Size is important in liver exchange

 Liver exchange has a lot in common with kidney exchange, in the sense that the issues involved in forming cycles and chains once you know which donors are compatible with which patients are very similar.  But a big difference is what constitutes a compatible donor: for livers, size (of the donor, and the donor liver) is very important, sensitively so.

Here's a paper forthcoming in the American Journal of Transplantation, by a team of transplant physicians and economists (with kidney exchange experience), on the importance of size.

"Enhanced Role of Multi-Pair Donor Swaps in Response to Size Incompatibility: The First Two 5-Way and the First 6-Way Liver Paired Exchanges" by Sezai Yilmaz, MD, FACS, Tayfun Sönmez, PhD, M. Utku Ünver, PhD, Volkan Ince, MD, Sami Akbulut, MD, PhD, FACS, Kemal Baris Sarici, MD, and Burak Isik, MD, American Journal of Transplantation, Brief communication, in press.

Abstract: A significant portion of liver transplantations in many countries is conducted via living-donor liver transplantation (LDLT). However, numerous potential donors are unable to donate to their intended recipients due to factors such as blood-type incompatibility or size incompatibility. Despite this, an incompatible donor for one recipient may still be a viable donor for another patient. In recent decades, several transplant centers have introduced liver paired exchange (LPE) programs, facilitating donor exchanges between patients and their incompatible donors, thereby enabling compatible transplants. Initially, LPE programs in Asia primarily involved ABO-i pairs, resulting in 2-way exchanges mainly between blood-type A and B recipients and donors. This practice has led to a modest 1-2% increase in LDLTs at some centers. Incorporating size incompatibility alongside blood-type incompatibility further enhances the efficacy and significance of multiple-pair LPEs. Launched in July 2022, a single-center LPE program established at Inönü University Liver Transplant Institute in Malatya, Türkiye, has conducted thirteen 2-way, nine 3-way, four 4-way, two 5-way, and one 6-way LPEs until February 2024. In 2023 alone, this program facilitated 64 LDLTs, constituting 27.7% of the total 231 LDLTs performed. This paper presents the world's first two 5-way LPEs and the first 6-way LPE.

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Another (not entirely unrelated) domain in which size is important, and exchange involves many pairs, involves the exchange of shells among hermit crabs. See these earlier posts (which included this short video):

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Saturday, July 21, 2012

Hermit Crab Vacancy Chains

Thursday, January 16, 2020

Vacancy chains and hermit crab housing, revisited

Tuesday, January 18, 2022

Evictions and coalitions in the housing market of hermit crabs--shell trafficking in the wild

Vacancy chains in urban housing

 Vacancy chains occur not just in labor markets, but also in housing markets. (Earlier this week I wrote about housing chains for hermit crabs that result from evictions.)  A vacancy chain in a housing market can be thought of as a moving chain: someone moves into a vacant house or apartment (perhaps a newly constructed one), and someone else moves into the home they vacated, and so on, until the chain ends when a person who was in some different market (e.g. in rental housing, or in a distant location) moves into the last identifiable home in the chain.

Here are two papers that explore what happens when newly constructed housing is relatively expensive. They find that the chain often reaches much more moderately priced housing, i.e. adding to the stock of expensive housing also makes more affordable, existing housing available to new occupants.

The first paper draws on data from a dozen American cities (from Atlanta to San Francisco):

The effect of new market-rate housing construction on the low-income housing market, by Evan Mast, Journal of Urban Economics, Available online 27 July 2021, https://doi.org/10.1016/j.jue.2021.103383

Abstract: I illustrate how new market-rate construction loosens the market for lower-quality housing through a series of moves. First, I use address history data to identify 52,000 residents of new multifamily buildings in large cities, their previous address, the current residents of those addresses, and so on for six rounds. The sequence quickly reaches units in below-median income neighborhoods, which account for nearly 40 percent of the sixth round, and similar patterns appear for neighborhoods in the bottom quintile of income or percent white. Next, I use a simple simulation model to roughly quantify these migratory connections under a range of assumptions. Constructing a new market-rate building that houses 100 people ultimately leads 45 to 70 people to move out of below-median income neighborhoods, with most of the effect occurring within three years. These results suggest that the migration ripple effects of new housing will affect a wide spectrum of neighborhoods and loosen the low-income housing market.

