The performers behind these waves, which can last for minutes at a time, are sulfur mollies (Poecilia sulphuraria), a slight, silver fish less than five centimeters long. These fish are endemic to the harsh sulfur streams and spend most of their time close to the water’s oxygen-rich surface. Space is limited, so the sulfur mollies must get cozy–in some spots, there can be well more than 2,000 mollies per square meter.
Wave behavior in sulfur mollies appears to be a defense against predatory bird. Credit: Juliane Lukas
To create waves, the sulfur mollies in mass move in unison. Although they appear to be gracefully moving across the pond, they actually stay in one place. Each fish splashes its tail on the surface and dives down. The ripples spread out to other fish, creating a wave. Jens Krause, another researcher from IGB, compares this illusion to falling dominoes. “The dominoes won’t disappear, but your eyes will follow .”
the dominoes movement.
Although the mole’s penchant for making waves has been well-documented, researchers have not yet discovered the reason behind this behavior. Lukas Krause, their colleagues, and others braved extreme conditions to see the sulfur molly wave up close. They soon realized that they weren’t the only ones watching the mollies. A multitude of piscivorous birds were attracted to the massive crowds of fish, including kingfishers, herons and large flycatchers called kiskadees. Lukas states, “It’s almost like a buffet for birds.”
The birds were eager to eat, and they swooped down so often that the team was sometimes observing several attacks per minute. Although picking up the swimmers on the surface should be easy, researchers speculate that the synchronized actions of the mollies may be a defense mechanism.
A kingfisher with a successful catch of a sulfur molly. Credit: Juliane Lukas (CC BY-SA)
To determine whether the waves bothered the birds, the team set up perches around the springs to give kingfishers and kiskadees prime real estate to launch their attacks. The researchers set up cameras to record the wave action and then used them to train cameras on diving birds. Kingfishers would dive into the water to chase a molly. However, kiskadees skimmed the surface of the water with their beaks, creating fewer waves. Researchers used slingshots to launch pebbles filled with fish waves and trigger the mollies.
Their findings show that synchronized waves have a profound effect on both kingfishers as well as kiskadees. The birds were able to wait twice as long between strikes when waves pulsated through the spring. This significantly reduced the frequency of strikes. The waves made it harder for kiskadees successfully nab moles. Both birds also changed their perches frequently in response to waves. This could be because they were searching for an angle of attack.
Although the waves had a clear impact on birds, it was difficult to understand their true effects. The team speculates that predators may be confused by the rippling motion which is directed away from birds approaching. The fish could also be signaling to birds that they have been detected, by rippling. Krause believes that this could be beneficial for both the fish and birds. He says that fish don’t need to swim away from the bird to stop it attacking. The bird also benefits, because it can be detected by the fish and it will know to move elsewhere .
No matter what the mechanism, this research team believes it is the first example of collective behavior directly linking to the reduction of predator efficiency. Bertrand Lemasson is a biologist who studies predator-prey interactions in animals like zebrafish at U.S. Army Engineer Research and Development Center, Newport, Ore. The sulfur springs of southern Mexico are an interesting setting to explore this relationship. Lemasson, who wasn’t involved in the study, said that the fish being forced to remain at the surface puts them at an interface between birds and predators. These boundaries are always fascinating places in ecology because it’s often where you see most interactions .
Lukas and Krause believe that other animals may use similar synchronized movements in complex systems such as starlings avoiding falcons or sardines steering away from dolphins. The sulfur mollies show that it is possible to work together and stay off the menu. We love the collective patterns of bird flocks and fish schools, as well as herds of antelope. Krause says, “But you often wonder: What is the significance of these beautiful patterns?” “And I think this study shows it quite strongly it decreases mortality in these animals.”