War, pandemics and stress: when we’re in danger, we stick together – the glassfish teaches us this

‘I’ becomes ‘you’. ‘We’ turns into ‘you all’. And perhaps we behave like swarms of bees moving together towards the hive, or birds chasing one another to reach the same destination, if we feel in danger – because we are social animals. By coming together, we benefit from being in a group, and just as animals do in the presence of predators, so too must we move quickly. But how do we convey this information in a matter of moments? It is fish that teach us this, and in particular the glassfish or Danionella cerebrum. It has a tiny, transparent body (about 12 millimetres long, less than the width of a little finger), which allows the animal’s brain activity to be assessed using optical microscopes. By analysing this animal, experts at the University of California, San Diego, led by Jo-Hsien Yu, who works in Matthew Lovett-Barron’s laboratory, have unravelled the mystery in a study published in *Nature*. There appears to be a genuine ‘neural signature’ for the detection of social behaviour in an ancient visual circuit of the midbrain found in fish, birds and primates. “The study highlights how ancestral defence mechanisms aimed at safeguarding a species—already present in the simplest animals—have been preserved even in humans,” comments Matteo Balestreri, co-president of the Italian Society of Neuropsychopharmacology and a long-standing lecturer in Psychiatry at the University of Udine. “Processing a danger by first assessing the severity of the threat, in order to subsequently organise a defensive behaviour, would be extremely disadvantageous due to the time required to mount a coherent response. However, in humans too, there are both rapid and more complex preservation mechanisms.”

Researchers have examined the group response of this fish species when faced with danger. When an object simulating a predator’s attack approaches, a collective escape proves more effective than the individual fish moving about in confusion – even for those furthest from the enemy – thanks to their observation of their neighbours’ movements and their shoaling behaviour. “The ability to pay attention to one another helps these fish detect danger,” explains a statement from Lovett-Barron University. So far, one might say, this falls within the realm of normal reactions. What is striking is that group behaviour leads them to follow others even when the threat is not visible, but is perceived precisely through their fellow fish. The study demonstrates this: using a sort of fish ‘video game’, virtual specimens of Danionella cerebrum were created, whose realistic appearance, posture and movements attracted real fish that swam alongside them on a screen. If these virtual fish suddenly fled, the real fish would scatter from the screen as if there were a real threat. “The example of the Danionella brings to mind images of mass stampedes in frenzied crowds caused by dangers arising at large gatherings,” explains Balestreri. There is no need to understand the nature of the danger: if the people around you start running, everyone tries to do the same. It is undoubtedly the visual stimulus that drives the flight behaviour because it is what conveys signals most rapidly. If, on the other hand, let us suppose, there is a smell of smoke, people do not flee, but begin to assess its source, delaying their escape. The source of the smoke cannot be seen, but the presence of a danger is cognitively perceived. What is striking about the behaviour of fish is that it is coordinated and very different from that of humans. When danger strikes, people crowd together and sometimes trample over one another. In such cases, the individual takes precedence over the group.”

Using optical microscopy, the activity of thousands of neurons in the brains of glassfish was studied whilst they observed the actions of these virtual fish. The visual neurons in the midbrain proved to be highly responsive to the actions of social partners and were strongly activated when the virtual partners ‘fled’. It must be said, however, that there is also a behavioural context to this: the small fish are, in fact, guided by jerky movements. In the absence of these visual ‘cues’ – that is, with unnatural movements – the glassfish’s brain is, in short, tuned to identify social companions based on the natural movement patterns of their species and is highly sensitive to their sudden disappearance from their expected position and to the movements they make. The lesson to be learnt, however, is simple. For these fish, much of their natural visual experience consists of observing one another, and their brains are extremely sensitive to the actions of these social partners. Animals and humans share a common trait: their brains have evolved to pay attention to one another and to each other’s actions. In short: the study’s contribution is to show us how paying attention to the actions of others can transform an individual perception into a collective response. “In a dangerous situation, what the other does can tell us something we cannot yet see,” says Balestreri.

“Observing the behaviour of others is central to every person’s development,” concludes Balestreri, pointing out that the role models provided by parents and educators, as well as those of peers, form the basis of each person’s identity. Moreover, it must be said that modern man, on the other hand, is generally not equipped to live in a state of alarm. “Of course, we might think that certain populations accustomed to living in danger (think of Ukraine or Gaza) can react more quickly when faced with a threat, but this is because they have learnt certain behaviours that have proved safer and less harmful to their neighbours,” concludes the expert. The difference between humans and fish lies in the type of defensive behaviour that can be employed. Schools of fish are unified entities; that is, they behave as if they were a single organism. If a predator attacks, they disperse only to regroup immediately. For humans, however, coordinated behaviour is exemplified by military parades, which are not a natural movement, but a constructed and idealised one.”