Abstract
A major benefit of being in a group is the possibility of integrating social information with directly-perceived information about the environment to guide behaviour. Across the animal kingdom, social information acquired via specific signals or cues is used for decisions in reproduction, foraging and protection against predation. Acute fitness benefits of the usage of social information are flagrant in the context of a response to a potential threat: failure to detect a predator can lead to an animal’s immediate demise, whereas needless engagement in metabolically costly defence responses can unnecessarily impact fitness. Given its role as a primordial driver of group formation, and importance for immediate survival, we study social modulation of behaviour in groups in the context of predation threat. We previously showed that female flies exposed to a predator-mimicking visual threat in groups show a ‘safety in numbers’ effect — they decrease their active immobility defence responses (freezing) compared to when they are alone. Crucially, we identified that exposing wild-type female flies to graded social cues of safety, by controlling the movement of surrounding flies, led to graded freezing responses, pinpointing social motion cues as key mediators of the ‘safety in numbers’ effect. Recently, we have shown that a model of bounded accumulation of evidence accurately predicts our graded freezing data, a crucial step for understanding the mechanisms of evidence integration in a group setting. Furthermore, we have also identified that flies scale their responses to social motion cues with group size, hence uncovering a complex interplay between detection of biological movement and numerosity in group-modulated behaviour. Interestingly, these two aspects of the perception of social cues seem to be mediated by different neuronal circuits. Our work opens up the possibility to study sensory-motor transformations in the context of group behaviour in detail, which will provide insight into the origin and evolution of social behaviour.