Influence of Body Size on Leadership in Shoals of Golden Shiners, Notemigonus Crysoleucas

In shoals of uniformly-sized golden shiners (Notemigonus crysoleucas), a minority of individuals who know when and where food is available can lead their naïve shoalmates to food. The present study investigated whether such leadership still takes place when leaders and followers are of different body size. Shoals of either 3 small and 9 large shiners, or 3 large and 9 small ones, were trained to expect food around midday in one of the corners of their large tank. The shoals revealed their learning by anticipating food arrival, i.e. by spending an increasing amount of time in the food corner up to midday. The 9 similarly-sized shiners were then replaced by 9 others of the same size who had never been in the tank before. When the remaining minority of knowledgeable fish were the large individuals, the new small fish followed them to the food corner the next day. These large leaders occupied front positions, though a few days earlier when the whole shoal had been experienced they had tended to stay at the back of the shoal as it entered the food corner. When the remaining minority of knowledgeable fish were the small individuals, the new large fish refused to follow them on at least half of the six trials. Knowledgeable small fish tended to be at the front of the shoal, either when the whole shoal was experienced or on the few occasions when naïve large fish accepted to follow them. The behaviour of the large fish suggests that they were motivated primarily by wariness, whereas that of the small fish is more compatible with a motivation to find food. Reluctance by large fish to follow small ones may contribute to the body size assortativeness of fish shoals in the wild.