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A more recent working paper draws on data from metropolitan Helsinki and reaches similar conclusions:

Bratu, Cristina and Harjunen, Oskari and Saarimaa, Tuukka, City-wide Effects of New Housing Supply: Evidence from Moving Chains (August 31, 2021). VATT Institute for Economic Research Working Papers 146, Available at SSRN: https://ssrn.com/abstract=3929243 or http://dx.doi.org/10.2139/ssrn.3929243

Abstract: We study the city-wide effects of new, centrally-located market-rate supply using geo-coded total population register data from the Helsinki Metropolitan Area. The supply of new market rate units triggers moving chains that quickly reach middle- and low-income neighborhoods and individuals. Thus, new market-rate construction loosens the housing market in middle- and low-income areas even in the short run. Market-rate supply is likely to improve affordability outside the sub-markets where new construction occurs and to benefit low-income people.

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Earlier:

Vacancy chains in housing for hermit crabs   

Blum, Y., A.E. Roth, and U.G. Rothblum "Vacancy Chains and Equilibration in Senior-Level Labor Markets," Journal of Economic Theory, 76, 2, October 1997, 362-411.

Evictions and coalitions in the housing market of hermit crabs--shell trafficking in the wild

 I've previously blogged about the observation that hermit crabs, who live in the shells of other animals and have to get new shells as they grow, sometimes engage in chains of exchange, that resemble kidney exchange chains, or vacancy chains in labor markets.

In particular, they resemble kidney exchange chains initiated by a deceased donor, in this case initiated by an empty shell.

 Here's a new article about hermit crabs which reports that they also engage in something that looks like organ trafficking, with a hermit crab being forcibly removed from its shell by two smaller crabs acting in concert, so that one of them may occupy the now vacant shell while the other moves into the shell of its partner in crime.

Laidre, Mark E. "The Architecture of Cooperation Among Non-kin: Coalitions to Move Up in Nature’s Housing Market." Frontiers in Ecology and Evolution (2021): 928.

"Coalitions typically involve two individuals (a pair), with a third individual being the target that the two-member coalition seeks to evict from its shell (Figure 1). Both members of the coalition have shells of their own, but these individuals and their shells are virtually always smaller than that of the target individual and its shell. Sometimes, based on the commotion and struggle generated during an attempted eviction, additional individuals—beyond the target and the core two-member coalition—are attracted to the area. These additional individuals—referred to as “third parties” or “bystanders”—are not part of the actual coalition, since they do not help at all to evict the target. Generally, third parties simply wait in the vicinity and sometimes position themselves in a social chain, which emanates from the back of the shell of one or both of the coalition members (Figure 2). This positioning in a social chain enables third parties to indirectly benefit, since in the event an eviction succeeds, it can catalyze a succession of back-to-back shell swaps (see Laidre, 2019a). Third parties are thus, in effect, “free riders” (Sigmund, 2010), since their positioning around the coalition offers no advantage whatsoever to the coalition itself as it works to evict the target. Indeed, whether third parties are positioned in a chain or not, they merely wait, performing no pulling actions and never adding any strength or providing any help to the two-member coalition. Interestingly, based on precisely where third parties position themselves, some may potentially even undermine the coalition (see below), effectively acting not merely as “free riders” but as “cheaters” (Sigmund, 2010). Finally, if too many bystanders accumulate, it can lead to chaotic jockeying and repositioning, with the original coalition separating.

"Whether with third parties present or not, the two members of the coalition attempt to physically evict the target. The target remains flipped on its back (i.e., with the dorsal side of its shell on the ground) and the opening of the target’s shell faces upward, allowing both coalition members to use their claws and legs to grab at and pull the anterior portion of the target’s body. As the coalition forcibly pulls, the target attempts to resist by clinging inside its shell. Typically, the two coalition members both pull simultaneously; though at times the two may alternate attempts at pulling, each doing so sequentially as one or the other member briefly rests. Both members of a coalition appear strongly involved, in terms of time and effort. Yet coalitions are not always successful. In some cases, one or both coalition members may give up; or the target individual may manage to flip itself over, escape from being pinned down, and run away. If a coalition is successful at evicting the target, the time till eviction occurs can vary widely, from just minutes up to hours (Laidre, personal observation). Once a coalition is successful and the target individual is evicted from its shell, then the evictee is pushed to the side and remains naked and shell-less as one of the coalition members moves into its now empty shell."

