If a hermit crab finds a vacant shell that's too big or too small, it sits patiently and waits for others to join. One by one, they try out the shell while other arriving crabs line up from biggest to smallest in a kind of "conga line." Once the Goldilocks crab arrives ("this shell is juuuuuust right!"), the crabs all swap shells in sequence to enjoy their new, upgraded homes.
These "vacancy chains" have been observed for many years in certain species of hermit crab, but they are not always as orderly and peaceful as I've described above. In some cases, there are scuffles over new, nice shells. And research has found that hermit crabs who currently live in damaged shells are more likely to win scuffles over new shells-- this is likely because they have more motivation to fight for a new home (since damaged shells make them more vulnerable to predation).
Hermit crabs are not the only animals to use vacancy chains, or so preliminary evidence suggests. Clown fish, lobsters, octopuses, and birds also seem to exhibit this sort of social, adaptive behaviour.
This behaviour raises many obvious and interesting questions. Through what stepwise pattern could this kind of behaviour evolve? It is likely that population density and pre-existing waiting behaviour both play a role in the development of vacancy chains. And on a more philosophical level-- do these hermit crabs (and other species) understand what is going on? Or are the individual behaviours relevant here-- testing the shell, waiting, lining up, and occupying a new shell-- each rewarding for the crab, which has no broader conception of the overall process and goal? It is difficult to say. But at least in birds-- one group that has been observed performing vacancy chain behaviour-- there is strong evidence for future planning, social know-how, and the ability to predict and understand a cascading chain of events.
Many thanks to Elizabeth Allan for sending along this article!
When a lone crab encountered one of the beautiful new shells, it immediately inspected the shelter with its legs and antennae and scooted out of its current home to try on the new shelter for size. If the new shell was a good fit, the crab claimed it. Classic hermit crab behavior. But if the new shell was too big, the crab did not scuttle away disappointed—instead, it stood by its discovery for anywhere between 15 minutes and 8 hours, waiting. This was unusual. Eventually other crabs showed up, each one trying on the shell. If the shell was also too big for the newcomers, they hung around too, sometimes forming groups as large as 20. The crabs ... clamped onto one another in a conga line stretching from the largest to smallest animal—a behavior the biologists dubbed "piggybacking."