Intermittent collective motion in sheep results from alternating the role of leader and follower

Flocking behaviour is often presented as an example of a self-organized process, where individuals continuously negotiate on the direction of travel and compromise by moving along a local average velocity until the group reaches a consensus. Such a collective behaviour does not take advantage of the benefits of hierarchical organizational strategies that confer the leader of the group full control over it with a reduced information flow overhead. Here we study the spontaneous behaviour of small sheep flocks and find that sheep exhibit a collective behaviour that consists of a series of collective motion episodes interrupted by grazing phases. Each motion episode has a temporal leader that guides the group in line formation. Combining experiments and a data-driven model, we provide evidence that group coordination in these episodes results from the propagation of positional information of the temporal leader to all group members through a strongly hierarchical, directed interaction network. Furthermore, we show that group members alternate the role of leader and follower by a random process, which is independent of the navigation mechanism that regulates collective motion episodes. Our analysis suggests that it is possible to conceive intermittent collective strategies that take advantage of both hierarchical and democratic organizational schemes.