Researchers in Spain have discovered that in collectives of moving fire ants, rippling “waves” of density and activity are likely triggered by local regions where ants collectively travel in the same direction as their neighbors.
Described in a new paper published in Journal of Applied Physics, Alberto Fernandez-Nieves and colleagues at the University of Barcelona are hopeful that their predictions could be confirmed in future experiments—potentially leading to deeper insights into the complex motions of active materials.
Movement and social interaction
Many physicists are fascinated by active matter—where each particle uses stored or harvested energy to propel itself into motion. The complex collective motions that emerge can be found in certain types of materials, but are perhaps best known in living systems, including ant collectives where each individual ant behaves as a self-propelling active particle. When ants move collectively, they can create patterns of motion vastly more complex than any one insect could conceive on its own.
For collectives of fire ants, these motions are affected by social interactions: When they come into close contact with each other, they will instinctively slow down. And the more they slow down, the more they will become concentrated—driving more regular social interactions into a spiraling process.
Yet as Fernandez-Nieves’ team discovered in their previous research, this behavior doesn’t simply bring the collective to a standstill. “The result is the formation of clusters of essentially stationary ants,” Fernandez-Nieves explains. “But we have also determined that, on average, the ants spend half the time moving and half the time not moving. This means that these clusters are always in coexistence with moving ants.”
Phases of motion
Much like many non-living systems, complex living systems, including ant collectives, can undergo phase changes, where the system rapidly transitions from one dominant state of motion to another. When a fire ant collective becomes dense enough, the phase of coexisting clusters can rapidly give way to a phase where the clusters break up and almost all ants are moving.
Based on their previous work, Fernandez-Nieves’ team believe that this all-moving phase is connected to a fascinating phenomenon observed when a column of ants is confined between vertical sheets of plexiglass. In this environment, waves of density, activity, and alignment propagate upward from the base of the column—where the density of ants is highest—much like ripples on water.
Number fluctuations
In both phases, collective ant behaviors can be characterized by “number fluctuations”—describing how the number of ants in a given space varies relative to the average number of ants. “Since the number fluctuations of systems of active particles relate to the phase the system is in, we decided to look at these fluctuations in the two phases that the ants exhibit: the cluster phase and the all-moving phase,” Fernandez-Nieves states.
Through their latest analysis, the researchers discovered that regardless of phase, these fluctuations are significantly higher compared with a system in equilibrium—like air trapped in a box. In each phase, however, the reasons behind them are completely different. “In the cluster phase, they happen due to the inhomogeneous structure of the system,” Fernandez-Nieves explains. “But in the all-moving phase, we believe they result from local alignment.”
Confirming alignment
This observation suggests that in the all-moving phase, large number fluctuations stem from ants moving in the same direction as their local neighbors—closely mirroring the collective motions seen in flocks of birds or schools of fish, and consistent with the alignment thought to drive the activity waves in ant columns.
For now, however, this interpretation couldn’t be confirmed directly. “In our current experiments, a global alignment could not yet be measured,” Fernandez-Nieves says. “We believe this is due to the small system size that causes the local alignment to be different in different regions of the cell—hence washing away the features of alignment.” On top of this, the ants’ all-moving phases didn’t last long enough for the team to take long enough measurements to find this alignment on local scales before the ants transitioned back to the cluster phase.
All the same, the results offer a valuable framework for future studies of collective motion in fire ants. If confirmed, they could yield deeper insights into how density, activity, and alignment interact to produce the remarkable emergent behaviors of active matter.
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Publication details
C. J. Anderson et al, Giant number fluctuations in fire-ant collectives, Journal of Applied Physics (2026). DOI: 10.1063/5.0326020
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Ripples in fire-ant collectives suggest motions are driven by neighbor alignments (2026, May 28)
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