Dynamics on the web: spiders use physical rules to solve complex tasks in mate search and competition

A long-standing question in animal behaviour is how organisms solve complex tasks. Here we explore how the dynamics of animal behaviour in the ubiquitous tasks of mate-search and competition can arise from a physics-based model of effective interactions. Male orb-weaving spiders of the genus Trichonephila are faced with the daunting challenge of entering the web of a much larger and potentially cannibalistic female, approaching her, and fending off rival males. The interactions that govern the dynamics of males within the confined two-dimensional arena of the females web are dominated by seismic vibrations. This unifying modality allows us to describe the spiders as interacting active particles, responding only to effective forces of attraction and repulsion due to the female and rival males. Our model is based on a detailed analysis of experimental spider trajectories, obtained during the approach of males towards females, and amidst their interactions with rival males of different sizes. The dynamics of spider particles that emerges from our theory allows us to explain a puzzling relationship between the density of males on the web and the reproductive advantages of large males. Our results provide strong evidence that the simple physical rules at the basis of our model can give rise to complex fitness related behaviours in this system.