When a group of nematodes gets together in any kind of liquid, they put on quite a show. Each worm waves its body rhythmically to propel itself along. But the waving isn’t all random—in a dense group of Caenorhabditis elegans, animals synchronize their swimming to prevent a traffic jam, a team of researchers has reported in a PNAS Early Edition paper. The observation, and findings on what mediates the synchronization, shed insight on how other entities—from molecules to larger animals—coordinate behavior.
“There’s a whole spectrum of phenomenon in biology and physics that exhibit coordinated or collective behavior,” says mechanical engineer Haim Bau of the University of Pennsylvania, an author of the new work. And in many cases, exactly how that coordination is mediated is poorly understood, he says.
Bau doesn’t usually study C. elegans—his lab teamed up with biologists who wanted microfluidics devices to contain nematodes for unrelated experiments. Once the devices were created, Bau and his colleagues became interested in how the few-millimeters-long worms behaved inside them.
“Over the course of our experiments, we observed that the animals exhibited certain synchronized behavior,” Bau says. “So we decided to probe more deeply into the questions of how large numbers of independent entities can coordinate their behavior.”
First, the researchers recorded video of worms swimming in sync—they analyzed the tapes to determine that worms most often synchronized their swimming when they were relatively close together. This ruled out hydrodynamics as a mediating force of the synchronization—if water movements provided the basis for the coordination, the worms wouldn’t have to be as close together to get into sync. To test whether the synchronization instead relied on the C. elegans sensory system, Bau and his colleagues repeated the experiments with touch-insensitive mutant nematodes. But the animals—despite having no ability to feel physical touch on the sides of their bodies—still swam in sync.
Bau knew that the coordination of movement between many smaller entities—molecules that align their movements, for instance—relies on short-range steric hindrance. Essentially, collisions between individuals leads to an aligning of motion without any sensory or adaptive behaviors required by the entities. When Bau’s team developed a model of how such steric hindrance could mediate nematode swimming, they found that it could fully explain the patterns they saw.
Despite the fact that the C. elegans have nervous systems, “the mechanics that are involved are actually similar to what’s been observed in non-living, molecular systems,” Bau says.
The findings have implications for not only nematode biology but the design of man-made entities that must coordinate movement with each other, and also could help researchers better understand how steric hindrance could shape the motions of molecules or cells. “This could even be extrapolated to social situations,” Bau says, “And how individual people move in a group.”