The Role of Very Low-Reynolds Hydrodynamics on the Transfer of Information Among Active Agents

We investigate the role of hydrodynamic interactions on the decision-making and leader-identification processes within a group of fifty small-size active individuals, immersed in a viscous fluid at very low Reynolds number, Re ∼10^-2. A fraction of the individuals is informed about the spatial location of the target, and moves accordingly along a privileged trajectory. The rest of the group has no access to this information, but may draw indirect benefit by following the trajectory of the informed individuals, through a process of leader-identification. Such process responds to simple behavioral rules (“social” interactions) discussed previously in the literature (Krause and Ruxton in Living in groups, 2002). The above scenario is enriched with two mechanical ingredients: the presence of an obstacle, preventing the informed individuals from following a straight trajectory to the target and hydrodynamic interactions with the surrounding fluid. It is found that hydrodynamic interactions are particularly effective in steering the uninformed individuals towards the target under neutral conditions, i.e. whenever the fraction of informed individuals is around 50 %. At lower fractions, only the informed individuals manage to reach the target, regardless of hydrodynamic interactions. Likewise, at higher fractions, all individuals reach the target, independently of hydrodynamic effects. This shows that, while hydrodynamics is subdominant under most circumstances, it may nonetheless take on a strategic role whenever the informed and uninformed individuals become comparable in number.