Architectural modification of shells by terrestrial hermit crabs alters social dynamics in later generations.

Organisms architecturally modify environments and these modifications may persist across generations, potentially strongly shaping social behavior. However, few experiments have directly tested the impact of architectural modifications from earlier generations on social behavior in later generations. Here, I report experiments using extremely durable resources, shells, which endure for decades to centuries in stable form. Terrestrial hermit crabs (Coenobita compressus) architecturally remodel shells and pass these modified shelters to subsequent generations, which reuse them long after the original architect's death. I conducted controlled field experiments in a population of these crabs in which shells have been individually marked and tracked for a decade. I examined the impact of architectural modifications by contrasting social behavior around introduced shells, either remodeled shells (whose internal architecture was modified by earlier generations) or unremodeled shells (whose architecture had never been modified). Remodeled shells generated radically different social dynamics than unremodeled shells, catalyzing vacancy chains in which shells were socially redistributed across the population. Social groups that formed around remodeled shells consisted of size-ordered queues, with precise timing and social coordination required if individuals were to acquire superior shells. Interestingly, comparative experiments in two non-architect species (Clibanarius albidigitus and Calcinus obscurus) failed to show any impact of architectural modifications on social behavior; such impacts were only found in the architect species (C. compressus). Broadly, architecture from earlier generations can thus play a major role in driving social dynamics among later generations.