Group orientation and social order versus disorder: Perspective of outsiders toward experimental chains of social hermit crabs

Social organisms form groups with diverse configurations. While the dynamics within social groups have been explored extensively, the perspective of individuals outside of groups has been considered less. Here, we studied social hermit crabs (Coenobita compressus) and experimentally tested the perspective of "outsiders"-individuals that were not yet part of a group but were actively searching for a group to join. Using architectural arrays of shells, we simulated a natural social structure-a "social chain"-in which all group members piggyback in sequence. We then tested how outsiders perceived experimentally simulated social chains, which we varied in orientation (horizontal vs. vertical) and which we arranged to be either socially ordered or disordered (i.e., all members arranged in order of size vs. randomly). Outsiders were more attracted to horizontally vs. vertically oriented chains, consistent with a specialization for detecting groups along the horizon. Outsiders were also more attracted to the long rather than the short side of horizontally oriented chains, consistent with the greater perceptual salience of the long side. Finally, outsiders were only marginally capable of determining where they belonged within groups-in terms of size matching-for ordered chains; and they showed no such capability for disordered chains. Broadly, our experiments suggest that a group's configuration, including both its orientation and social ordering, influences outsiders' perceptions and decisions.