Walls can turn a group or make it circle
When a school of fish meets a wall, the wall is not just a passive obstacle. A model that included both water flow and wall interactions found new collective phases in which the school follows the tank wall or forms a double mill—two groups circling in opposite directions [3]. These are not simply the same schooling pattern squeezed into a smaller space; they are distinct arrangements that emerge because the boundary changes the forces each fish experiences.
The same modeling work found that confinement can induce repeated switching between two collective states, schooling and milling, and described the group's behavior probabilistically as bistable—meaning both states are possible and the group can flip between them [3]. The authors suggest these transitions could occur spontaneously, without any individual fish adjusting its behavior [3]. That is a hypothesis about mechanism, not a settled fact, but it points to a useful idea: the shape of the space itself can act as a kind of switch.
How crowded the space is changes how often the group switches
Confinement is not only about walls; it is also about how many individuals share the available area. In experiments with rummy-nose tetra fish, increasing confinement density—the number of fish per unit area—produced a continuous transition from a polarized state, where fish align and move together, to a milling state, where they circle [4]. During this transition the school was bistable: polarization and milling coexisted, and the group randomly alternated between them [4].
A simple two-state Markov process—a model that treats the group as flipping between two states with certain probabilities—described the observed transitions remarkably well, and confinement density shaped the statistics of that bistability, including the distribution of transition times between states [4]. In plain terms, crowding the fish more often did not just push them into one pattern; it changed the rhythm of switching between patterns.
Even a weak pull toward the group center can hold a group together
Cohesion—how strongly individuals are drawn toward the group's center—can be measured separately from environmental effects. A modeling framework applied to 11 reindeer distinguished a group-level movement model from individual decisions made relative to the group centroid, and found a directional bias toward the centroid when the reindeer were in an encamped state [1]. The attraction was relatively weak, yet the model still captured group-influenced movement and predicted the non-diffusive behavior of a cohesive mobile group better than a regular mixture of correlated random walks [1].
That result is a reminder that cohesion does not have to be strong to matter. A modest bias toward the group center can be enough to keep a group from spreading out like a random walk, and the modeling approach shows how social attraction can be separated from environmental factors such as confinement [1].
The same boundary effects appear in swimming microbes and migrating cells
Confinement shapes collective movement beyond fish and mammals. In simulations of microorganisms swimming in viscoelastic fluid inside circular chambers, both pushers and pullers accumulated near the walls, and fluid elasticity enhanced that accumulation [2]. Wall accumulation slowed their swimming speeds, and at high concentrations pullers formed multilayer structures near the boundary while also aggregating into clusters in the central area—two behaviors that seemed to compete [2]. The squirmers' orientations inclined toward the walls, strengthened by fluid elasticity, and the authors believe this is connected with the wall accumulations [2].
Work on cell migration also treats 3D geometric confinement as a factor shaping collective modes of directed cell migration and how animal cells interact with different extracellular matrix and biomaterials [5]. Together these studies suggest that boundary effects on group direction and cohesion are not limited to one kind of animal or one scale, though the mechanisms differ—wall accumulation and orientation for swimming microbes [2], and matrix interactions for migrating cells [5].
Sources used in this answer
Modelling group dynamic animal movement
A group-dynamic movement model applied to 11 reindeer found a directional bias toward the group centroid in the encamped state; the attraction was relatively weak but the model predicted non-diffusive cohesive group behavior better than a mixture of correlated random walks.
Collective motions of microorganisms in geometric confinement
In simulations of squirmers in viscoelastic fluid within circular chambers, pushers and pullers accumulated near walls (enhanced by fluid elasticity), wall accumulation slowed swimming, and at high concentrations pullers formed boundary multilayers and central clusters while orientations inclined toward walls.
Collective phase transitions in confined fish schools
A model incorporating geometric confinement and wall interactions produced new collective phases in fish schools—wall-following and double milling—and induced repeated switching between schooling and milling, described as bistable collective states.
Confinement-driven state transition and bistability in schooling fish
In rummy-nose tetra fish, increasing confinement density produced a continuous transition from polarized to milling states, with a bistable period where both coexisted and the group randomly alternated between them, well described by a two-state Markov process.
Profiling Collective and Individual Cell Migration Using Biomaterial and Microfluidic Technologies
The paper examines how 3D geometric confinement affects collective modes of directed cell migration and how animal cells interact with different extracellular matrix and biomaterials.