The mystery of the swarm’s behavior came to the fore

The mystery of the swarm's behavior came to the fore

Thousands of individuals move in a coordinated manner: what is the function of the often complex schooling behavior of fish? In the case of a species of fish hit by “la ola waves,” researchers are now able to document how the effect works against birds of prey: When they see the fish repeatedly emerging and budding, they lose their senses. lose aggression. However, what exactly this effect is based on is not clear. The robbers may be confused, but the fish can also give them this message: “It’s not worth attacking anymore, because we’re on our guard now”.

A herd also speaks of intelligence: structures made up of many individuals can generate meaningful responses to each other through cues and certain behaviors. This interesting phenomenon has attracted a lot of scientific attention in recent years. However, the focus was mainly on interactions between individuals that lead to collective behaviour. However, the functions it fulfills are not well researched. It is believed that herd behavior serves to protect against predators – but there is hardly any empirical evidence for this effect, especially in the wild. A German-Mexican research team is now helping to bridge the gap.

A curious example in sight

As part of their study, they observed fish found in the springs of Baos del Azufre, near the Mexican city of Tipa. The water there contains a lot of toxic hydrogen sulfide and little oxygen. But Sulfur Molly (Poecilia sulphurea) can handle it. These two-centimeter small fish appear there in large schools that often contain more than 100,000 individuals. To get enough oxygen, they usually stay near the surface of the water. But it does attract predatory birds. When attacked, the fish exhibit an interesting schooling behavior: they respond en masse by diving in stages, each fish touching the surface of the water with its tail. From afar it looks like “La Ola Waves” running through the water.

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Fish diving is a normal response to avoid birds, but the behavior is special in this case, the researchers explain. Because if the attacking bird no longer attacks directly, the fish will allow the waves to flow through the water several times in a row – sometimes up to two minutes. “Since the observed waves are distinct, frequent and regular and the intervals between individual waves are always of equal length, it was clear that the wave speed is greater than a pure escape response,” co-authors of the Leibniz Institute -As author David Bierbach explains. For Aquatic Ecology and Inland Fisheries in Berlin (IGB). The researchers investigated whether these wave movements had an effect on the birds’ behavior. To do this, they evaluated recordings of activities in the water and also used experimental stimuli to monitor fish behavior.

A protective function is emerging

As the team reports, their assessment shows: The longer kingfishers (Chloroceryl americana) wait before another attack, the more waves they experienced after their first attack. “Sometimes the birds leave before they move on to the next attack,” reports Carolina Doran, first author of IGB. In addition, the researchers specifically examined the effect of the waves on the sulfur tyrant (Pitangus sulphuratus). When these birds begin their hunt, they will deliberately throw small objects into the water and make repeated waves of fish. When faced with several waves, the sulfur dictators also delayed their attacks. Furthermore, the success of their attack sank and they proceeded to other parts of the river as assessed.

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Overall, the results speak for the protective function of fish waves. Researchers say this is the first strong evidence that collective behavior is responsible for reducing an animal’s risk of being hunted. According to him, studies play an important role in the study of collective behavior of animals in general. “So far, scientists have mainly explained how collective patterns arise from the interactions of individuals, but it was not clear why animals form them in the first place,” says IGB senior author Jens Krauss. “Our study shows that collective behavior patterns can be very effective in protecting themselves from predators”.

Exactly what effect Sulfur Molly’s La Ola waves have is unclear. It is possible that the visual effects may confuse invading birds. But this may not be the only reason: the behavior may also have evolved from fish to birds as a signal. That’s why it might convey the message: “We know you’re there, don’t waste your time attacking us!” Says IGB co-author Julian Lucas. Therefore, the researchers want to put the exciting behavior of Sulfur Molly in their sights.

Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries, expert article: Current Biology, doi: 10.1016/j.cube.2021.11.068

Video Credit: Julian Lucasso

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