One early morning in June of 1986, I waded into a shallow tide pool on Long Island, squatted on a plastic milk crate and dropped an empty snail shell into the water. In a few minutes a small hermit crab skittered toward the shell, probed the opening with its claws to measure the size of the interior space and rotated the spiral casing several times to look for holes. Almost quicker than I could follow, the crab pulled itself out of its old refuge and thrust its vulnerable abdomen into the snail shell I had dropped. Satisfied with the exchange, the animal strolled away, leaving its previous, smaller shell behind. A few minutes later another hermit crab discovered the first one’s discarded dwelling and, after the same inspection ritual, scuttled away with its newfound lodging. About 10 minutes later a third crab found the second’s old home and claimed its prize, abandoning a small shell with a large hole.

It may seem strange, but this was one of the happiest moments in my life as a researcher. For nearly 10 years I had been wondering whether hermit crabs take up residence in one another’s vacated shells. I finally had my confirmation. I was the first person to observe an animal making use of what sociologists and economists call a “vacancy chain”—an organized method of exchanging resources in which every individual benefits by claiming a more desirable possession abandoned by another individual. Even though hermit crabs have relatively simple brains and nervous systems, they have evolved sophisticated social behaviors to make the most of vacancy chains.

3 Comments

1. 1. Pete2100 12:42 PM 6/12/12

   I think that the reason the vacancy chains tend to have a length of about 3 is that the underlying resource structures tend to be hierarchical. This can be seen easily in the case of the military or a company, but it applies even in something like the housing market, where there are more inexpensive houses than expensive houses.
   
   Thus, the vacancy chain doesn't extend through the entire population, with each participant getting an incremental improvement. The chain extends down through a hierarchy. There are costs and risks involved in switching, and each participant wants to make sure that the switch will be worth doing. Thus, for each individual, the improvement will need to be large enough to account for the costs and the risks.

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2. 2. Floridacrack 09:56 AM 6/18/12

   Very sweet when you suppose you were "the first person to observe an animal making use of what sociologists and economists call a “vacancy chain” but I hardly think you were the first person to observe hermit crabs trading up their shells. More like the first person to use the term 'vacancy chain', but every boy like me who grew up catching hermit crabs and keeping them in aquariums or even buckets always knew they did this. We just didn't think it was all remarkable or in any way useful.
   
   So maybe you were the first person to see useful information in the behavior, and more power to you for doing so. But no way were you the first person to observe this behavior.
   
   Seems to me you could also make an argument that human divorce could also, or at least often, be modeled as a vacancy chain.
   
   
   
   

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3. 3. Colin den Ronden 01:40 AM 10/23/12

   I would be tempted to drop a fossilized snail shell into the water just to see their reaction to that.

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