These worms stack together to form living towers, new study finds

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Nature seems to offer an escape from the hustle and bustle of city life, but the world at your feet may tell another story. Even in the shade of a fruit tree, you could be surrounded by tiny skyscrapers — not made of steel or concrete, but of microscopic worms wriggling and writhing into the shape of long, vertical towers.

Even though these miniature architects, called nematodes, are found all over Earth’s surface, scientists in Germany recently witnessed their impressive building techniques in nature for the first time.

After months of closely inspecting rotten pears and apples in local orchards, researchers from the Max Planck Institute of Animal Behavior and the University of Konstanz were able to spot hundreds of the 1-millimeter-long (0.04-inch) worms climbing onto one another, amassing structures up to 10 times their individual size.

To learn more about the mysterious physics of the soft, slimy towers, the study team brought samples of nematodes called Caenorhabditis elegans into a lab and analyzed them. There, the scientists noticed the worms could assemble in a matter of hours, with some reaching out from the twisting mass as exploratory “arms” sensing the environment and building accordingly. But why the worms formed the structures wasn’t immediately clear.

The team’s findings, published Thursday in the journal Current Biology, show that even the smallest animals can prompt big questions about the evolutionary purpose of social behaviors.

“What we got was more than just some worms standing on top of each other,” said senior study author Serena Ding, a Max Planck research group leader of genes and behavior. “It’s a coordinated superorganism, acting and moving as a whole.”

Living towers: A closer look

To find out what was motivating the nematodes’ building behavior, the study team tested the worms’ reactions to being poked, prodded and even visited by a fly — all while stacked in a tower formation.

“We saw that they are very reactive to the presence of a stimulus,” said the study’s first author, Daniela Perez, who is a postdoctoral researcher at the Max Planck Institute of Animal Behavior. “They sense it, and then the tower goes towards this stimulus, attaching itself to our metal pick or a fly buzzing around.”

This coordinated reaction suggests the hungry nematodes may be joining together to easily hitch a ride on larger animals such as insects that transport them to (not so) greener pastures with more rotten fruit to feast on, Perez said.

“If you think about it, an animal that is 1 millimeter long cannot just crawl all the way to the next fruit 2 meters (6.6 feet) away. It could easily die on the way there, or be eaten by a predator,” Perez explained. Nematodes are capable of hitchhiking solo too, she added, but arriving to a new area in a group may allow them to continue reproducing.

The structures themselves may also serve as a mode of transport, as evidenced by how some worms formed bridges across gaps within the petri dishes to get from one surface to another, Perez noted.

“This discovery is really exciting,” said Orit Peleg, an associate professor of computer science who studies living systems at the University of Colorado Boulder’s BioFrontiers Institute. “It’s both establishing the ecological function of creating a tower, and it really opens up the door to do more controlled experimentation to try to understand the perceptual world of these organisms, and how they communicate within a large group.” Peleg was not involved in the study.

The unknowns in stacks of worms

As the next step, Perez said her team would like to learn whether the formation of these structures is a cooperative or competitive behavior. In other words, are the towering nematodes behaving socially to help each other out, or are their towers more akin to a Black Friday sale stampede?

Studying the behaviors of other self-assembling creatures could offer clues to the social norms of nematodes and help answer this question, Ding said.

Ants, which assemble to form buoyant rafts to survive floodwaters, are among the few creatures known to team up like nematodes, said David Hu, a professor of mechanical engineering and biology at Georgia Tech. Hu was not involved in the study.

“Ants are incredibly sacrificial for one another, and they do not generally fight within the colony,” Hu said. “That’s because of their genetics. They all come from the same queen, so they are like siblings.”

Like ants, nematodes didn’t appear to display any obvious role differentiation or hierarchy within the tower structures, Perez said. Each worm from the base to the top of the structure was equally mobile and strong, indicating no competition was at play. However, the lab-cultivated worms were basically clones of one another, so it’s not clear whether role differentiation occurs more often in nature, where nematode populations could have more genetic differences, she noted.

Additionally, socially cooperative creatures tend to use some form of communication, Peleg said. In the case of ants, it may be their pheromone trails, while honeybees rely on their ritual dance routines and slime molds use their pulsing chemical signals.

With nematodes, however, it’s still not clear how they might communicate — or if they are communicating at all, Ding said. “The next steps for (the team) are really just choosing the next questions to ask.”

Notably, there has been a lot of interest in studying cooperative animal behaviors among the robotics community, Hu said. It’s possible that one day, he added, information about the complex sociality of creatures like nematodes could be used to inform how technology, such as computer servers or drone systems, communicates.

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