Hydrodynamics of sheep herds

The collective motion of living entities, from cells to wildebeests, is a fascinating and aesthetic phenomenon which continues to intrigue physicists as well as any curious observer. Over the past decade, many compelling findings have come out of the rigorous study of systems such as bird flocks and mosquito swarms, where individuals are scattered and interact remotely. In parallel, slowly-moving aggregates of grasping individuals, such as ants and bees, have been successfully investigated from a material perspective. A third category of interest is now surfacing, in which individuals move collectively while remaining in a near-jamming state. This includes for example crowds of humans, or herds of sheep, which are the focus of this presentation. By employing optical flow methods to measure the velocity field of dense sheep herds, we construct a hydrodynamics framework to study the collective motion of living systems flowing like liquids. The proposed methods are versatile and can be adapted to other flocking systems, such as flamingoes, penguins, etc.