[Un article de The Conversation écrit
par Guy Théraulaz – Chercheur au CNRS au Centre de Recherches sur
la Cognition Animale, Centre de Biologie Intégrative, Université de
Toulouse]
In animal societies, as in human crowds, the coordination of individual behaviors does not result from orders from above, but often from the capacity of individuals to perceive and react to what their neighbors are doing. These chain reactions, sometimes subtle, give rise to emerging collective behaviors, often extremely effective.
The contagion of yawning wonderfully illustrates the capacity of certain behaviors to spread from one individual to another. Although seemingly innocuous, this phenomenon observed in humans, chimpanzees or wild dogs – African wild dogs organized in very social groups – is a formidable coordination tool. Among the wild dogs (Lycaon painted) repeated yawns often precede going hunting. The more yawns, the more likely the group is to get moving. Each individual thus becomes a link in a distributed decision-making mechanism: no need for a leader, action emerges from mutual imitation.
This logic extends to other species. The giant bees of Asia (Apis dorsata), in response to hornet attacks, trigger a spectacular visual and sound wave by rhythmically straightening their abdomen. This behavior, called “flickering,” acts as a synchronized collective alarm that scares away predators.
In humans, the ola in the stadiums illustrates a similar dynamic. A small group of supporters stand up, arms outstretched, urging their neighbors to do the same. The movement, if launched in a collective state of intermediate excitability (when the spectators are neither too calm nor too agitated), will propagate like a wave across the arena. Each spectator acts as a cell in an excitable system, moving from inactive to active to refractory (temporarily inactive) states, as do heart or nerve cells.
Behavior Synchronization
A step above simple contagion is synchronization, where all individuals in a group align over time. The fireflies Photinus carolinus are famous for this. Each male emits flashes of light to attract females, but when the density becomes sufficient, these flashes synchronize in collective bursts every 12 seconds. The mechanism is simple: each firefly acts like an oscillator adjusting its rhythm to that of the others. If she perceives a flash early in her cycle, she delays hers; if it is late, she brings it forward.
It is the same principle among human applause : at the end of a concert, spectators often end up clapping their hands in unison. This does not require a conductor or explicit instructions. Individual rhythms gradually align, each person acting like a metronome influenced by their neighbors. The effect is even stronger as the frequency of applause decreases, facilitating synchronization.
Propagation dynamics in motion
The propagation of social information becomes even more complex when groups are on the move. In fish or birds, a change of direction initiated by a single individual can involve the entire group. In this case, there is no notion of “leader”.
For example, in the yellow shiner (Notemigonus chrysoleucas), a species of freshwater fish native to North America, a single individual changing tack can set off a chain reaction without any external threat. This behavioral plasticity is also observed in desert locusts which, even in a homogeneous environment, spontaneously change direction of movement.
Fine studies that we carried out at the Animal Cognition Research Center in Toulouse, made it possible to characterize these interactions. Thanks to the analysis of fish trajectories filmed in the laboratory, we now know that each individual adjusts its direction according to a small number of close neighbors. He aligns himself more with those he sees in front or to the sides, but almost not with those behind him. This selective attention lightens the cognitive load, while ensuring rapid propagation of information by domino effect.
And if the intensity of these social interactions varies, the whole form of collective movement changes: dispersion, circular vortex (circular collective movements in which they swim around an empty central area), polarized shoal (swimming in the same direction), or disorganized swarm. By simply modulating the attraction and alignment between them, fish can adapt their collective behaviors to circumstances. Better yet, they can collectively fall into a critical state where the slightest disturbance such as a sudden change in light or stress induced by the presence of a predator is enough to trigger a rapid transformation of the entire group. This state of “maximum vigilance” constitutes a form of distributed intelligence, ready to react even before everyone has perceived the danger.
The power of traces
Information does not always come through direct observation. In social insects like ants or termites, it often spreads through the environment via traces left by individuals. This is what the biologist Pierre-Paul Grassé called the stigmergy. When an ant discovers a food source, it leaves a trail of pheromones on the ground as it returns to the nest. Other ants follow this trail, reinforce the trace, and self-reinforcing traffic is set up.
This simple mechanism allows remarkable collective decisions. For example, faced with two sources of sugar of different qualities, ants of the species Lasius niger favor the richest, without needing to compare it explicitly. The chemical marking varies depending on the perceived quality: the more concentrated the sugar, the more reinforced the track.
But this system can also trap the colony. If a lead is well established to a poor source, the subsequent introduction of a better source is not enough to cause the group to change. Collective chemical memory becomes a behavioral lock. The same logic applies in choosing the shortest path between a nest and a source; the ants “choose” by differential reinforcement, amplifying the option whose signals grow the fastest, in this case, the quickest way to go.
The art of collective adjustment
From the dance of fireflies to the trails of ants, from schools of fish to crowds, the propagation of social information always relies on the same fundamental principle. Each individual, with their limited perceptions and simple rules, adjusts their behavior to that of others. From these local adjustments emerge collective decisions, forms of coordination, and sometimes even intelligent behavior without a leader or preconceived plan. These mechanisms, long reserved for the natural sciences, are now of interest to urban planners, designers of interactive systems, or specialists in artificial intelligence. Because understanding how social information circulates, filters, amplifies or disappears in a group also means laying the foundations for a more responsive, fairer and more cooperative society.
