Thursday, December 19, 2019

Property and Wealth Inequality Among Hermit Crabs

This video shows hermit crabs exchanging shells.  A hermit crab who is too cramped in his shell will move into a larger shell, and then the shell that he has discarded will be taken over by another hermit crab.  Sometimes hermit crabs will line up, and the shells will be passed down the line.  In Darwinian Natural Right, I wrote about this in the section on "The Normative Structure of Animal Movement," because hermit crabs are like human beings and other animals in that their behavior conforms to the same normative structure: they have natural desires, they have natural capacities for gathering information relevant to their desires, and they are naturally inclined to do whatever seems to satisfy their desires according to their evaluation of the information.  Aristotle lays out this normative structure in his biological works--particularly, in On the Movement of Animals and the Nicomachean Ethics.

Hermit crabs are small crustaceans who occupy the empty shells of dead snails.  They are most easily seen in tidal pools along ocean coastlines.  Their shells protect them from predators, reduce their physiological stress from desiccation, and promote their reproductive success.  Some shells are better than others in satisfying these desires, and as the animals grow they need to move to larger shells.  Finding the right shell means the difference between life and death, or at least the difference between being cramped or cozy in one's portable domicile.

A hermit crab will carefully inspect a new shell to assess its weight, size, and structure.  Even after moving into a new shell, the animal will continue to test the shell for suitability, and it will sometimes decide to move back into the old shell (Elwood and Neil 1992).  The process of evaluation becomes even more intricate when hermit crabs fight over shells.  Then they must assess not only the relative value of their shells but also the size, strength, and resoluteness of their opponents.  This competition often displays a hierarchy in which the most dominant crabs gets first choice of a shell.  If the dominant crab moves into a new shell, the old shell is occupied by a less dominant crab, which creats another vacancy for a third crab, and so on down the hierarchy.  Sociologists who study the social structure of "vacancy chains," in which resources are passed from one individual to another down a social hierarchy, have discovered remarkable similarities between hermit crabs occupying vacant shells left behind by more dominant crabs and human beings occupying jobs and houses left vacant by those of higher status (Chase 1991; Chase and DeWitt 1988).

We can see here the evolutionary origins of property.  Like all organisms, hermit crabs have evolved to control, preserve, and invest in their own bodies; and thus, as John Locke saw,. the sense of property begins in the self-ownership of one's own body.  This self-ownership then extends to the ownership of external resources when animals privatize those resources and thereby convert them into property.  For hermit crabs to preserve and protect their bodies, they must privatize a shell by carrying it on their body (Strassmann and Queller 2014).  (I have written about the evolutionary neurobiology of Lockean self-ownership herehere, and here.)

We can also see here the first steps in the evolutionary history of possessive behavior. Hermit crab conflicts over shells show first the priority of power, in which the dispute is settled by relative fighting power.  The second step is for the priority of power to become the priority of dominance, so that individuals can yield to those with higher status without the need for fighting.  The third step would be for hermit crabs to recognize prior possession, so that individuals learn that those with prior possession of a resource generally prevail over raiders (Tibble and Carvalho 2018).

Remarkably, Ivan Chase and his colleagues have recently reported a study that shows a distribution of these shells in one hermit crab population that follows a pattern similar to the wealth inequality in human societies (Chase et al. 2020; Preston 2019).  They collected 297 crabs from a tidal pool on a beach on Long Island, New York.  They removed the crabs from their shells.  They then weighed each crab, and they weighed and measured each shell.  Looking at how shells of different weights were distributed across the group, they saw a distribution curve that peaked around the medium-sized shells and then dropped as the shells got larger, then tapering off through the largest shells into a long tail.

They found that this inequality of wealth was similar to what one sees in small primitive human groups, such as hunting-gathering bands, but not as great as in large modern societies today.  The top 1 percent of hermit crabs owned about 3 percent of the total shell weight.  The top 1 percent of people in large human societies typically own a larger proportion of the wealth.  Among the hermit crabs, Chase and his colleagues observed, "there are no Warren Buffets or Jeff Bezoses."  But still the distributional pattern of inequality is similar.

One might think that the distribution of shell sizes among hermit crabs is determined simply by biological factors such as the survival and growth of either the crabs or the snails whose shells the crabs occupy.  Shell distribution might reflect the size distribution of surviving crabs over their life history or the size distribution of shells from dying snails.  But Chase and his co-authors did not find this to be the case.

It is common today to measure economic inequality in human societies with a number called the Gini coefficient, which ranges from 0 (if everyone in a society had exactly the same level of wealth) to 1 (if one person held all of the wealth, and everyone else had no wealth at all).  Chase and his colleagues estimate that the Gini coefficient for the hermit crabs is 0.32, which is only a little higher than some estimates of human hunter-gatherer societies as showing a Gini around 0.25, but much lower than the 0.41 Gini for the United States, and close to the 0.27 Gini for the Nordic social democracies such as Norway and Sweden (Borgerhoff Mulder et al. 2009).  Amazingly, the Gini for the hermit crabs coincides with what Thomas Piketty has proposed as the "ideal" Gini number for low inequality--0.33 (Piketty 2014, 247-60).

If even hermit crabs show such wealth inequality, does this indicate that inequality is "natural"?  And if so, what exactly are the natural causes of inequality?  Even in Locke's account of the state of nature as a state of natural equality, he recognized that all human beings could never be absolutely equal in all respects, because differences in age, talents, birth, merit, and social relationships would always elevate some above others.  Some researchers today explain inequality as arising from individual differences due to personality, training, education, or the random events of luck.  Others emphasize the importance of the intergenerational transfers of wealth, so that wealth accumulates by inheritance within some families.

Among hermit crabs, there is no intergenerational transfer of the wealth in shells.  Some of the inequality among hermit crabs might be explained by individual differences in size, fighting prowess, and dominance.  But Chase and his colleagues emphasize the transfer of shells through vacancy chains, and they imply that some similar vacancy chain process might explain human inequality.  Surely, however, there must be more than that at work in human wealth inequality.

I will have more to say about the evolution of wealth inequality in human societies in my next post.

I have written about the debates over inequality and the possible argument for "good inequality" herehereherehereherehere, and here.


Borgerhoff Mulder, Monique, et al. 2009. "Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies." Science 326: 682-88.

Chase, Ivan. 1991. "Vacancy Chains." Annual Review of Sociology 17: 133-154.

Chase, Ivan, and Theodore H. DeWitt. 1988. "Vacancy Chains: A Process of Mobility to New Resources in Humans and Other Animals." Social Science Information 27: 83-98.

Chase, Ivan, Raphael Douady, and Dianna K. Padilla. 2020. "A Comparison of Wealth Inequality in Humans and Non-Humans." Physica A 538: 122962.

Ellwood, R. W., and S. J. Neil. 1992. Assessments and Decisions: A Study of Information Gathering by Hermit Crabs. London: Chapman and Hall.

Piketty, Thomas. 2014. Capital in the Twenty-First Century. Cambridge: Harvard University Press.

Preston, Elizabeth. 2019. "Even Hermit Crabs Have Wealth Inequality." The New York Times, December 13.

Strassmann, Joan E., and David C. Queller. 2014. "Privatization and Property in Biology." Animal Behaviour 92: 305-311.

Tibble, Lucy, and Susana Carvalho. 2018. "Rethinking the Evolution of Property and Possession: A Review and Methodological Proposition." Evolutionary Anthropology 27: 285-296.

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