Leaders of spontaneous group movements influence whole-group social organization: an experimental study on grazing heifers

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Group-living animals have to reach consensus decisions through recruitment processes while collectively exploiting food resources in heterogeneous environments. In farming conditions, the group compositions are frequently modified, which can force the animals to readjust their collective strategies, e.g., when group leaders are removed. We, therefore, conducted a group-manipulation experiment for testing how the loss of the individual with the highest leadership score in spontaneous group movements is likely to influence decision-making and whole-group social organization. Observations were carried out in two groups of nine familiar heifers, as using animals of the same age-class simplifies the analysis of the proximate determinants of leadership. Under the hypothesis of distributed leadership, we expected the emergence of a new leader but no change in social relationships due to the stability of dominance hierarchy and affinities among the familiar remaining heifers. Both group leaders were heifers that had a low cohesion index and expressed few allogrooming interactions. Leadership did not relate to position in the dominance hierarchy. After removing the leader and replacing it with a familiar peer, the number of spontaneous group movements decreased by 40% in both groups, and no new leader emerged. Both groups maintained a structure after leader removal but the distribution of pairwise association was modified. Allogrooming interactions dropped by nearly 60% in one group and their overall distribution were modified in the other group. These findings demonstrate interrelations between various social traits even in the case of same-age herbivore groups - and difficulties with rapidly reaching a new consensus for maintaining synchronized activity and group cohesion.