Nature: Fish in Mexico work together to produce waves to protect them from predatory birds

Working together in their hundreds of thousands, fish that live in sulphidic springs in Mexico — aptly enough — can produce a Mexican wave that protects against birds.

This is the conclusion of researchers from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Germany who studied sulphur mollies in Teapa Municipality.

What remains unclear, however, is exactly why the waves seem to both deter birds from attacking and also make executed offensives less likely to succeed.

While further studies will be needed to explore this, the team have suggested it could be that the waves confuse the birds — or tell them they’ve been noticed.

Birds that prey on sulphur mollies include kingfishers and kiskadees.

The study was undertaken by biologist David Bierbach of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries and his colleagues.

The mollies — whose sulphuric spring habitat is too toxic for most fish — not only make waves in response to birds, but also when they catch sight of people, too. 

‘At first we didn’t quite understand what the fish were actually doing,’ Dr Bierbach said of the Mexican wave-like behaviour.

‘Once we realized that these are waves, we were wondering what their function might be,’ he added.

The team suspect that the action might be some form of defensive measure after noticing how many fish-eating birds could be found around the fishes’ river.

Investigating the mollies’ behaviour, the researchers found that the waves could be characterised as conspicuous, repetitive and rhythmic.

Furthermore, experimentally-induced waves served to generally double the time that birds near the river waited until launching their next attack on the mollies — while some even switched perches, suggesting they had decided to focus elsewhere.

For kiskadees, but not kingfishers, the probability that they would successfully capture a fish decreased with increasing wave number. 

‘The dive response in sulphur mollies probably first evolved as an individual escape behaviour toward attacking birds,’ the researchers explained in their paper.

‘We hypothesize that fish that used dive responses of close neighbours as a cue indicating predator presence (prompting them to dive too) had a fitness advantage.

‘They could respond even without having detected the predator themselves. In turn, this led to the evolution of synchronized diving behaviour.’ 

The mollies (D) — whose sulphuric spring habitat (pictured) is too toxic for most fish — not only make waves (E) in response to birds like kingfishers (B) and kiskadees (C), but also when they catch sight of people, too, the researchers discovered

‘The surprises came once we realized how many fish can act together in such repeated waves,’ added paper co-author and ecologist Jens Krause, also of the Leibniz Institute.

‘There are up to 4,000 fish per square meter and sometimes hundreds of thousands of fish participate in a single fish wave.

‘Fish can repeat these waves for up to two minutes, with one wave approximately every three to four seconds.’

For kiskadees, but not kingfishers, the probability that they would successfully capture a fish decreased with increasing wave number. Pictured: a kingfisher with a molly in its beak 

‘At first we didn’t quite understand what the fish were actually doing,’ Dr Bierbach said of the Mexican wave-like behaviour. ‘Once we realized that these are waves, we were wondering what their function might be,’ he added. Pictured: a human Mexican wave at a sporting event

‘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,’ Professor Krause explained.

‘Our study shows that some collective behaviour patterns can be very effective in providing anti-predator protection.’

The full findings of the study were published in the journal Current Biology.