They slide together through the water with synchronous movements – but what are the advantages of fishing in a group? With the help of movable replicas of the animals, researchers have now been able to confirm experimentally for the first time that swimming together creates hydrodynamic effects that enable energy savings. The researchers were then able to confirm that fish actually use the concept demonstrated by the robots.
From small groups in ponds and lakes to gigantic swarms in the oceans: In many species of fish, the individuals form associations and display synchronous movement behavior. One purpose of this strategy is to confuse predators, but it has also been believed that swimming in a school has energetic benefits. So far, however, there have only been theoretical models and calculations. The extent to which fish actually save energy by swimming in a bandage only seemed to be verifiable by directly measuring their energy balance. However, such investigations have so far been considered hardly feasible.
Fish robots illustrate the energy consumption
An international team of researchers has now mastered this problem with the help of robotics. The scientists developed a fish robot that has a soft tail fin and propels itself forward with undulating movements that perfectly imitate the swimming behavior of its natural models. In contrast to these, however, the robots allow a direct measurement of the energy consumption when moving. “By developing these bionic robots, we were able to solve the fundamental problem of determining energy consumption when swimming,” explains first author Liang Li from the Max Planck Institute for Animal Behavior in Constance. “When we then let several robots interact with each other, we were able to examine how different strategies of swimming together affect the costs of transportation.”
Specifically, during the experiments, the researchers recorded the energy consumption of the following fish robots in many different positions relative to the specimen swimming ahead. There were clear differences between fish robots swimming alone and those that moved in pairs. It also became apparent how this is related to the influence on hydrodynamics. The energy consumption of a subsequent fish is therefore determined by two factors: by its distance from the lead fish and by the relationship between its tail and this animal.
Synchronization with a delayed cycle
This means: In order to make optimal use of the vortex created by the lead fish, the following fish must adjust its tail stroke to the beat of the lead fish depending on its distance from it. The secret is an adaptive synchronization: In order to save energy, the animals have to adapt their tail flaps to that of the lead fish, whereby a corresponding time delay is necessary depending on their position. This is a previously unknown strategy that the research team calls “vortex phase matching”. “We discovered a simple rule for synchronizing with neighboring fish, which allows fish swimming afterwards to continuously use eddies created by the school. Before our robot experiments, we just didn’t know what to look for, and so this rule has so far remained undiscovered, ”says senior author Iain Couzin from the University of Konstanz.
In order to make the hydrodynamic effects in the system visible, the scientists let tiny hydrogen bubbles rise in the water and illuminated them with a laser during the experiments. This made the vortices generated by the robots’ swimming movements visible. It turned out that the lead fish create whirlpools that then migrate downstream. It was also shown that the robots could use these vortices in different ways. “Obviously it’s not just about saving energy. By adjusting their pace, fish swimming behind can also use the whirlpools created by other fish to generate and accelerate forward thrust, ”says co-author Máté Nagy from the Eötvös Loránd University in Budapest.
Concept proven in real fish
Until then, however, the results for the robots only suggested that their natural role models also use the strategy of “vortex phase matching” to save energy. To confirm this, the researchers used artificial intelligence methods. They used the adaptive system to analyze the posture of goldfish swimming together. The scientists were able to confirm that the strategy is actually used in nature.
“Schools of fish are highly dynamic, social systems,” says Couzin. “Our results now provide an explanation of how fish can benefit from the eddies of water that are generated by neighboring fish without having to maintain fixed distances from one another,” the scientist sums up. In addition to the biological significance of the findings, the concept could now also be used in the development of swarms of swimming robots that move through the water in an energy-saving manner, the researchers conclude.
Source: University of Konstanz, specialist article: Nature Communications, doi: 10.1038 / s41467-020-19086-0
Video: Dr. Liang Li, Max Planck Institute for Animal Behavior