GROUP DECISIONS. Shared decision-making drives collective movement in wild baboons

Abstract

Conflicts of interest about where to go and what to do are a primary challenge of group living. However, it remains unclear how consensus is achieved in stable groups with stratified social relationships. Tracking wild baboons with a high-resolution global positioning system and analyzing their movements relative to one another reveals that a process of shared decision-making governs baboon movement. Rather than preferentially following dominant individuals, baboons are more likely to follow when multiple initiators agree. When conflicts arise over the direction of movement, baboons choose one direction over the other when the angle between them is large, but they compromise if it is not. These results are consistent with models of collective motion, suggesting that democratic collective action emerging from simple rules is widespread, even in complex, socially stratified societies.

Fig. 1. Extracting pulls and anchors from movement data

Baboon trajectories (25 individuals) during the first day of tracking. (inset, left) Successful initiation (pull), where the initiator (red) recruits the follower (blue). (inset, right) Failed initiation (anchor), where the initiator (red) fails to recruit the potential follower (blue). Other individuals’ trajectories are in gray.

Fig. 2. Probability of following depends on the number of initiators and their directional agreement

Baboons are most likely to follow when there is high agreement among many initiators. When agreement is low, additional initiators do not improve the chances of following, and may decrease them. Surface plot shows a GEE fit to the data (Table S2).

Fig. 3. As predicted by collective movement models (A), as the angle between initiation directions increases, baboon followers exhibit a transition from compromising (moving in the average of the two directions) to choosing one direction over the other

(B–D). Plots show the empirical distribution of follower movement directions as a function of the angle of disagreement between two initiators (B) or two clusters of initiators (C). Regions divided by dotted lines are statistically assigned to (i) compromise, (ii) transitional, and (iii) choose (see Fig. S9). Solid white lines show the median of the directions taken for each mode. Dashed white lines represent the expected direction when compromising (middle line) or choosing (top/bottom lines). When the number of individuals in the clusters differs by 1, followers are more likely to move towards the majority (i.e. along the horizontal line) (D).

Fig. 4. When initiation directions conflict, followers choose the direction of the largest subgroup of initiators

(A) Empirical data are in black; error bars are 95% confidence intervals estimated by 1000 bootstrapped replications of the data. Red line shows a sigmoidal fit to the data. The tendency to follow the majority is maintained regardless of the total number of initiators (B–D), or whether the troop is moving or stationary (Fig. S6).