But, interestingly, the soldier crabs also rotate regularly. Leader crabs at the edges cycle back into the interior, and interior crabs rotate to the outside, into leadership roles. All this happens without discussion of course. The direction and speed the crabs travel is often dictated by outside stimuli, such as the shadow of a crab-eating bird being cast on the group. But they more or less move as a unit, regardless of which crabs are in charge at any particular time.