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76 Scientific American, June 2012 Photographs by Adam Voorhes
O
1986, I waded into a shal-
low tide pool on Long Is-
land, 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 ab-
domen into the snail shell I had dropped. Satis-
fied 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 new-
found 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.
sociobiology
Life Is a
Shell Game
Like people, hermit crabs and other animals
trade up by treasuring what others leave behind
By Ivan Chase
June 2012, ScientificAmerican.com 77
78 Scientific American, June 2012
It may seem strange, but this was one of the happiest mo-
ments in my life as a researcher. For nearly 10 years I had been
wondering whether hermit crabs take up residence in one an-
other’s vacated shells. I finally had my confirmation. I was the
first person to observe an animal making use of what sociolo-
gists and economists call a “vacancy chain”—an organized meth-
od of exchanging resources in which every individual benefits by
claiming a more desirable possession abandoned by another in-
dividual. Even though hermit crabs have relatively simple brains
and nervous systems, they have evolved sophisticated social be-
haviors 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 econom-
ic strategies. Some tactics, it seems, do not require human-level
intelligence or altruism—they are far more universal.
CRABS IN QUEUE
1986, as well as the next summer, I
brought groups of students to West Meadow Beach on Long Is-
land to observe vacancy chains in Pagurus longicarpus—a her-
mit 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 observa-
tions, 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 se-
lection 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 aver-
age. Some people are disappointed to hear this number. They ex-
pect 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 vacan-
cy 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 obtain-
ing a resource compared with more typical competition.
After our studies, other researchers reported vacancy chains
in various species of hermit crabs, including Caribbean land her-
mit 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 chemi-
cals released by the injured snail. When the predatory snail final-
ly pulls its prey from its protective casing—a process that can
take as long as an hour—the nearest crab dives into the now emp-
ty shell. In turn, another crab immediately snatches the first
crab’s old shell, and so on. Instead of following the careful inspec-
tion 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 ben-
efits, but the immediacy of the competition speeds everything up.
Recently researchers have made further surprising discover-
ies 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 set-
tles on a new shell, the crab behind him takes his shell, and so
on, within seconds. Such well-orchestrated behaviors suggest so-
phisticated social cognition, especially for an animal with a rela-
tively small and simple brain.
Few published studies focus on vacancy chains in animals be-
sides hermit crabs, but preliminary observations suggest that
the strategy has evolved in many dierent species. Like hermit
crabs, several species of octopuses and cichlid fish live in and de-
fend 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 oth-
er animals. People do exactly the same thing.
WHAT PEOPLE DO
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 Kris tof, then
head of planning and research for the New York City Housing and
Redevelopment Board, realized that the construction of new
Ivan Chase is an emeritus professor at Stony Brook
University, where he directs the Laboratory for the
Study of Social Organization. Chase studies domi-
nance hierarchies and distribution of scarce resourc-
es, among other subjects in sociology, ecology and
evolution. He is also an avid sea kayaker.
IN BRIEf
Sociologists and economists use the term “vacancy chain” to describe a se-
quential exchange of resources that benets every individual in the sequence.
In recent decades scientists have gathered evidence that hermit crabs—and
possibly other animals—use vacancy chains, too.
Studying how these animals behave may help us improve how we distribute
resources—such as apartments, cars and jobs—among ourselves.
June 2012, ScientificAmerican.com 79
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. Follow-
ing Kristof’s work, other research-
ers 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 pro-
fessor of sociology then at Harvard
University who coined the term “va-
cancy chain,” independently discov-
ered such sequences within reli-
gious groups—specifically, Methodist, Presbyterian and Episco-
pal 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, ocers in the armed forces and syndicates selling il-
legal 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 eect 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 individu-
als or groups, both in dierent species of hermit crabs and in
humans? My guess is that some as yet undiscovered correspon-
dence between the demography of humans and hermit crabs
explains the eect—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
these properties. First, such resources are cov-
eted and relatively hard to get; jobs, cars and houses are not ly-
ing 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 ob-
tained. 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. Near-
ly all adult crabs have shells to leave behind when they get an-
other, 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 sociol-
ogists usually think about who gets what and whether the dis-
tribution 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 signifi-
cant in their own right. But they sometimes prevent us from dis-
covering other processes that influence how resources get dis-
tributed, 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. Re-
searchers could, for example, give a group of hermit crabs shells
of dierent 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 situ-
ations 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. Experi-
ments 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 re-
vealed insights and connections that I could never have anticipat-
ed 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.
MORE TO EXPLORE
Vacancy Chains. Ivan D. Chase in Annual Review of Sociology, Vol. 17, pages 133–154; 1991.
Forging the Vacancy Chain: Law Enforcement Eorts and Mobility in Criminal Economies.
H. R. Friman in Crime, Law & Social Change, Vol. 41, pages 53–77; 2004.
Social Context of Shell Acquisition in Coenobita clypeatus Hermit Crabs. R. D. Rotjan,
J. R. Chabot and S. M. Lewis in Behavioral Ecology, Vol. 21, No. 3, pages 639–646; 2010.
SCIENTIFIC AMERICAN ONLINE
To watch a video of hermit crabs exchanging shells, visit
ScienticAmerican.com/jun2012/vacancy
Some social
patterns
are so fun-
damental
that we
share them
even with
primitive
creatures.
