La Jornada: Schools of Tabasco mollies can remember their predators

The schools of sulfur mollies that inhabit the sulfur springs of El Azufre, in Tabasco, seem to retain information about recent attacks by their predators. It is a kind of “collective memory” that strengthens their defenses and forces birds of prey to modify their hunting strategies.

This was revealed by a study led by Korbinian Pacher, from the Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin, published in the journal Proceedings of the Royal Society B.

“It is often asked how prey can defend themselves from these predators, but the opposite is almost never asked: what predators do or what they can do to overcome the defenses of their prey,” Pacher tells The Day.

And he adds: “our results show that predators can develop specific strategies against the collective defenses of their prey. Now we can look for this in other systems where these strategies could be present, although they are less evident.”

Sulfur mollies live in the sulfur springs of El Azufre, in Tabasco, an extreme environment where few organisms can survive. When birds of prey try to capture them, the fish respond with synchronized dives that create waves on the water surface.

These waves formed collectively by small fish make hunting difficult and force predators to wait longer before attempting a new attack. Until this new research, little was known about how birds respond to this group defense.

For this study, in which members of the Cluster of Excellence in Intelligence Science also participated, the researchers recorded 120 hours of video and analyzed 867 attacks by three species of birds of prey.

The scientists found that kingfishers preferred to attack the edges of the schools, where defensive waves were less intense, rather than the center, where the chances of capture were higher, but also generated stronger collective responses that made subsequent attacks more difficult.

“Our results show that predators seem, at least, to anticipate that their prey have this effective defense and consider it before attacking,” the scientist points out.

The most novel finding occurred when observing the behavior of the prey. The scientists found evidence that the schools reacted more intensely when an attack occurred shortly after a previous one.

“A school of fish represents a connected network of individuals; like the brain, which is a network of billions of neurons. Like an individual fish, a single neuron can do very little, but connected together they form a network capable of performing complex tasks, including remembering things,” explains the expert.

Pacher points out that, although a fish only perceives events that occur directly next to it, the interaction with thousands of neighbors allows the group to respond in a coordinated manner to a predator.

“The fish that obtained information about the bird’s attack (because they saw the bird directly) enter a state of high alert. When they see the bird again, they instantly know what is happening and begin the defense. Other fish see this and copy the behavior, which leads to more collective waves,” he details.

So far it is unclear how this works and how the collective memory effect is connected to and influenced by individual memories. “This raises many interesting questions for the future, such as how long this collective memory lasts and whether fish also forget collectively,” he adds.

According to the researcher, although predator-prey dynamics exist in many places, the conditions of the sulphurous springs of Tabasco turn the site into a natural laboratory to study collective intelligence.

Due to the extreme water conditions, very few species can survive in these springs, so it is inferred that the collective behavior of the fish evolved mainly to respond to birds attacking from outside the water.

Another characteristic is that, since there is no oxygen, the fish must remain close to the surface and the schools consist of a single layer of fish. So “all interactions occur almost in two dimensions, which makes it easy to observe and understand them.”

Furthermore, the schools usually remain in the same area and do not move great distances, as happens with other populations. Researchers can even cause waves without the presence of the predator, if the research requires it.

However, he warns, this natural laboratory faces various environmental threats, such as the loss of riparian vegetation and pollution. According to Pacher, the disappearance of these springs would mean the loss of a unique system for studying collective intelligence.

“The future challenge is to spread the idea that not only the sulfur springs are unique and deserve protection, but also their little inhabitants, who are a true natural treasure of Tabasco.”