The influence of nutritional state on individual and group movement behaviour in shoals of crimson-spotted rainbowfish (Melanotaenia duboulayi)

Groups of animals are often heterogeneously structured and may be composed of selfish individuals responding to different internal stimuli. Group-level behaviour can be determined by the slight differences in simple behavioural movement parameters structuring local interactions between conspecifics. To accurately understand individual behaviour within groups and how it affects whole-group behaviour, we need to measure the responses of individuals in groups to changes in internal state and examine the outcome of these responses within the social context. Therefore, we quantified the influence of nutritional state on the individual and group movement parameters of free swimming shoals of eight rainbow fish, Melanotaenia duboulayi. Individual fish were experimentally manipulated to be in one of two nutritional states, hungry or satiated, and were assayed in three group compositions: all-hungry (8:0 hungry/satiated), mixed (4:4) or all-satiated (0:8). We showed that the internal nutritional state of individual fish affected basic behaviours relating to spatial positioning. The interaction between pairs of fish was dependent on the nutritional state of both fish, and there was an additive effect of individual behaviour on group behaviour, which meant that group behaviour reflected the motivations of its individual members in such a way that allowed individuals to fulfil their own behavioural needs whilst still attaining the benefits of grouping.