Some of the most stunning examples of animal behavior seem to be driven by predators. A massive cloud of starlings darting away from a hawk that looks like swirling plume of smoke, for example. Or a shiny school of sardines twirling as dolphins circle around. Biologists have long believed such behavior confuses predators, giving each member of the group more protection. Despite the wild kingdom drama, scientists have struggled to prove this kind of collective action makes individual prey any safer. Now, a study of fish in a toxic stream in Mexico has provided the evidence they’ve been looking for.
It’s a “really nice” study, says Christos Ioannou, a behavioral ecologist at the University of Bristol, who was not involved with the research. “It’s a very different way of thinking about what we normally see in bird flocks or fish schools.”
The sulfur molly (Poecilia sulphuraria), native to southeastern Mexico, has made the best of a deadly environment. The 2.5-centimeter-long fish has genes that allow it to survive in streams that have naturally high concentrations of hydrogen sulfide, a toxic compound that leaches from nearby soil and rocks. (Researchers must wear gas masks near some of the streams.) Few other creatures can survive these conditions, so the mollies have no predators within the streams. There is danger in the air, however. Egrets and other kinds of birds flock to the shore to hunt the mollies, which swim in schools of thousands.
Jens Krause, a fish biologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries, and colleagues came to streams near Teapa, Mexico, to study an entrancing behavior of the mollies. The fish usually hang out at the surface of the stream, where there is more oxygen. But when they feel threatened, they dive about 10 centimeters below the surface and stay submerged for a few seconds. The diving prompts other nearby mollies to also take cover, which triggers a cascade of action. On the surface, this appears as a glimmering wave that darts away from a predator in various directions for up to 2 minutes (see video, below).
“It’s very much like dominoes that fall,” Krause says. “The individual domino doesn’t go anywhere. But the line of falling dominoes moves away.”
After studying the behavior of the birds, the team realized there was a rare opportunity to investigate the potential antipredator benefits of escape waves. That’s because various bird species hunt in different ways. Green kingfishers (Chloroceryle americana) swoop over, plunge into the water, and almost always trigger an escape wave. But great kiskadees (Pitangus sulphuratus) are more discrete and only dash their beak into the water—a behavior that rarely triggers an escape wave.
So the researchers waited until a kiskadee was perched near a stream. Just as it was about to fly over the water to hunt, they fired a slingshot into the water, alerting the mollies to begin an escape wave. They reasoned that if the escape wave is an effective defense against predators, then it should cause the kiskadees to catch fewer fish than when they conduct their normal stealthy hunts.
That’s exactly what they found. After recording 718 attacks by kiskadees in 2017 and 2018, the scientists discovered that in their normal hunts, the kiskadees snagged a fish in 54% of attacks. But when the researchers triggered a burst of escape waves, the birds succeeded only 21% of the time, they report today in Current Biology.
The escape wave might make the attacks less effective by startling the predators, the researchers say. The shimmering motion probably also confuses the birds, because it makes it appear the fish are darting away in different directions.
There may be a consolation prize for the predators, however. In the kingfisher’s case, for example, a wave might indicate the fish are aware of their presence, which could make an attack less likely to succeed. So the wave could cue the birds to conserve their energy and wait until they have the element of surprise.
The researchers hope to test this idea with a new experiment. They would like to build a temporary cage around part of a stream and introduce kingfishers from other parts of the country. These birds would presumably not realize that the conspicuous wave is a signal that the fish are forewarned, and so go ahead with what is likely to be an unsuccessful attack.
It would be noteworthy if the sulfur mollies are indeed sending a message to their predators through a collective effort, the researchers say, because the fish are not closely related to each other. The evolution of a group alarm signal has only been found in species where the individuals that signal together are closely related, such as in a bee colony. “I think it really raises some interesting possibilities,” Ioannou says.