Image copyright Carin Bondar, 'The Nature of Human Nature'
Moving is both exciting and stressful at the same time for members of our species…finding (and being able to obtain) a home that meets the needs of a growing family is something that requires careful consideration and a lot of work. One person’s discard is another person’s dream: as you move out of that one bedroom apartment into a 3 bedroom condo, someone from the 3 bedroom condo may be moving to a 5 bedroom house. Another individual (likely a first time buyer) is happy to take over your small apartment. This kind of property exchange represents a complex process that involves several (unrelated) individuals or groups and can ultimately be beneficial for all of them at the same time. All participants are satisfied with occupying the vacancy left by the previous tenants. Termed ‘Vacancy chain theory’, the process of a sequential distribution of resources (houses in this example) across multiple individuals1 occurs in our species as well as in several others. After all, there can be serious consequences for one’s health and survival for living in quarters that are too cramped: spread of disease and detection by predators are two important ways in which survival can be jeopardized by living in a home that is too small.
Many animals build their own homes out of available materials (e.g. birds use twigs to build nests, snails secrete shells from their mantles, spiders spin their webs from silk glands) making their home construction a process that is independent from conspecifics in the population (i.e. if a larger home is necessary, grab some more supplies and make it happen). However, there are other animals that rely on the availability of homes in their surrounding environments (akin to the Homo sapien). Clownfish that inhabit anenmones, tree-cavity nesting birds and hermit crabs that inhabit abandoned snail shells are all examples of animals that make their homes out of discrete, reusable resources that are limited in use to one individual or family at a time. Individuals of such species may therefore stand to benefit if a new home becomes available.
Researchers examining this phenomenon in the terrestrial hermit crab (Coenobita clypeatus) in Belize found that there were benefits to be gained for several individuals along a vacancy chain when a new shell was introduced to a group2. Crabs ranging in size from large to small were placed in experimental arenas in groups of 8. A new shell that approximated the size of the largest crab was added to the arena, and the crabs left for 24 hours to proceed with shell inspections and possible shell-switching. At the end of the trials the crabs that had changed shells were measured for the differences in shell crowding between their old and new homes (overcrowding represents a considerable survival threat for a hermit crab since being unable to fully retract into its shell increases susceptibility to predators).
It was found that an average of 3.2 crabs in each group switched shells, and that a whopping 89% of those that switched shells gained a significant reduction in the amount of shell crowding. In other words, 3.2 individuals benefitted (aka increased their ability to survive) from the addition of a single resource to the arena, a phenomenon termed the ‘chain multiplier effect’. More than one individual stands to ‘move on up’ the housing ladder by virtue of the fact that others have as well. The vacancy chains observed in the hermit crab population were abruptly terminated if one crab had a damaged shell. This makes sense in the human world too: damaged homes require a lot more attention and sell for a much lower price, if at all. Not entirely unlike requesting a home inspection prior to taking that last subject off of your offer to purchase, crabs undertake careful consideration on the qualities of a new home prior to moving in. The size, weight, internal volume and amount of damage to the shell are all critical to the survival of the crabs living within them, and therefore shell selection is a careful process in this and other hermit crab species3.
The dynamics of home selection in the terrestrial hermit crabs studied here have definite parallels to the struggles faced by the Homo sapienswhen it comes to finding and obtaining a new home. Vacancy chains can have reverberating benefits down the property ladder in both species. However, if a home is sub-standard or damaged in some way the benefits of a vacancy chain come to an abrupt halt. We all look for high quality, non-damaged places to live, especially when there are threats to our well-being created by having outgrown the one we currently inhabit. A major difference between ‘us’ and ‘them’ in terms of moving up the property ladder is the universal human-based concept of finance. Hermit crabs have no need for mortgages, loans or banking, no need for individuals to carefully budget the financial cost of the new home. Once an individual crab has out-competed others for a certain shell, it is that individuals’ home for the taking. In the human world, out-competing others for a certain home generally translates to making a better offer (a higher price). In the hermit crab world, out-competing others for a certain home means that an individual physically fought for it and won. Perhaps it’s a little on the violent side but at least it’s simple…and mortgage free!
1Chase, I.D. 1991. Vacancy chains. Annual Review of Sociology 17: 133-154.
2Lewis, S.M. and Rotjan, R.D. 2009. Vacancy chains provide aggregate benefits of Coenobita clypeatus hermit crabs. Ethology 115: 356-365.
3Rotjan, R.D., Blum J. and Lewis, S.M. 2004. Shell choice in Pagurus longicarpus hermit crabs: does predation threat influence shell selection behavior? Behavioral Ecology and Sociobiology 56: 171-176.