Symmetry breaking of escaping ants

The phenomenon arises in the experiments described as follows:

Inspired by earlier computer simulations that predicted a symmetry-breaking phenomenon when panicked humans escape from a room with two equivalent exits, E. Altshuler and coworkers from the University of Havana designed the experiment described in the section above, which revealed the symmetry-breaking effect in the leafcutting ant Atta insularis.[1]

The A. insularis ant (colloquially known as bibijagua in Cuba), like most social insects, displays a high degree of swarm intelligence, which has made them great survivors through millions of years of evolution. In Cuba, a person may be described as "smarter than bibijaguas" (sabe mas que las bibijaguas). The poor evacuation efficiency illustrated by the phenomenon of symmetry breaking in escaping ants is one of the few examples where bibijaguas do not seem to behave collectively in a smart way.

Geng Li and coworkers from Beijing Normal University tested whether and how the density of a group influenced symmetry breaking in escaping ants, using the red imported fire ant (Solenopsis invicta) to repeat the experiment mentioned above with different total of ants. The results show that the symmetry breaking increases at low density, but decreases after a peak. In other words, when density is low, the ant group produces a collective escaping behavior, while at high density, they behaves more like random particles.[2]

The common idea is that the action of the repellent induces herd behavior in the ants. When ants are in "panic", they experience a strong tendency to follow each other. As a result, if a random fluctuation in the system produces a locally large amount of ants trying to reach one of the two doors, the fluctuation can be amplified because ants tend to follow the direction of the majority of individuals, leading to that door getting crowded.

E. Altshuler and coworkers were able to reproduce their symmetry breaking experiments in ants using a simplified version of the theoretical model proposed earlier by Helbing et al. for humans,[3] based on the fact that walkers tend to follow the general direction of motion of their neighbors ("Vicsek's rule"[4]), and such herd behavior increases as the so-called "panic parameter" increases. In the case of ants, the panic parameter is supposed to be low when no repellent is used, and high when the repellent is used.

A more "biologically sensible" model based on the deposition of an alarm pheromone by ants under stress also reproduces the symmetry breaking phenomenon, with the advantage that it also predicts the experimental output for different concentrations of ants in the cell.[2]

The pheromone mechanism shares the key element of the previous models: stressed ants tend to "follow the crowd".

From the statistical point of view, human beings and ants seem to behave similarly in certain scenarios, such as evacuation from a room or cell under stress, resulting in symmetry-breaking phenomena; while ants increase their tendency to follow each other under stress, human beings seem to forget about emergency evacuation strategies, and just run towards the door to where most people run. However, the enormous physical and behavioural differences between humans and ants imply also big differences in their collective behavior.[5][6]

In the symmetry-breaking experiment on ants, for example, the individuals are relatively "polite"; ants are rarely crushed by others in the process of evacuation. When humans are in panic, however, deaths by overpressure or suffocation are common, as in the case of the Love Parade disaster in Germany in 2010, or the Astroworld Festival crowd crush in 2020.