How swarm size affects soldier crab swarming behavior

Abstract Collective vigilance has commonly been considered as one of the possible hypotheses explaining swarming in (social) animals. According to this theory, by engaging in cooperative monitoring of the environment, groups of social animals can enhance threat detection and overall survival. At the same time, this enables them to spend more time foraging and engaging in social activities. In this study, we report a behavior observed in soldier crabs challenging some aspects of collective vigilance. Laboratory experiments were performed at three swarm density levels (low, medium, and high) to check whether crabs are able to collectively adapt their motion to the changing environment (a moving light). Results show that only medium densities allow dynamic realignment with the moving light, whereas no collective motion is shown at low densities, and changes do not occur at high density. Experimental results were qualitatively backed by ecological observations, in which we show that above a specific threshold of swarm sizes, individuals fail to detect external threats (such as a passing bus) and/or do not change behavior in response to this environmental stimulus. In general, our results hint at the fact that the ability of swarms to collectively adapt to external conditions depends on swarm density. Further, we show that the ability to self-organize has a non-linear relationship with swarm density. Our study highlights the importance of considering complex systems from a closer microscopic perspective based on the ability of individuals to detect changes in the surrounding environment along with the interactions among them.