attracted a great deal of interest in both natural and social sciences. While the role of leadership has been closely scrutinized, the rules used by joiners in collective decision making have received far less attention. Two main hypotheses have been proposed concerning these rules: mimetism and quorum. Mimetism predicts that individuals are increasingly likely to join collective behavior as the number of participants increases. It can be further divided into selective mimetism, where relationships among the participants affect the process, and anonymous mimetism, where no such effect exists. Quorum predicts that a collective behavior occurs when the number of participants reaches a threshold. To probe into which rule is used in collective decision making, we conducted a study on the joining process in a group of free-ranging Tibetan macaques (Macaca thibetana) in Huangshan, China using a combination of all-occurrence and focal animal sampling methods. Our results show that the earlier individuals joined movements, the more central a role they occupied among the joining network. We also found that when less than three adults participated in the first five minutes of the joining process, no entire group movement occurred subsequently. When the number of these early joiners ranged from three to six, selective mimetism was used. This means higher rank or closer social affiliation of early joiners could be among the factors of deciding whether to participate in movements by group members. When the number of early joiners reached or exceeded seven, which was the simple majority of the group studied, entire group movement always occurred, meaning that the quorum rule was used. Putting together, Macaca thibetana used a combination of selective mimetism and quorum, and early joiners played a key role in deciding which rule should be used.

Social rank versus affiliation: Which is more closely related to leadership of group movements in Tibetan macaques (Macaca thibetana)?

“…To more fully explore group movements in this species, one needs to consider that the decision‐making process not only refers to leaders’ initiations, but also includes other components that comprise the movement (e.g., recruitment and preliminary behavior: Sueur & Petit []). We also documented previously that Tibetan macaques’ social rank is positively correlated with eigenvector centrality coefficients of early joiners during collective movements [Wang et al, ]. Clearly, one needs to take into account the entire decision‐making process to understand the many factors that influence collective movements in macaque species.…”

“…We recorded the process of movements at set observation sites that afforded a good view (Fig. from Wang et al []). The provisioning area was marked with a systematic grid of reference points and divided into four zones (Fig.…”

“…When less than two‐thirds of all the group members followed, we coded the initiation as failed. We chose a criterion of two‐thirds of members because this represented a majority of individuals [Wang et al, ]. We focused on adult initiators because no immature individuals initiated successful group movements during our preliminary observations.…”

“…Similar to other macaques, Tibetan macaques show female philopatry, male dispersal, and linear dominance hierarchies [Berman et al, ]. Previous studies on this species showed that higher rank and closer social affiliation of early joiners can influence whether other group members participate in group movements [Wang et al, ]. However, the factors influencing the leadership of group movement remain unknown.…”

Collective decision making in Tibetan macaques: how followers affect the rules and speed of group movement

“…The quorum hypothesis states that once a certain number of individuals join a movement, a quorum is reached and entire group movement will occur. This mechanism can help promote accuracy and cohesion during group decision making (Passino & Seeley, 2006;Pratt, Mallon, Sumpter, & Franks, 2002;Sumpter & Pratt, 2009) and has evolved separately in many organisms such as bees (Passino & Seeley, 2006), ants (Cronin, 2013) and monkeys (Wang et al, 2015). Alternatively, the mimetism hypothesis proposes an individual's probability of joining a movement increases with the number of individuals who have already joined the movement (Wang et al, 2015).…”