First description of “wealth” inequality in an animal (hermit crabs), which may provide an animal model of the dynamics generating wealth inequality

A comparison of wealth inequality in humans and non-humans. Ivan D. Chase, Raphael Douady, Dianna K. Padilla. Physica A: Statistical Mechanics and its Applications, September 26 2019, 122962. https://doi.org/10.1016/j.physa.2019.122962

Highlights
•    We present the first description of “wealth” inequality in a non-human animal.
•    We describe the distribution of snail shells occupied by a hermit crab species.
•    The distribution of shells resembles the common form of human wealth distributions.
•    Hermit crabs may provide an animal model of the dynamics generating wealth inequality.
•    Shell distribution in hermit crabs provides a baseline to compare to human inequality.

Abstract: Inequality in the distribution of material resources (wealth) occurs widely across human groups. The extent of inequality, as measured by the Gini coefficient, is less in small-scale societies, such as hunter-gatherers and pastoralists, and greater in large-scale ones like current nation states. In many societies, the statistical distribution of wealth takes a characteristic form: unimodal, skewed to the right, and fat-tailed. However, we have relatively little systematic information about the distribution of material resources in nonhuman animals even though such resources are vital to their survival and fitness. Here we present the first description of inequality in material resources in an animal population: the distribution of gastropod (snail) shells inhabited by the hermit crab Pagurus longicarpus. We find that the shell distribution for the crabs strongly resembles the characteristic form of wealth distribution in human groups. The amount of inequality in the crabs is more than that in some small-scale human groups but less than that in nations. We argue that the shell distribution in the crabs is not simply generated by biological factors such as survival and growth of either crabs or gastropods. Instead, the strong resemblance in the human and hermit crab distributions suggests that comparable factors, not dependent upon culture or social institutions, could shape the patterns of inequality in both groups. In addition to the similarity in their inequality distributions, human and hermit crabs share other features of resource distribution, including the use of vacancy chains, not seen in other species. Based upon these parallels, we propose that P. longicarpus could be used as an animal model to test two factors – individual differences and intergenerational property transfers – that some economists theorize as major factors influencing the form of wealth distributions in humans. We also propose that inequality in hermit crabs could provide a baseline for examining human inequality.