Aggregation of individuals, such as people, bacteria or sperm, is an ubiquitous phenomenon and is often attributed to direct attraction between the agents. In this talk I will present a basic model that suggests how aggregation can also be a consequence of interactions with an elastic environment.

I will start with a stochastic individual-based model (IBM) of collectively moving self-propelled swimmers and elastically tethered obstacles. Simulations reveal at least three distinct large-scale patterns: travelling bands, trails and moving clusters. This motivates the derivation of a macroscopic partial differential equations (PDE) model for swimmer-obstacle interactions, for which we assume strong obstacle springs. The result is a coupled system of non-linear, non-local PDEs. Linear stability analysis allows to investigate pattern appearance and properties. Close inspection of the derived convolution operator in the PDE model reveals short-ranged swimmer aggregation, irrespective of whether obstacles and swimmers are attractive or repulsive.