by Samuel Scopel
In “Signals use by leaders in Macaca tonkeana and Macaca mulatta: group-mate recruitment and behaviour monitoring,” the authors examine the interplay between visual cues and coordinated social movements. In primates that live in groups, there are often specific areas designated for a given activity such as foraging or resting. To retain the benefits conferred by performing activities as a social unit, movement between these areas must be done as a group and requires consensus among the individuals composing the group. In this study, Sueur and Petit examine the visual cues utilized by the individual initiating the movement as well as how those cues are modulated based on the members of the group that chose join.
In Tonkean and rhesus macaques, the primate species observed during the study, the individual wishing to initiate a collective movement will begin moving in the desired direction and pauses and back-glances cue other group members of the intent. The authors monitored how the frequency of both these cues affected the actions of fellow group members, and how the number and identity of the joiners affected the behavior of the initiator. In general, pauses and back-glances decreased significantly when the desired individuals joined the initiator in both species.
The authors grappled with whether pausing was a direct cue to specific group members to join the effort or an expression of uncertainty on the part of the individual initiating the movement. If pausing was an expression of general uncertainty, then pausing should decrease with the number of contemporaries joining the movement regardless of who the joiners were. Interestingly, the authors observed that pausing only decreased when certain members joined the group, suggesting that the pause signal was intended to recruit specific group members to join the action. The only substantive difference noted between species was that Tonkean macaques tended to emphasize recruiting affiliated individuals and rhesus macaques decreased pauses when kin-related individuals joined. The authors hypothesized back-glances were mainly used to monitor the number/identities of the group members joining the collective effort.
One limitation noted by the authors was the semi-free ranging conditions in which these observations were performed. The distances between individuals was lower and general visibility higher than what would be found in natural populations. Other behavioral factors, such as calls, may play a more significant role in natural conditions.
This study highlights just one example of how visual indicators contribute to social efforts and facilitate group cohesion. It stands to reason, given the number and variety of activities in which primates take part in a social context, that the expansion of the area of the brain responsible for visual processing observed in primates would provide a selective advantage. While this study alone is insufficient evidence for this proposition, one could imagine how sociality could function as a positive feedback loop driving evolution of regions of the brain that are critical for operating within it. Primates that are more effective at coordinating social activities (ie resource management, group defense, etc.) would lead to fewer individuals being lost to predation or other miscellaneous selective pressures that are more evident in species that can only operate in a solitary capacity. This would, in turn, lead to a more pronounced emphasis on the refinement of inter-individual communication and the physical structures or processes associated with it.
Sueur, C. & Petit, O. Signals use by leaders in Macaca tonkeana and Macaca mulatta: Group-mate recruitment and behaviour monitoring. Animal Cognition 13, 239–248 (2010).