Strange Behavior of Pillbugs: How Streetlights Make Them Run in Circles
Israeli biologists have described a previously unknown behavior of terrestrial pillbugs: at night, they gather in funnels of thousands and run in circles endlessly. The cause of this phenomenon is light pollution. The vertical beam of a streetlight disrupts the navigation of these crustaceans, forcing them to move along the edge of the illuminated area.
Collective circular movements in nature are often associated with disorientation. A well-known example is the ant "death carousel," where insects follow a pheromone trail and walk in circles until they die from exhaustion. Mass synchronous movement in terrestrial isopods has not been officially recorded before. At night, they may gather under stones to retain moisture, but directed collective movement has not been observed.
The issue was noted by amateur naturalists who filmed strange living whirlpools in the Golan Heights and the Jezreel Valley. Researchers from the Hebrew University of Jerusalem digitized the recordings using tracking algorithms.
The authors of the study, published in the journal Ecology and Evolution, identified the trajectories of 60 individual pillbugs from a crowd of about 5500 of the species Armadillo sordidus. The analysis confirmed that the animals form a strict synchronous circular flow rather than just moving chaotically.
To determine the cause of the phenomenon, zoologists captured some of the circling pillbugs and conducted field experiments. They tested three hypotheses: magnetic disorientation, light exposure from a UV lamp, and a bright white LED lamp. Examination of the captured individuals ruled out the mating dance hypothesis: the ratio of males to females in the crowd was one to four, with many females carrying offspring. Experiments with the magnet and UV light also yielded no results: pillbugs either ignored them or gathered in a chaotic pile.
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The solution lay in the geometry of white light. When researchers shone the lamp from the side, pillbugs simply clustered at the source. But when the beam was directed vertically—downward, simulating a streetlight—the animals formed a ring and began to run continuously.
The authors reconstructed the mechanics of this process: at night, pillbugs emerge in search of food and are attracted to artificial light. Once under the streetlight, the animal reaches the edge of the illuminated area and begins to move strictly along its boundary. When a critical mass accumulates at the edge of the light circle, they push each other and initiate a continuous conveyor. This navigational error becomes an ideal feeding ground for predators: during observations, researchers noted a large centipede that appeared to be hunting the running pillbugs.
The study demonstrates that anthropogenic light pollution disrupts the basic algorithms of spatial orientation in invertebrates. A regular streetlight acts as a geometric trap, causing entire populations to expend energy on meaningless movement.
