Cockroach groups can behave as though they remember a collective decision, even when no individual insect retains any memory of it, according to new research published in the journal PLOS One.
A team led by social ecologist Mariano Calvo Martin of the Universite Libre de Bruxelles studied the American cockroach (Periplaneta americana), which instinctively seeks darkness and humidity. Researchers placed the insects in an enclosure with two identical shelters, one lit in red light, which cockroaches perceive as near-total darkness, and one in green light.
As expected, most cockroaches gathered under the red light. Scientists then swapped the illumination: the crowded shelter turned green and the empty one turned red.
The response depended entirely on group size. A lone cockroach moved immediately to the red-lit shelter. Large colonies, however, stayed put, ignoring the changed conditions entirely.
According to the study’s authors, the group’s memory was not stored in any individual insect’s brain. Instead, it emerged from social attraction: each cockroach preferred to remain close to the others, and that collective pull overrode any environmental signal to move.
To test the explanation, the team ran computer simulations using virtual insects programmed with only two rules: prefer red light and stay near others. No individual memory of the original shelter was coded in. The models reproduced the laboratory results.
The authors said the phenomenon may extend well beyond cockroaches, potentially shaping collective decisions in bird flocks, fish schools, ant colonies, and humans.
