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Just as my office would not stay empty for long were I to move out, nature's real estate changes hands all the time. Potential homes range from holes drilled by woodpeckers to empty shells on the beach. A typical example of what economists call a "vacancy chain" is the housing market among hermit crabs. To protect its soft abdomen, each crab carries its house around, usually an abandoned gastropod shell. The problem is that the crab grows, whereas its house does not. Hermit crabs are always on the lookout for new accommodations. The moment they upgrade to a roomier shell, other crabs line up for the vacated one.
One can easily see supply and demand at work here, but because it plays itself out on a rather impersonal level, few would view the crab version as related to human economic transactions. The crab interactions would be more interesting if the animals struck deals along the lines of "you can have my house if I can have that dead fish." Hermit crabs are not deal makers, though, and in fact have no qualms about evicting homeowners by force. Other, more social animals do negotiate, however, and their approach to the exchange of resources and services helps us understand how and why human economic behavior may have evolved.