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.

In all likelihood, researchers will soon discover the same thing about other animals; already preliminary evidence hints that in addition to hermit crabs, limpets, lobsters, fishes, octopuses and woodpeckers also take turns upgrading their homes. Studying these animals may help us recognize and improve vacancy chains in our own communities, providing new insights for problems such as Manhattan apartment shortages and drug crime. The fact that hermit crabs and other critters depend on vacancy chains is also changing the way sociologists think about economic strategies. Some tactics, it seems, do not require human-level intelligence or altruism—they are far more universal.

CRABS IN QUEUE
From June to September 1986, as well as the next summer, I brought groups of students to West Meadow Beach on Long Island to observe vacancy chains in Pagurus longicarpus—a hermit crab common to the East Coast. I wanted to discover basic facts about the chains, such as how many crabs acquired new shells in the average sequence and whether the availability of bigger shells created longer chains. After a morning’s observations, we drove to my laboratory and immersed the crustaceans in warm water so that they would relax and we could remove them from their shells without hurting them. We weighed and measured the crabs and their shells to determine their sizes at various positions in the chains. When we had what we needed, we put each crab into a tank filled with cool water and a large selection of empty shells. When the animals had chosen a shell, we returned them to the beach and set them free.

We found that the crabs usually traded up to bigger shells and that the chains we initiated with large shells were indeed longer—allowing more crabs to get new shells—than the chains we started with small shells. Between two and three crustaceans upgraded to a new home in the chains we started—2.5 on average. Some people are disappointed to hear this number. They expect it to be larger—something on the order of 10 or even 50 crabs benefiting in each chain. I tell them that this number is large if you look at it in the right way. Usually when we think about competition, we presume that one individual or group is successful and that the other competitors are not. But in a vacancy chain, even a short one, more than one individual obtains a new possession. If only two hermit crabs acquired new shells, that figure would still be twice the number of individuals obtaining a resource compared with more typical competition.

After our studies, other researchers reported vacancy chains in various species of hermit crabs, including Caribbean land hermit crabs, which are sometimes sold as pets. One of the strangest examples involves a predatory snail that attacks other kinds of snails, including some whose shells hermit crabs particularly like. As the predatory snail grasps the prey snail, drills a hole in its shell with a rasplike tongue and injects digestive enzymes, nearby hermit crabs gather around, following the scent of chemicals released by the injured snail. When the predatory snail finally pulls its prey from its protective casing—a process that can take as long as an hour—the nearest crab dives into the now empty shell. In turn, another crab immediately snatches the first ­crab’s old shell, and so on. Instead of following the careful inspection rituals that we observed on Long Island, crabs at the scene of a mollusk murder make split-second decisions—choosing new homes based on vision alone. Everyone in the vacancy chain benefits, but the immediacy of the competition speeds everything up.

Recently researchers have made further surprising discoveries about vacancy chains in hermit crabs. It turns out that crabs use at least two kinds of chains: synchronous and asynchronous. In the asynchronous type (the kind we observed), usually one crab at a time comes across a vacant shell. But in synchronous chains, the animals queue up by size in descending order behind a crab examining a vacant shell. When the first crab in line settles on a new shell, the crab behind him takes his shell, and so on, within seconds. Such well-orchestrated behaviors suggest sophisticated social cognition, especially for an animal with a relatively small and simple brain.

Few published studies focus on vacancy chains in animals besides hermit crabs, but preliminary observations suggest that the strategy has evolved in many different species. Like hermit crabs, several species of octopuses and cichlid fish live in and defend empty snail shells. Limpets hunker down in the recesses of rocks, and clown fish snuggle up to sea anemones. Maine and southern spiny lobsters occupy small caves in rock or coral. And the red-cockaded woodpecker carves nest hollows out of the trunks of pine trees. As many of these creatures grow larger and older, they seek better-suited shelters, creating vacancies for other animals. People do exactly the same thing.

WHAT PEOPLE DO
The first studies of vacancy chains in people took place in the 1960s in Manhattan, only 60 miles from the beach where I watched hermit crabs exchange shells. The late Frank Kris­tof, then head of planning and research for the New York City Housing and Redevelopment Board, realized that the construction of new apartments created chain reactions that enabled families to move from smaller, substandard apartments to larger, more adequate ones. Kristof found that about 2.4 families moved to better apartments for each newly constructed housing unit. Following Kristof’s work, other researchers described real estate vacancy chains in the U.S. and abroad. One of the most comprehensive of these studies, examining the national housing market, discovered that the average chain helped about 3.5 families to move.