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Earlier:

Saturday, July 21, 2012

Hermit Crab Vacancy Chains

Thursday, January 16, 2020

Vacancy chains and hermit crab housing, revisited

Vacancy chains and hermit crab housing, revisited

Here's the story from the NY Times (with a nice video):
Even Hermit Crabs Have Wealth Inequality  by Elizabeth Preston, Dec. 13, 2019


"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."

And here's the paper in Physica A.

A comparison of wealth inequality in humans and non-humans
 Ivan D.Chase, Raphael Douady, Dianna K.Padilla

"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.
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(297 crabs were re-homed to produce this paper about economics, published by biologists in a physics journal.)

See earlier post:

Saturday, July 21, 2012 Hermit Crab Vacancy Chains

Hermit Crab Vacancy Chains

From Scientific American:
On a Tiny Caribbean Island, Hermit Crabs Form Sophisticated Social Networks [Video]

In 2009, Lewis and Rotjan surveyed the entire hermit crab population on Carrie Bow Cay. Many crabs were living in shells that were a tight fit or had one too many holes. As they grow, hermit crabs must move into larger shells, so they are always on the lookout for a more spacious dwelling. And an undamaged shell is preferable to a broken one, even if the shells are the same size. Knowing this, the researchers decided to dramatically change the available hermit crab real estate on Carrie Bow Cay. They placed 20 beautifully intact shells that were a little too big for most hermit crabs at various spots around the island and watched what happened.

When a lone crab encountered one of the beautiful new shells, it immediately inspected the shelter with its legs and antennae and scooted out of its current home to try on the new shelter for size. If the new shell was a good fit, the crab claimed it. Classic hermit crab behavior. But if the new shell was too big, the crab did not scuttle away disappointed—instead, it stood by its discovery for anywhere between 15 minutes and 8 hours, waiting. This was unusual. Eventually other crabs showed up, each one trying on the shell. If the shell was also too big for the newcomers, they hung around too, sometimes forming groups as large as 20. The crabs did not gather in a random arrangement, however. Rather, they clamped onto one another in a conga line stretching from the largest to smallest animal—a behavior the biologists dubbed "piggybacking."

Only one thing could break up the chain of crabs: a Goldilocks hermit crab for whom the shell introduced by Lewis and Rotjan was just right. As soon as such a crab claimed its new home, all the crabs in queue swiftly exchanged shells in sequence. The largest crab at the front of the line seized the Goldilocks crab's abandoned shell. The second largest crab stole into the first's old shell. And so on.

No one had ever documented such well-orchestrated shell swapping before, but similar behavior was not unknown. In 1986, Ivan Chase of Stony Brook University made the first observations of hermit crabs exchanging shells in a "vacancy chain"—a term originally coined by social scientists to describe the ways that people trade coveted resources like apartments and jobs. When one person leaves, another moves in. Since then, several researchers—including Lewis and Rotjan—have studied the behavior in different hermit crab species. Some preliminary evidence suggests that other animals use vacancy chains too, including clown fish, lobsters, octopuses and some birds. As Chase explains in the June issue of Scientific American, vacancy chains are an excellent way to distribute resources: Unlike more typical competition, a single vacancy chain benefits everyone involved—each individual gets an upgrade. So it makes sense that hermit crabs and other animals have evolved sophisticated social behaviors to make the most of vacancy chains.

The orderly vacancy chain that Lewis and Rotjan observed is called a synchronous vacancy chain, which is different from an asynchronous vacancy chain in which a lone crab encounters a shell, claims it and leaves behind its old home, which is later seized by a different crab that never interacts with the first animal. As the above video makes clear, however, synchronous vacancy chains are not always civilized affairs. Sometimes crabs fight each other for the best shell or gather in violent groups. And the exchanges often happen extremely quickly. Lewis and Rotjan had to slow down the footage just to see what was happening and it is still difficult to make out: three hermit crabs crowd a large green shell; the largest claims the green shell and the other two swiftly trade up. Lewis thinks the chain would have been more orderly if the crabs were not disturbed by two biologists filming them.

 Sara Lewis and Randi Rotjan 

HT: Benjamin Kay

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Update: here's another video https://www.facebook.com/video.php?v=844190408934712 (HT: Yingua He)
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And on YouTube (HT Joshua Gans)