Hermit crabs face a uniquely competitive real estate market. They need bigger and bigger shells throughout their lives, but can’t grow these homes themselves. So they rely on castoff snail shells, and are constantly on the lookout for better properties entering the market.
A study that will be published next month in the journal Physica A found that the distribution of these shells in one hermit crab population was surprisingly similar to the distribution of wealth in human societies.
That may make hermit crabs one of the first animals known to experience wealth inequality.
Ivan Chase, an emeritus professor at Stony Brook University in New York and the study’s lead author, researches social systems in animals and described a phenomenon called the vacancy chain in hermit crabs in the 1980s.
When a snail dies on the beach, a crab that comes across the empty shell will inspect it closely, turning the shell over in its claws. If the crab decides this home is better than its current shell, it trades up. Another, usually smaller crab may soon find that crab’s castoff and move in. Each vacancy lets about three crabs upgrade their shells, Dr. Chase said.
Dr. Chase had always wondered whether this system led to a kind of inequality among hermit crabs, with a few crustaceans hoarding the biggest homes. So in 2017, he and his co-authors started testing the idea. They gathered almost 300 hermit crabs from a Long Island beach and briefly removed the crabs from their shells. They weighed and measured each crab and its residence. Then they looked at how shells of different weights were distributed among the population.
The distribution curve they found peaked around medium-sized shells, then dropped as the shells got larger, before tapering off very gradually through the largest shells of all. This matches the shape of wealth distribution curves in many human societies.
The team used a number called the Gini coefficient to measure overall inequality among the crabs. It found a value similar to that in small human populations, though not as great as in today’s large countries. The top 1 percent of hermit crabs owned only about 3 percent of the total shell weight, Dr. Chase and his co-authors noted: “There are no Warren Buffetts or Jeff Bezoses.” There is also no transfer of shells between crabs and their offspring.
What they discovered suggested that the distribution of shell sizes did not simply depend on crab biology. They did not find similar numbers of crabs in every size of shell, which would be expected if most crabs survive to old age (and if longevity determines shell distribution). Nor did they find that the smallest shells were most abundant, which might occur if crabs most often die young, or are preyed on at a steady rate throughout their life span.
Dr. Chase thinks the resemblance between crab and human inequality might come from similarities between crab vacancy chains and the ways people pass on wealth. While smaller crabs don’t exactly inherit their wealth from bigger crabs, the largest shells are a scarce resource that only a few crabs are privileged enough to get their claws on.
“Vacancy chains are just another way of transferring property,” he said.
Although he’s hesitant to draw any societal lessons from the crustaceans, he hopes hermit crabs can one day become a kind of model organism, like lab rats, for scientists studying wealth inequality.
“The authors have nicely shown that the wealth distribution in crabs is humanlike,” said Monique Borgerhoff Mulder, an anthropologist who studies human wealth inequality at the University of California, Davis. The pattern is very much like what researchers have found in small-scale human societies, both historic and contemporary, she said.
Although the transfer of wealth and property between people is an important contributor to inequality, Dr. Borgerhoff Mulder said many other factors matter too, such as social connections and individual differences in skills and education. She doubts vacancy chains are the whole story in crab society, either.
The notion that crabs can teach us about human wealth distribution “may be a little preposterous,” Dr. Borgerhoff Mulder added. But she said this kind of idea sharing between studies of humans and other animals is making social science, as a whole, richer.