Biology

There is probably a gradient of smell or other chemicals to motivate their general direction of progression. If with initiating or extending a bridge, an ant can climb the gradient r times higher than without (where r is a threshold that varies ant by ant), the ant will choose to bridge. The kick is the gradient has to be constrained by where the ant can go: there is much less choice at the tip of the bridge than on the ground, in other words the action of building the bridge itself introduces constraints on the base manifold the ant can explore to climb the gradient. By introducing movable platforms, the authors are basically changing the base manifold calculus again without consent from the ants. I imagine there's quite a bit of confusion and grudge on the ants' end.