Scope and impact: In this work, we studied the intermittent and spontaneous collective motion observed in small groups of sheep. By performing experiments and acquiring the tracking of the positions of each individual over time, we found that these groups move by forming files, which resulted to be a strongly hierarchical structure. We called such events Collective Motion Phases (CMPs) and discovered that the leader of each CMP was not necessarily the same individual, but there was a change in leaders. On a larger time scale, this change of leadership resulted to be a rather democratic structure only at work after a sequence of CMPs. We studied the possible benefits of these two opposite mechanisms and found that the hierarchical structure (one file during one CMP) is optimal to guide a group in complex landscapes, and that the democratic structure (change of leaders after multiple CMPs) is optimal for tasks like the exploration of multiple targets in space.

Scope and impact: In this publication we studied the highly synchronized diving motion observed in large shoals of sulphur mollies (Poecilia sulphuraria). This small fish is endemic of south eastern Mexico and is adapted to live in sulphuric springs, where the individuals face two types of "pressure": the sulphur in the water (which pushes the individuals to swim close to the surface to have access to oxygen) and the presence of natural predators like birds (which makes the individuals dive collectively to avoid being eaten). We analysed the spatio-temporal dynamics of the system and concluded that the empirical observations strongly suggest that the system operates at criticality, i.e. at the edge between to different dynamics. We used state-of-the-art machine learning algorithms and studied possible ecologic benefits to the individuals inherited by the fact that the system (as a whole) operates at criticality. We found that the individuals could potentially benefit of a property of critical systems, which is the susceptibility to external perturbations. This means that the system reacts stronger to stronger external perturbations, something which makes the individuals optimize their diving patterns when facing predator attacks.