Synchronous and asynchronous vacancy chains have been found to occur amongst both terrestrial and marine populations of hermit crabs.[4] The crabs live in shells left behind by other species but need to move into a new, larger, shell as they grow. When a vacant shell becomes available the crabs may slowly form a chain, ranging from the largest to the smallest crab until one arrives that is just the right size for the shell. Once the “Goldilocks” crab claims the new shell leaving its old shell vacant, the crabs then all rapidly exchange shells, in sequence, until they all have a larger shell, allowing them to grow without restriction.[5] This behavior makes Hermit crabs an ideal animal model for testing predictions of vacancy chain theory. Similar shelter-based vacancy chains are likely to take place in many animal groups that use discrete, reusable resources that are limited to occupancy by a single individual or group at a time; some other examples include anemone-dwelling clownfish, and cavity-nesting birds.[4]