Speaker
Description
Milling behavior—such as the vortex-like motion observed in reindeer herds, fish schools, and army ant colonies—represents a striking example of collective dynamics in nature. The milling structure is formed spontaneously and maintained continuously, but the underlying mechanisms are still unclear. We think that animals' anisotropic perception or attention is important; therefore, we account for the vision range, depicted by a vision angle and the vision distance, of each particle when it is aligned with others in active Brownian particle (ABP) simulations. By varying the vision angle and activity, we reproduce the milling motion and also obtain aster and flocking states. The aster state typically occurs for small vision angles, but it hardly exists in the macroscopic world, implying that animals' sensing angles have a lower bound. For the transition from flocking to milling, our model may provide insight that individuals become hyper-aroused and their vision narrows under threat, leading to the formation of a milling structure to defend against the predator. In addition, we propose an approximation method that may relate the milling morphology observed in nature to its corresponding vision range of the species. Furthermore, we show that the spatial distribution of particles within the milling structure depends on individuals' activity, which sheds light on the evolutionary advantage of the defensive strategy.