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Understanding interactions between individuals is imperative for predicting how groups may react to changing environmental landscapes. Animal populations have displayed variation in behaviour when responding to different environmental cues. Variation in behaviour has been linked to differences in physiology, including metabolic phenotypes and locomotor performance. Understanding how these differences in individuals present themselves in groups provides insight into how physiology affects group behaviour, and how this may change in different contexts. Collective movement in animals is an increasingly prevalent theme in behavioural research, and understanding how and why groups decide to move is critical to our knowledge of animal life. Group movement may emerge from the decisions of one or few individuals, i.e. leadership, or be a shared decision by all individuals. Leadership has been previously linked to individual behavioural traits, which has also been related to physiological differences, however the specific links between physiology and leadership are understudied. Using laboratory experiments, I investigated the role of physiology in leadership of schools of fish, and how different contexts altered leadership in groups in order to examine how groups move and the mechanisms underpinning leadership. In the first data chapter, I tested whether metabolic composition of groups affected leadership by compiling groups of nine fish according to their standard metabolic rate and recorded their swimming behaviour. We measured behaviour at 15 °C, and again at 18 °C to see how temperature increases affect leadership and group dynamics. We found that metabolic composition had no consistent effect on group behaviour and leadership, but increases in temperature caused fish to be less synchronised and leadership to be disrupted. The metabolic cost of digestion has been shown to affect individual behaviour. Our second experiment investigated how group behaviour changed with feeding and time since feeding. Before and during feeding showed relationships between behaviour and meal size, where fish that ate the most were found to be followers when a leader was accelerating, however a fish who has eaten more food is more likely to be a leader when turning. There was no association between meal size and leadership after feeding, however leadership in groups changed before and after feeding events. Our results from chapter 3 and 4 indicated that different environmental contexts disrupted group behaviour, rather than creating consistent differences in specific individual leadership ability. To see how social context affected these metrics, I tested individual swimming performance testing how cost of transport related to leadership and see how individuals alter their voluntary swim speeds to stay within groups and how this relates to their physiological optimum. We found that higher cumulative costs are found when swimming alone compared to groups. Leadership is also not linked to deviation from optimum swim speed, showing that leaders in groups do not influence groups to swim at their optimum swim speed. This study confirms that leadership is not more costly in terms of transport speed, and overall swimming in groups is less costly than swimming alone. These results provide evidence that changing contexts affect group behaviour and leadership in schools of fish. Leadership may not be attributed to one or few specific individuals however how leadership is distributed among individuals may still change in different contexts. Chapters 3 and 4 suggest that physiological processes affect leadership behaviour, and chapter 5 shows that social context will affect group behaviour. Our results provide insight into how leadership in groups change in different contexts and how I may expect collective behaviour to change with environmental variation groups may experience in the wild.