Some fish start Mexican waves to keep themselves safe from predators

New research reports that at least one species of fish engages in similar behavior to sports fans — collective waves.

Kingfisher bird with Sulphur molly. Image credits Juliane Lukas.

It’s not uncommon to see collective — also known as ‘Mexican’ — waves on arenas hosting football (soccer) matches around the world. These involve large groups of fans successively standing up in unison, as a display of solidarity between them and for their favorite teams.

Sulphur mollies (Poecilia sulphuraria), however, do it for a completely different purpose. A new paper describes this incredible collective behavior in the wild fish species, detailing how hundreds of thousands of individuals coordinate, likely to protect themselves from predatory birds.

Stronger together

“At first we didn’t quite understand what the fish were actually doing,” said David Bierbach, co-first author of the study. “Once we realized that these are waves, we were wondering what their function might be.”

The study showcases just how many of the fish partake in such behavior — there can be up to 4000 fish per square meter of ‘wave’, and each can include hundreds of thousands of individuals, according to the team.

Sulphur mollies are small animals, who stand out due to their preferred environment: sulphuric springs whose chemical make-ups make them toxic to most other species of fish.

The team explains that they likely use this living wave behavior as a way to confuse or maybe deter predators, especially birds. Mollies engage in this behavior when a person’s shadow falls on the water as well, further reinforcing this hypothesis. Individual waves last three to five seconds each, but the mollies have been recorded as repeating the behavior for up to two minutes.

The team first had to rule out the possibility that this behavior was random — their experiments showed that the fish would engage in ‘waves’ in a conspicuous, repetitive, and rhythmic fashion in response to stimuli associated with the presence of predators.

Then, they examined whether this behavior had any effect on the predators themselves: it does. The team reports that experimentally-induced fish waves dramatically reduced the frequency of attacks from birds of prey, and doubled the time these birds took between attacks. For one of their predator species (kiskadees, Pitangus sulphuratus), wave patterns also decreased capture probability.

Birds exposed to these wave patterns would switch perches more often than control individuals, suggesting that they may prefer to focus their attention on other prey when confronted with the mollies’ wave behavior.

According to the team, this is the first time a collective behavior has been shown to be directly responsible for reducing a species’ chances of being attacked and preyed upon. It is an important discovery for the study of collective behavior in animals more broadly, they add.

“So far scientists have primarily explained how collective patterns arise from the interactions of individuals but it was unclear why animals produce these patterns in the first place,” says co-author Jens Krause. “Our study shows that some collective behavior patterns can be very effective in providing anti-predator protection.”

Something that the team can’t yet explain is why such behavior helps protect the mollies from attacks. It’s possible that the motions confuse the birds, or perhaps they work as a signal to the bird that they have been spotted, making it consider another target altogether. The team plans to explore these questions in the future.

The paper “Fish waves as emergent collective antipredator behavior” has been published in the journal Current Biology.

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