What makes a living being “social”? From flocks of birds that sweep across the sky to schools of fish that move like liquid silver, the natural world is full of coordinated groups. Individuals respond to one another, forming patterns that appear almost choreographed. Yet beneath these collective movements lies a dynamic chemical balance unfolding inside individual brains. Social behaviour often begins with signals that shape how an animal senses and responds to the world around it. In humans the answer is layered with culture, cognition, and experience. Yet the foundations of social interaction may lie in far simpler biological systems. Across species, from insects to mammals, conserved neurochemicals such as serotonin influence mood, motivation, and social behaviour. Understanding how these signals are regulated inside the brain may reveal fundamental rules that shape how animals interact with one another. One of the most powerful systems for studying the roots of behaviour is a tiny nematode worm called Caenorhabditis elegans. Barely a millimetre long, this worm carries a nervous system of just 302 neurons. By comparison, the human brain contains nearly a hundred billion. Yet the worm’s behaviour is far from simple. It explores its surroundings, senses chemical cues, and changes its movement depending on food availability and internal state. Because every neuron in its nervous system has been mapped and its genes can be manipulated with precision, C. elegans provides scientists with a remarkable opportunity to trace how genes shape neural circuits and how those circuits generate behaviour. Our recent work began with a deceptively simple question: why do some animals gather together while others remain dispersed?
“Look closely, and you’ll see that every ‘us’ is actually a ‘them’ a billion chemical signals deciding whether to stay together or to drift apart“
Under normal conditions, worms spread out across a bacterial lawn while feeding. Dispersal allows individuals to access food efficiently and reduces competition with neighbours. During the course of an unrelated experiment, however, a mutant strain of worms behaved very differently. Instead of spreading out, these animals remained tightly clustered at the edge of the food source. Even when food was available nearby, they continued to aggregate in dense groups.
At first glance, the behaviour looked almost accidental. Biological experiments often produce irregular patterns that disappear when conditions change. But the clustering persisted. It appeared repeatedly across experiments and across generations of worms. What initially seemed like a curiosity began to look like a consistent behavioural trait. The question soon shifted from observation to mechanism. What could cause animals to remain together when dispersal would clearly be more advantageous? The answer turned out to lie in neuromodulation. Neural circuits do not rely only on fast electrical signals passing from neuron to neuron. They are also shaped by chemical messengers that adjust how circuits respond to incoming information. These molecules, known as neuromodulators, influence behavioural states that can last seconds, minutes, or even longer.
Serotonin is one of the most well-known neuromodulators. In humans it regulates mood, appetite, and sleep. In simpler organisms it influences behavioural states such as roaming and dwelling. When serotonin levels change, animals often shift from exploratory behaviour to more stationary activity. The mutant worms in our study appeared to be locked into one such state. Genetic analysis revealed that the animals lacked a functional copy of a gene called CASY-1. This gene encodes a synaptic protein related to the calsyntenin family found across many species, including humans. Although its molecular role had been studied in other contexts, its involvement in social behaviour had not been explored.
Further experiments showed that losing CASY-1 disrupted communication between two neuromodulatory systems. One system involved serotonin, which promotes dwelling and reduced movement. The other involved a neuropeptide called pigment dispersing factor, or PDF, which normally counterbalances serotonin signalling and encourages animals to move more freely. In healthy worms, these signals maintain a dynamic balance that allows animals to switch between exploring and feeding. In the mutants, that balance tipped. Serotonin signalling became dominant, pushing the animals toward a persistent dwelling state. Instead of spreading across the food source, the worms gathered together. What looked like social behaviour was emerging from a shift in internal brain chemistry. To understand how the behaviour of individual worms produced these group patterns, movement data were analysed using computational models developed in collaboration with physicists. The models revealed how subtle changes in movement could ripple through a population. When one worm slowed down and remained in place, nearby worms were more likely to encounter it and pause as well. Over time these local interactions generated stable clusters that resembled swarming. This principle appears throughout the natural world. Complex patterns often emerge not because individuals coordinate their actions deliberately, but because simple behavioural rules are repeated across many individuals. Similar phenomena can be seen in desert locusts, where serotonin signalling helps trigger the transition from solitary insects into enormous migrating swarms. Despite vast differences in size and brain complexity, the underlying neuromodulatory signals appear remarkably conserved. Another striking insight came from experiments that allowed neural activity to be controlled in real time. Using optogenetics, a technique that activates neurons with pulses of light, it became possible to manipulate the worm’s behavioural state within seconds. When specific neurons were stimulated, aggregated worms immediately spread apart across the plate. When neural activity was suppressed, animals that usually disperse began to cluster together. The transformation could occur almost instantly. These experiments revealed how rapidly behavioural states can shift when neural signalling changes. Patterns that appear stable at the level of a group may actually arise from dynamic processes unfolding inside individual brains.
The broader significance of these findings lies not in the behaviour of worms alone, but in the principles they reveal. Many animals rely on neuromodulators to adjust how neural circuits respond to environmental information. Small differences in how these signals are regulated can lead to profound differences in behaviour. Social behaviour therefore emerges from an interplay between individual neural states and interactions with others. Each animal carries its own internal chemistry, shaped by genes, environment, and experience. When individuals encounter one another, these internal states combine to produce patterns that extend beyond any single organism. From this perspective, collective behaviour becomes less mysterious. What appears as coordinated activity across a group may begin with molecular events inside individual neurons. The nervous system of C. elegans may be tiny, but the questions it helps answer are vast. By tracing how genes influence neural circuits and how those circuits shape behaviour, small model organisms reveal principles that extend far beyond their size. How do brains generate the behaviours that allow individuals to coexist, cooperate, and compete within a social world?
