The physiological cost of leadership in collective movements

Individuals can gain substantial benefits from collective actions.^{1,2,3,4,5,6,7} However, collective behaviors introduce new challenges, like coordinating actions, maintaining cohesion, and meeting the needs of different individuals. When making collective movements, leaders are typically thought to gain disproportionate benefits through the choice of more beneficial resources³ and/or earlier access to resources.⁸ However, reaping these benefits can also come with costs. Attempting to influence group movements can increase energy expenditure^4,9,10 and predation exposure.^11,12 Moreover, leadership involves a process of negotiation in many animal groups. Within-group differences in directional preferences are typically resolved by some individuals initiating directional movements, after which they are either followed (if they are successful in leading) or return to the group (if they fail).^{13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30} By combining data on movement initiations (using whole-group global positioning system [GPS] tracking³¹) and individual heart rate (from implanted electrocardiogram [ECG] loggers) in wild vulturine guineafowl, we found significant increases in heart rate (and decreases in heart rate variability) during collective movements. Further, we found that attempting-and failing-to initiate directional movement was physiologically costly, especially for leadership attempts when consensus among group members was low and when potential leaders acted against the majority. The scale of these costs far outweighed those arising from increased movement speeds alone, suggesting that leadership can induce physiological stress, entailing increased energy expenditure and potential physiological damage. These results suggest that behaviors often thought beneficial to individuals (by influencing group behaviors) are also physiologically costly, representing a constraint on group-living and explaining why sometimes individuals opt out of contributing to leadership.