But Kristof was not the only one interested in vacancy chains in the 1960s. Harrison White, a professor of sociology then at Harvard University who coined the term “vacancy chain,” independently discovered such sequences within religious groups—specifically, Methodist, Presbyterian and Episcopal congregations. He found that the retirement or death of a preacher, the opening of a new church or a pastor’s decision to switch careers all created vacancy chains.

After White’s work, sociologists and economists investigated vacancy chains among a variety of professions: football coaches, state police, officers in the armed forces and syndicates selling illegal drugs. White and other researchers found that typically about 2.5 to 3.5 people moved to new and usually better-paying jobs in the chains. That domino effect was not always a good thing, though. Research into drug sales revealed that when the police arrest high-ranking drug dealers, they unwittingly create long vacancy chains that allow many people to advance within the illicit organization.

Vacancy chains are probably at work when people purchase some types of major consumer goods as well, particularly cars. I know of no recent published studies on this subject, but some early work points in that direction. In 1941 business scholar Theodore H. Smith carried out a massive study of the new and used car market in the U.S. Although he did not actually use the term “vacancy chain,” he concluded that such exchanges are crucial for the automobile industry. In the early 20th century car dealers realized that to sell new cars more easily, they would have to take the old vehicles of the new car buyers in trade and then sell those old cars to yet other buyers, and so on. Using Smith’s data, I estimate that about three people got cars in the average chain in his era.

Why do vacancy chains tend to benefit about three individuals or groups, both in different species of hermit crabs and in humans? My guess is that some as yet undiscovered correspondence between the demography of humans and hermit crabs explains the effect—their birth and death rates, perhaps, or the rates at which new resource units are produced and used. But these are hunches. What is clear, however, is that vacancy chains in both animals and people cannot happen with any old kind of resource—they are made possible by resources that share a distinct set of properties.

PRINCIPLES EMERGE
White defined these properties. First, such resources are coveted and relatively hard to get; jobs, cars and houses are not lying around unoccupied in large numbers, waiting to be freely taken. Second, they are the kind of thing that can be occupied or owned by only one individual or family group at a time, and these “resource units” get left behind when a new one is obtained. Finally, and most important, a resource unit cannot be taken unless it is vacant. White was thinking about people, but the same features characterize hermit crab chains. Shells are relatively scarce; only one crab at a time occupies a shell. Nearly all adult crabs have shells to leave behind when they get another, and crabs must wait for shells to become vacant before they move in.

Focusing on resources themselves turns the typical way of looking at their distribution on its head. Economists and sociologists usually think about who gets what and whether the distribution of valuable items is fair. We wonder, for example, how important intelligence, ethnicity, education or socioeconomic status is for getting jobs or homes. These questions are significant in their own right. But they sometimes prevent us from discovering other processes that influence how resources get distributed, and they can obscure commonalities across species.

Because the type of resource defines vacancy chains in both people and animals—not the kind of individuals participating in the chains—studying hermit crabs might clarify how best to maximize resource redistribution in human populations. Researchers could, for example, give a group of hermit crabs shells of different sizes and conditions, vary their birth rates, death rates and “retirement ages” by adding and removing crabs, and generally manage them and their shells to determine what situations result in the most individuals or groups moving up in the world most quickly. After all, we can ethically manipulate groups of hermit crabs in ways we cannot apply to people. We humans already rely on various small creatures to understand ourselves—we study fruit flies to learn about our genetics, rats and mice to investigate some of our diseases, and sea slugs to pin down the molecular basis of learning and memory. Experiments with hermit crabs could now become among the first to model human social systems with simpler animals.

Not long ago I returned for inspiration to the beach where I first began my observations. I walked down to the tide pool and watched the hermit crabs slowly crawling along the sand below the water. I looked at them with what I can only call gratitude. What began as a fun pursuit to satisfy my curiosity ultimately revealed insights and connections that I could never have anticipated that first day on Long Island. Most of all, I have been delighted to learn that some patterns of our social life are so fundamental that we share them even with rather primitive creatures. 

This article was published in print as "Life is a Shell Game."