Vacancy Chains Provide Aggregate Benefits To Coenobita clypeatus Hermit Crabs

Abstract

Vacancy chain theory describes a unique mechanism for the sequential distribution of animal resources across multiple individuals. This theory applies to any resources, such as shelters or nest sites, that are discrete, reusable, and limited in use to single individuals or groups at one time. Hermit crabs rely on gastropod shells for shelter, and a single vacant shell can initiate a chain of sequential shell switches that distributes new resources across many individuals. Using the terrestrial hermit crab Coenobita clypeatus, we examined the previously untested theoretical prediction that this process will yield trickle-down resource benefits to vacancy chain participants (aggregate benefits). In laboratory experiments, we measured improvements in shell quality when a single vacant shell was provided to groups of eight crabs. We found that crabs participating in vacancy chains (averaging 3.2 individuals) gained significant reductions in their shell crowding. In addition, vacancy chains terminated early when experimental groups included a single crab occupying a damaged shell, because damaged vacancies always remained unoccupied. Hermit crabs in damaged shells were more likely to win resource contests for high quality shells against size-matched hermit crabs in crowded shells. Finally, field additions of many new shells to an island population of C. clypeatus hermit crabs reduced average shell crowding for crabs of all sizes, possibly from propagation of benefits through vacancy chains. These results provide empirical support for the theoretical prediction that vacancy chains should provide benefits distributed across many vacancy chain participants. Since shelter-based vacancy chains likely occur in other animals, additional studies of vacancy chain processes should provide new insights into resource acquisition behaviors in diverse animal groups.