Game theory models based on repeated interactions between two individuals have often been the framework for understanding cooperative interactions in humans, but these models rarely apply in nature. Non-human animals, after all, rarely find themselves in situations like the “prisoner’s dilemma.”

Instead, partner choice and competition are emerging as the framework for understanding cooperation in the natural world. Some mutualisms (biological interactions between organisms where each individual derives a fitness benefit) can be described as “biological markets,” where organisms exchange goods or services. These markets and the animals that participate in them share some similarities with humans and our markets: animals preferentially interact with partners that provide the highest-quality goods or services; animals sometimes cheat each other; competition is often a good thing, and threatening to take your business elsewhere can lead to more cooperative behavior from your partner.

In many cleaner mutualisms among fish, cleaner fish occupy cleaner “stations” where they remove parasites from cooperating client fish. Buyer beware, though, because clients often have to wait for service from a cleaner and when it’s finally their turn, they may be cheated by cleaners that feed on tissue or mucous instead of parasites. Clients don’t have many options for ensuring good service. They can’t demand their mucous back or complain to management. What they can do is go get cleaned somewhere else.

Thomas C. Adam, a graduate student at the Department of Ecology, Evolution, and Marine Biology at the University of California, Santa Barbara, investigated cleaner-client interactions involving the territorial butterflyfish Chaetodon ornatissimus . In the Maharepa lagoon on the north shore of Moorea, French Polynesia, C. ornatissimus (at left) is the preferred client of bluestreak cleaner wrasse (at right), but has the option of partnering with several other species of cleaners common to the area. Snorkelers mapped the territorial boundaries of C. ornatissimus and conducted hour-long observations of their interactions with their cleaners (in total, individual fish in 32 territories were observed for 43 hours).

The results of the study indicate that not only do bluestreak cleaner wrasse compete for access to their butterflyfish clients (the amount of time cleaners had access to clients was negatively associated with the number of cleaner stations in a territory and individual butterflyfish with access to multiple cleaner stations did, indeed, shop around and were less likely to return to a cleaner station for their next cleaning than individuals with access to just one cleaner station), but the ability of butterflyfish to take their business elsewhere got them higher-quality service from cleaners. To wit, (1) the observed clients were never ignored by cleaners (at left) when they had more than one cleaner station in their territory (in contrast, five of 11 fish with a single cleaner station in their territory were observed being ignored), (2) while there was no evidence that clients with access to multiple cleaner stations were cheated less frequently than clients without access, the clients with their choice of partners were less likely have interactions terminated early by cleaners and were inspected for significantly longer during each cleaning session.
See? The free market does work sometimes.

Reference: Adam, T. (2010). Competition encourages cooperation: client fish receive higher-quality service when cleaner fish compete Animal Behaviour, 79 (6), 1183-1189 DOI: 10.1016/j.anbehav.2010.02.023

I recently took part in what social scientists call a “vacancy chain” (a social structure through which vacancies in discrete, reusable, and limited resources propagate through a population) and all I needed was a moving truck, a few helpful relatives, a case of beer and a few pizzas. You see, when my girlfriend and I moved into a new house in May, we filled a vacancy left by the previous tenants. When we moved, someone moved into our old apartment and filled the vacancy we left. Their apartment, in turn, was filled by someone else, and their apartment was moved in to by someone else and so on and so forth. Somewhere (further up the chain than me), a vacancy was created and propagated down the socioeconomic order through a series of interdependent events and resulted in many individuals acquiring new, sometimes better (we have a patio, but no central air, so the jury is still out), resources and benefiting from them.

Hermit crabs, for whom really nice shells to call home are a scarce commodity, have evolved their own sorts of vacancy chains as way for optimizing shell acquisition and occupancy. While these shell vacancy chains have been described (and shown to provide aggregate benefits that are distributed across many participants) for several hermit crab species in previous research, not much was known about the behaviorial and ecological factors that lead to and influence them.

