I recently took part in what social scientists call a “vacancy chain” (a social structure through which vacancies in discrete, reusable, and limited resources propagate through a population) and all I needed was a moving truck, a few helpful relatives, a case of beer and a few pizzas. You see, when my girlfriend and I moved into a new house in May, we filled a vacancy left by the previous tenants. When we moved, someone moved into our old apartment and filled the vacancy we left. Their apartment, in turn, was filled by someone else, and their apartment was moved in to by someone else and so on and so forth. Somewhere (further up the chain than me), a vacancy was created and propagated down the socioeconomic order through a series of interdependent events and resulted in many individuals acquiring new, sometimes better (we have a patio, but no central air, so the jury is still out), resources and benefiting from them.
Hermit crabs, for whom really nice shells to call home are a scarce commodity, have evolved their own sorts of vacancy chains as way for optimizing shell acquisition and occupancy. While these shell vacancy chains have been described (and shown to provide aggregate benefits that are distributed across many participants) for several hermit crab species in previous research, not much was known about the behaviorial and ecological factors that lead to and influence them.
Cue the arrival of Randi Rotjan, Jeffrey R. Chabot and Sara M. Lewis (from the New England Aquarium in Boston, the Pfizer Research Technology Center in Cambridge, MA and the Department of Biology at Tufts University, respectively) at Carrie Bow Cay, a ¾-acre island located near the Belizean barrier reef that is home to Eighty-four palm trees and 1,084 purple-clawed hermit crabs of the terrestrial species Coenobita clypeatus.While the biologists were there study parrotfish, bad weather made the water too rough for diving, so they used their time to better understand shell vacancy chains. The researchers marked 20 locations around the island, set out a single vacant shell at dusk at each one and monitored them. Over the course of 24 hours they observed a total of 16 vacancy chains of two different types, asynchronous and synchronous.
An asynchronous chain occurs when one crab moves into a new, empty shell and abandons its old one to be found by another crab, which abandons its own for another crab to find, etc. With this type of chain, shell switching is sequential and the crabs experience little to no interference or competition. They have the opportunity to investigate any vacant shells they find and can directly compare their current shell with a new shell by switching back and forth between the two. The down side is that individual crabs aren’t very likely to just stumble upon a vacant shell that meets their specific size and quality requirements. It’s like if I told you that you could wander around your town, go into any unoccupied houses you wanted, check them out and pick your dream home, but you’d have to find the one with two bedrooms, a dishwasher and a fireplace on your own by chance, without the aid of Craigslist.
Synchronous shell vacancy chains are more social and much more interesting. They start off with “waiters,” crabs that hang around a shell that’s too big for them, and wait for a bigger crab to come along so that if the big crab moves in to the vacant shell, the waiter can grab their more appropriately-sized hand-me-down shell (the researchers note that the decision to wait, and how long to wait, based on previous experience, provides some evidence that the crabs are smarter than we thought). The chains that the researchers observed began with one to 20 waiters who spent anywhere from a few minutes to an hour-plus loitering around empty shells. As a crowd gathers, the crabs queue up by size, from largest to smallest, and once largest crab switches into the vacant shell, each crab climbs into a new shell as it’s vacated by the slightly larger crab ahead of it, quickly shuffling vacancies (literally) down the chain. In both chain types, the fun stops when the last shell vacated is so low in quality (too small or damaged) that all the crabs reject it.
A Synchronous Chain in Action
In addition to the waiting that kicks off synchronous chains, the researchers observed other unique shell acquisition behaviors that the crabs only exhibited in social contexts and appeared to be associated with the vacancy chains. At almost half of the observed locations, when the waiting crabs were all too small for they vacant shell they had gathered around, some would “piggyback,” or form lines with each crab grasping the shell of another crab from behind and frequently moving in and out of the line to jockey for a better positions. The researchers hypothesize that piggybacking may be help establish a dominance hierarchy among the waiting crabs and/or allow them to investigate some of the shells they might be able to move into. Theses piggyback lines often transformed into queues upon the arrival of crabs that were appropriately sized for the vacant shell.
At some of the locations, multiple queues formed when there were many similarly sized waiters, and the crabs in these queues appeared to engage in a “tug-of-war” for control of the vacant shell. The smallest crabs, positioned at the end of each queue, frequently switched back and forth between the lines in a possible attempt to stake its place in the winning line.
So what sets these theatrics off in the first place? Population density seems to be a key factor determining the length and type of vacancy chains. Using modeling software, the researchers created a simulated habitat space and a population of crabs of varying sizes. Rules for shell switches that realistically reflected hermit crab behavior were established and, after a while, a vacant shell appropriately sized for the largest crab in the population was placed the center of the habitat and the simulation was continued. During 100 model runs were at each combination of 2 parameters: population density (8 levels, from 10 to 900 crabs) and maximum waiting times for the waiters (2 levels), vacancy chain lengths increased along with population density at the highest population density, almost half of the shell switches that occurred were part of synchronous vacancy chains. How word about an available shell gets out among the crabs in the first place is still unknown, though. The researchers plan to address the question in a future study and speculate that the waiters may use aural or chemical signals to draw attention to the vacancy.
Reference: Rotjan, R., Chabot, J., & Lewis, S. (2010). Social context of shell acquisition in Coenobita clypeatus hermit crabs Behavioral Ecology, 21 (3), 639-646 DOI: 10.1093/beheco/arq027