  For group-living animals, reaching consensus to stay cohesive is crucial for
their fitness, particularly when collective motion starts and stops.
Understanding the decision-making at individual and collective levels upon
sudden disturbances is central in the study of collective animal behavior, and
concerns the broader question of how information is distributed and evaluated
in groups. Despite the relevance of the problem, well-controlled experimental
studies that quantify the collective response of groups facing disruptive
events are lacking. Here we study the behavior of groups of uninformed
individuals subject to the departure and stop of a trained conspecific within
small-sized groups. We find that the groups reach an effective consensus:
either all uninformed individuals follow the trained one (and collective motion
occurs) or none does it. Combining experiments and a simple mathematical model
we show that the observed phenomena results from the interplay between simple
mimetic rules and the characteristic duration of the stimulus, here, the time
the trained individual is moving away. The proposed mechanism strongly depends
on group size, as observed in the experiments, and though group splitting can
occur, the most likely outcome is always a coherent collective group response
(consensus). The prevalence of a consensus is expected even if the groups of
naives face conflicting information, e.g. if groups contain two subgroups of
trained individuals, one trained to stay and one trained to leave. Our results
indicate that collective decision-making and consensus in (small) animal groups
are likely to be self-organized phenomena that do not involve concertation or
even communication among the group members.
