Fasting and digestion exert opposite effects on collective behavior in goldfish across contexts

Animal groups are often composed of individuals with differences in their phenotypes (e.g. body size, personality, or internal nutritional state). Such differences in phenotypes between group mates can have significant consequences for the collective movement and behavior of a group. Here, we examined the effects of nutritional state and ecological context on the individual- and group-level behaviors of groups of shoaling goldfish (Carassius auratus) across an open water environment, an environment with food, and an environment with food and refuge. Fish were randomly manipulated to be in one of three nutritional states (control: fasted for 24 h, fasting: fasted for 1 week, and digestion: 2 h after feeding) and comprised five group compositions: the all-control (6 control fish), all-fasting (6 fasted fish), all-digestion (6 digesting fish), mixed con–fas (3 control fish and 3 fasted fish), and mixed con–dig treatment (3 control fish and 3 digesting fish). We found that compared to the control and fasted treatments, the fish in the digestion treatment had lower individual swimming speeds and nearest neighbor distances. However, group polarization was lower in the fasted treatment than in both the control and digestion treatments. For the two mixed treatments, individual and collective behaviors of the fish seemed to be intermediate in comparison to those of either the fasted or digestion treatments. Moreover, group-level differences in collective behaviors were maintained across different contexts. All five treatments exhibited considerable context-associated changes in collective behaviors, but behavioral repeatability was dependent on the nutritional state of the group composition. Our results suggest that nutritional state and ecological context influence both individual and collective behaviors and that fasting and digestion exert opposite effects on collective behaviors across ecological contexts.