Towering behavior and collective dispersal in Caenorhabditis nematodes

Dispersal behavior allows organisms to find new resources under harsh conditions^1,2,3,4,5; collective dispersal in group-living organisms raises interesting questions about kin selection, cooperation, and social conflicts that offer an exciting window into the evolution of sociality.^3,5 One type of collective dispersal is when individuals physically link their bodies into a super-organism and move as a group, but these phenomena are rare in nature and few empirical systems exist to enable their mechanistic dissection.^{6,7,8,9,10,11} Individuals of many nematode species can group together and self-assemble into a living tower of worms, which is hypothesized to be a collective dispersal structure.^{12,13,14,15,16} However, direct evidence demonstrating the occurrence and the function of towers in nature has been scarce. We documented towering behavior under natural, semi-natural, and laboratory conditions to confirm its existence and then manipulated these towers to confirm that they can bridge gaps and respond to external stimuli to confer group dispersal by phoresy. Having established the ecological and functional relevance of nematode towers, we developed a laboratory towering assay with the model organism Caenorhabditis elegans to exploit its experimental capabilities. Our lab assay rapidly and robustly induces towering and reveals several fundamental characteristics of both the towers and the constituent individuals, which together demonstrate the high experimental potential of using our model and the ample future research avenues that it opens. In summary, combining ecological relevance and empirical possibilities, our work sets the key foundations to establish nematode towering behavior as a powerful opportunity to elucidate the ecology, the mechanisms, and the evolution of collective dispersal.