The physiology of leadership in fish shoals: leaders have lower maximal metabolic rates and lower aerobic scope

The question of who leads and who follows is crucial to our understanding of the collective movements of group-living animals. Various characteristics associated with leadership have been documented across a range of social taxa, including hunger, motivation, dominance and personality. Comparatively little is known about the physiological mechanisms that underlie leadership. Here, we tested whether the metabolic phenotype of individual fish (x-ray tetras, Pristella maxillaris) determined their relative position within a moving shoal and their tendency to act as leaders. In contrast to previous work, we found that individuals with low maximal metabolic rates and low aerobic scope tended to be more likely to be found at the front of shoals and were more likely to act as leaders. We suggest that leadership by low-performing individuals leads to greater group cohesion. However, in more challenging environmental contexts, such as flowing water, higher performing animals may be more likely to become leaders while low-performing individuals seek the more favourable hydrodynamic conditions at the rear of the group. Hence, the travelling speed of the group may mediate the relationship between metabolic phenotype and leadership.