Just not out of pleasure, but out of dire necessity.

In a full football stadium it is a beautiful sight when visitors stand up in sequence in quick succession and throw their arms in the air. This creates a beautiful wave – wave – over the grandstand. Researchers now have in a new study discovered that even small freshwater fish, known as Poecilia sulphuraria, sometimes collectively venture into the ‘wave’. It’s just a matter of life or death for them.

Poecilia sulphuraria

The freshwater fish Poecilia sulphuraria is a remarkable beast. “It’s a viviparous fish endemic to a small area of ​​sulfur-rich waters in southern Mexico,” study researcher David Bierbach said. Scientias.nl. “Poecilia sulphuraria is one of only two species of fish to survive in these wells. Normally, water that is rich in sulphide is poisonous to animals; it hinders oxygen uptake through the blood. Moreover, such waters contain hardly any oxygen, with the exception of a thin oxygen-rich layer just below the surface of the water. Poecilia sulphuraria however, has developed special, sulfide-resistant proteins. In addition, these fish have a large head with large gills in order to be able to use the oxygen-rich surface layer very effectively.”

wave

However, this also has a rather big disadvantage. Because the fish often swim in that oxygen-rich top layer, they are an easy target for hungry birds. But it turns out they have come up with something clever. Because thanks to a beautiful collective wave, they manage to escape a fatal attack from above. A wave that resembles… the wave!


Watch the fish do the wave in this video.

Remarkable scene

It is a remarkable scene. “Especially because there are so many fish working together to produce the waves,” Bierbach says. “Hundreds of thousands of fish took part in the largest wave we saw. They then dive down in a waterfall-like manner.” In that sense, the observed behavior is very unique. “Fish diving down to escape birds is a well-known phenomenon,” Bierbach says. “But the repetitive, rhythmic and regular diving – where the intervals between individual waves are always the same length, no matter how many times the fish repeat the wave – is something we don’t see very often.”

People

Researchers tracked down this remarkable fish behavior after walking along the sulfur-rich waters. The behavior is hard to miss. That’s because the fish not only launch the wave when birds are nearby, but also in response to unsuspecting people passing by. “At first we didn’t understand what the fish were doing at all,” Bierbach says. “Once we realized they were waves, we wondered what their function might be.”

birds

The presence of many fish-eating birds around the river led the researchers to suspect that the fish are doing the wave in defense. And that works. Because the chance that a hungry bird will catch a fish during the wave is a lot smaller. Although the researchers think there may be more behind it. “We consider it likely that this is not the only way for the fish to escape an attack,” Bierbach says. “The wave may also be a method of confusing an attacking bird by ‘overwhelming’ its senses.”

Conscious

The question is whether the fish are actually aware of this. Are they really smart enough to know exactly how to take on predators? “Actually, we don’t think individual fish are aware that their own diving behavior is causing a giant wave,” Bierbach says. “Individual fish either react directly to an attacking bird – if within detection range – or copy the behavior of their closest neighbour. Although fish are smart, the behavior we observe can also be explained by some simple interaction rules.”

Defense

Taken together, the findings show that the freshwater fish have a strong weapon against predators thanks to a beautiful wave. “This is one of the first studies to show that a certain collective behavior of prey functions as a defense against predators,” Bierbach said. “A finding that has long been suggested, but which is difficult to test experimentally in wild animals.”

So it is clear that the wave of Poecilia sulphuraria rigorously reduces the bird’s chance of a successful attack. What scientists don’t yet know, however, is why this is so. Are the birds really confused? Or do the waves tell them they’ve been spotted and therefore have a smaller chance of catching a fish? In future studies, researchers plan to continue “hunting” for answers.