Intermittent collective dynamics emerge from conflicting imperatives in sheep herds

We report and analyze quantitative field observations of large groups of Merino sheep. While grazing, these sheep must balance two competing needs: (i) the maximization of individual foraging space and (ii) the protection from predators offered by a large dense group. We show that they resolve this conflict by alternating slow foraging phases—during which the group spreads out—with fast packing events triggered by an individual-level behavioral shift. This leads to an intermittent collective dynamics with large density oscillations triggered by packing events on all accessible scales: a quasi-critical state. All our findings are well accounted for by an explicit model with individual behavioral shifts and strong allelomimetic properties.