Trying to find a suitable home can be a battle, especially if you’re a hermit crab. These crabs have soft, vulnerable abdomens that they need to keep protected at all times by wearing abandoned snail shells. But as the crabs grow, they can find themselves living in cramped quarters. Once they’ve outgrown their shells, they need to search for another. If they’re lucky, they can find an empty shell with perfect proportions on the beach. Other times, they may find that another hermit crab is currently in possession of an ideal shell. When this occurs, the shell-seeking crab will confront to other, attempting to evict it by force.
Hermit crabs don’t engage in these fights on a whim. Researchers have found1 that a crab will make a careful assessment of multiple factors before deciding whether or not to engage in combat. These factors include the quality of its own shell compared to the quality of the other crab’s, as well as the other crab’s size.
Yet not every instance of crabs switching shells with each other is a case of bullying. In fact, it’s not uncommon for groups of hermit crabs to cooperate with each other so that all members can find a suitable shell. They form what scientists call a synchronous vacancy chain2, something that almost looks like a conga line. Each crab will line up in order of size, with the largest crab already having chosen a large, unoccupied shell. As soon as the largest crab moves into the new shell and out of the old, all the crabs behind it do the same, trading with those directly in front of them. In the context of hermit crab cognition, and more generally, invertebrate cognition, this is a highly interesting phenomenon. You can watch one of these group trades in the video below:
Vacancy chains involve unique patterns of resource acquisition behaviors that determine how reusable resources are distributed through animal populations. Shell vacancy chains have been described for several hermit crab species, both terrestrial and marine, but little is known about the ecological and behavioral dynamics of shell choice in social versus solitary contexts. Here, we present a novel conceptual framework that differentiates 2 types of shell vacancy chain in hermit crabs and discuss fundamentally distinct predictions concerning the behavioral and ecological costs and benefits associated with synchronous versus asynchronous vacancy chains. In laboratory studies of the terrestrial hermit crab Coenobita clypeatus, we found support for the prediction that social context alters shell acquisition behaviors. Field observations demonstrated that both synchronous and asynchronous vacancy chains are common and revealed previously undescribed waiting and piggybacking behaviors that appear to facilitate synchronous vacancy chains. Additionally, simulation results from an agent-based model showed that population density and waiting behaviors can both influence the likelihood of synchronous vacancy chains. Together, these results indicate that better understanding of hermit crab resource acquisition requires studying social behaviors, including vacancy chain formation.2
This article was written by Amanda Pachniewska, founder & editor of AnimalCognition.org
1 – Barbara M. Dowds, Robert W. Elwood
Shell Wars: Assessment Strategies and the Timing of Decisions in Hermit Crab Shell Fights
2 – Randi D. Rotjan et al.
Social context of shell acquisition inCoenobita clypeatus hermit crabs