A New Case of Consensual Decision: Collective Movement in Earthworms

Collective movements are reported for many species from microorganisms to humans. But except for a few soil-inhabiting species, intra-specific interactions in soil are poorly studied. Some intra-specific interactions occur in earthworms. Most of them are negative, concerning parameters like the rate of survival, maturation, food ingestion or growth. Virtually nothing is known about collective movement in earthworms that represent the dominant biomass of the soil. This study, the first one on annelids, highlights a consensual decision phenomenon based only on contact between followers. Using an olfactometer set-up and modelling, we show that earthworms Eisenia fetida influence each other to select a common direction during their migration. Experiments in a binary choice test showed that contacts between individuals are responsible for collective movement. This coordination in movement could allow earthworms to benefit from forming clusters. The resulting local higher densities, enhancing individual survival and favouring the cooperation, may be at the origin of Allee effects reported for these species.