Of hermit crabs and home sales
In 2005, my husband I bought a house in Birmingham, Alabama. I was working for mental_floss and we thought we'd live there for a few years. But, in 2006, my husband got an amazing job opportunity in Minneapolis. So we moved and we sold our house. After a few months in the Twin Cities, we bought another one. In order for me to buy the house I now live in, somebody else had to move. When I left my house in Birmingham, I opened a spot in my neighborhood there that was filled by somebody else.
This is one of those things that seems so basic and "duh" that it's easy to overlook. It's easy to think that it isn't important. But sociologists, and economists, care a lot about these patterns—called vacancy chains. That's because vacancy chains end up describing very similar situations that occur in all sorts of social systems across many, many species.
When a resource is exchanged in a sequence from one individual to another, and every individual in the sequence benefits from the exchange, that's a vacancy chain. You see these patterns in human home sales—I, the people I bought my house from, and the people who bought my old house all ended up with a home that better met our needs. And you see the same thing when hermit crabs trade out their old shells for new ones.
Ivan Chase, emeritus professor at Stony Brook University, studies vacancy chains in hermit crabs and people. He's written about his work for the June issue of Scientific American, and he recently spoke with me about how vacancy chains work and what we can learn about human social systems from watching animals like crabs.
Maggie Koerth-Baker: In your Scientific American article, you talk about documenting for the first time the fact that hermit crabs take up residence in one another's abandoned shells. Iwouldn’t have thought that the discovery of hermit crabs using other crab’s old shells was that new, something discovered in 1986. Am I understanding that correctly? What was the thinking before that?
Ivan Chase: People at least in some way knew that hermit crabs used other crab's shells. They would have said that might happen. But nobody thought about it as an organized thing that could be compared to humans. We were the first people to study hermit crab shell use as vacancy chains and describe them as that.
MKB: You talked about crabs inspecting the shells. Do they ever reject them?
IC: Oh, yeah. Sometimes they’ll actually get into it while holding onto their old shell. They'll try the new one out, but end up walking away and getting back into the old shell.
There is also evidence that hermit crabs will fight over shells if two of them want the same one. Sometimes a guy will bop someone else with his big claw, then he'll run away with the shell so he can inspect it in peace. There's even some documentation that crabs can negotiate a shell swap.
MKB: Does seeing another crab abandon a shell affect whether the second crab will choose to take it? And how do these things fit in with the idea of vacancy chains?
IC: There is some indication that crabs will wait by the side of a vacant shell that might be too large for them. In that case, it's as if they understand something about there being a possible vacancy chain they can take advantage of.
Vacancy chains are one of the ways that crabs get shells, but there are other ways. The negotiations I mentioned, for instance. Also, somebody might not get the shell they want, but that’s still a vacancy chain. If you had two people putting in bids for a house, only one person gets the house. But the chain continues on down the line, even though we both want it and only one gets it.
MKB: In the article, you say that, on average, each vacancy chain benefits three individuals or groups. Is that a rule across the board, no matter what kind of resource you're looking at or species you're looking at? Are there examples that don't fit the rule?
IC: In all the studies that I’ve looked at it’s the rule. There’ve only been a few studies of vacancy chains in crabs. But other studies of vacancy chains in getting jobs, and housing, and cars ... that’s all that I’ve seen. You can get to a higher number if you look at particular kinds of situations. Those chains that started with really big houses and really high level jobs for instance. Then you can get around four moves up. You can also get particular situations where you get fewer moves than that. For instance, really low level housing.
MKB: Why don’t the chains go longer?
IC: I really wish I had a great answer to that. If I did I’d have another paper. An organization can be too big. If you think about most chains, what you see is that the vacancy is moving from one status down to another. Mostly moving down a little bit in size. One crab is moving to something a little bigger, and vacancy is going down the chain a little bit. But it gets unweildy after too many levels of control.
This is speculating, but think about a crab population. They start off very small in very small shells the size of a pencil. The biggest ones are the size of a thumb. The sizes of the shells don't encompass an infinite range. There’s a limit on how big and how small a crab can be. The biggest shell is going to move down to the next smallest crab. Pretty soon you’re at a small shell.
MKB: What happens to a vacancy chain with the nature of the resource changes? In particular, I'm thinking about the sort of jobs that don't require a new position to open up in order for you to advance, like freelancing.
IC: One of the interesting things is that some kinds of resources go by vacancy chains. But some don’t. You have to have certain prerequisites. Freelance work doesn’t produce vacancy chains. It’s a different situation and it's more complicated.
MKB: In the article, you mentioned that when a vacancy chain happens, crabs will often make much quicker decisions about taking a new shell than they otherwise would—sometimes foregoing inspection entirely. Do vacancy chains lead to good decision making?
IC: I think that animals, just like us, aren’t perfect at making decisions. We’re fairly good and they’re fairly good. Like us, though, they probably make better decisions with time to slowly check things out.
MKB: What are some other sorts of things that vacancy chains wouldn't apply to?
IC: A classic case comes from the 1800s or 1900s in Ireland. Tenant farmers held a lot of the land at that time. When they died, the land was often taken over by a son or daughter. But when that happens, the son or daughter doesn't have anything to leave behind available for someone else. They start farming their parents land, but already there on that land. If I get rid of my shirt, somebody might pick it up, but they don’t necessarily leave their shirt behind at the same time.
MKB: Have there been times when you expected to see a vacancy chain happen and it didn’t appear?
IC: No. I don’t think I have. In particular cases we might have a rotten shell that ends the chain or a good shell that nobody finds. But those cases are blips here and there. Here’s sort of an example of that, though: In the summer there’s a lot of vacant shells lying around because snails die over the winter. If you had a population of crabs that has access to all the shells they wanted, then it would be harder to start a vacancy chain.
MKB: What makes vacancy chains valuable information, as opposed to just a curiosity that we can watch in nature?
IC: I think at a certain level it’s just a really interesting observation to make. There's entertainment value. But you also have to think about why something like a crab would have vacancy chains. They’re nothing like us. And it's from there that you get real insights.
If we can find a simple social system, then we can do experiments and observations with these simple animals that we could never do with humans.
For instance, think about the question, "Why do people get jobs?" Usually, we concentrate on the individuals and what helps them to move. But vacancy chain research in crabs is a way of turning that around. I don’t care too much about individuals, I care about the processes. Certain kinds of resources can be distributed in certain ways. It allows us to see social systems in a way we never could if we’re just looking at individuals. It’s weird to say that we’re in some ways like hermit crabs, but that's what we see if we're looking at society and resource distribution without all the stuff we layer onto human society. I’m looking at the processes that form social systems and it seems that the individuals aren’t always as important as we think.
Read Ivan Chase's article in the June issue of Scientific American (Full story is behind a paywall).
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