Separate research streams have identified synchrony and arousal as two factors that might contribute to the effects of human rituals on social cohesion and cooperation. But no research has manipulated these variables in the field to investigate their causal – and potentially interactive – effects on prosocial behaviour. We used in-vivo spatial tracking to examine the interactive effects of synchrony and arousal on groups of approximately 50 individuals. Near the beginning of the study, we manipulated the synchronous and arousing affordances of a group marching task, and observed participants' subsequent (a) group size, (b) dispersal distance, and (c) walking speed during a cooperation task where participants had to work together to collect 500 tokens from the ground and deposit them in a basin. Our hypothesis was that synchrony and arousal would independently facilitate more cohesion (larger groups and less dispersal) and more cooperation (faster walking speed during the foraging task). We had no a priori hypotheses concerning synchrony and arousal's interactive nature, given the scarce and mixed findings from previous research.