Experience and motivation shape leader-follower interactions in fish shoals

Leadership is an important process shaping collective movement in some species. Recent work has demonstrated that experienced or motivated individuals can emerge as leaders, and provides insight into the mechanisms by which this occurs. Ultimately, leadership depends upon the effectiveness with which would-be leaders can entrain followers, and while the properties of leaders have received much attention, less is known about the factors that affect the propensity of their groupmates to follow them. Here the roles of experience and state (hunger) in shaping leader and follower behavior were investigated using shoals of sticklebacks (Gasterosteus aculeatus). A first experiment revealed that individuals trained to approach a target could entrain and lead their naïve groupmates out of a refuge towards it, and that they did so more effectively when they (the trained fish) were food-deprived. In the second experiment the hunger level of the trained fish was held constant, while that of the naïve fish was varied. Here, leadership by trained fish was only apparent when the hunger levels of the naïve group members were intermediate. When naïve fish were recently fed they took a long time to visit the target and their arrival times were not affected by the presence of a trained individual. Very hungry groups recruited to the target most rapidly, but again with no evidence of influence by their trained groupmates. These experiments demonstrate that leadership in animal groups depends not only upon the state and experience of the leader but also upon that of the potential followers.