Cue the arrival of Randi Rotjan, Jeffrey R. Chabot and Sara M. Lewis (from the New England Aquarium in Boston, the Pfizer Research Technology Center in Cambridge, MA and the Department of Biology at Tufts University, respectively) at Carrie Bow Cay, a ¾-acre island located near the Belizean barrier reef that is home to Eighty-four palm trees and 1,084 purple-clawed hermit crabs of the terrestrial species Coenobita clypeatus.While the biologists were there study parrotfish, bad weather made the water too rough for diving, so they used their time to better understand shell vacancy chains. The researchers marked 20 locations around the island, set out a single vacant shell at dusk at each one and monitored them. Over the course of 24 hours they observed a total of 16 vacancy chains of two different types, asynchronous and synchronous.

An asynchronous chain occurs when one crab moves into a new, empty shell and abandons its old one to be found by another crab, which abandons its own for another crab to find, etc. With this type of chain, shell switching is sequential and the crabs experience little to no interference or competition. They have the opportunity to investigate any vacant shells they find and can directly compare their current shell with a new shell by switching back and forth between the two. The down side is that individual crabs aren’t very likely to just stumble upon a vacant shell that meets their specific size and quality requirements. It’s like if I told you that you could wander around your town, go into any unoccupied houses you wanted, check them out and pick your dream home, but you’d have to find the one with two bedrooms, a dishwasher and a fireplace on your own by chance, without the aid of Craigslist.

Synchronous shell vacancy chains are more social and much more interesting. They start off with “waiters,” crabs that hang around a shell that’s too big for them, and wait for a bigger crab to come along so that if the big crab moves in to the vacant shell, the waiter can grab their more appropriately-sized hand-me-down shell (the researchers note that the decision to wait, and how long to wait, based on previous experience, provides some evidence that the crabs are smarter than we thought). The chains that the researchers observed began with one to 20 waiters who spent anywhere from a few minutes to an hour-plus loitering around empty shells. As a crowd gathers, the crabs queue up by size, from largest to smallest, and once largest crab switches into the vacant shell, each crab climbs into a new shell as it’s vacated by the slightly larger crab ahead of it, quickly shuffling vacancies (literally) down the chain. In both chain types, the fun stops when the last shell vacated is so low in quality (too small or damaged) that all the crabs reject it.

A Synchronous Chain in Action

In addition to the waiting that kicks off synchronous chains, the researchers observed other unique shell acquisition behaviors that the crabs only exhibited in social contexts and appeared to be associated with the vacancy chains. At almost half of the observed locations, when the waiting crabs were all too small for they vacant shell they had gathered around, some would “piggyback,” or form lines with each crab grasping the shell of another crab from behind and frequently moving in and out of the line to jockey for a better positions. The researchers hypothesize that piggybacking may be help establish a dominance hierarchy among the waiting crabs and/or allow them to investigate some of the shells they might be able to move into. Theses piggyback lines often transformed into queues upon the arrival of crabs that were appropriately sized for the vacant shell.

At some of the locations, multiple queues formed when there were many similarly sized waiters, and the crabs in these queues appeared to engage in a “tug-of-war” for control of the vacant shell. The smallest crabs, positioned at the end of each queue, frequently switched back and forth between the lines in a possible attempt to stake its place in the winning line.

So what sets these theatrics off in the first place? Population density seems to be a key factor determining the length and type of vacancy chains. Using modeling software, the researchers created a simulated habitat space and a population of crabs of varying sizes. Rules for shell switches that realistically reflected hermit crab behavior were established and, after a while, a vacant shell appropriately sized for the largest crab in the population was placed the center of the habitat and the simulation was continued. During 100 model runs were at each combination of 2 parameters: population density (8 levels, from 10 to 900 crabs) and maximum waiting times for the waiters (2 levels), vacancy chain lengths increased along with population density at the highest population density, almost half of the shell switches that occurred were part of synchronous vacancy chains. How word about an available shell gets out among the crabs in the first place is still unknown, though. The researchers plan to address the question in a future study and speculate that the waiters may use aural or chemical signals to draw attention to the vacancy.

Reference: Rotjan, R., Chabot, J., & Lewis, S. (2010). Social context of shell acquisition in Coenobita clypeatus hermit crabs Behavioral Ecology, 21 (3), 639-646 DOI: 10.1093/beheco/arq027

Image: “Caribbean hermit crab (coenobita clypeatus)” by ZooFari, via Wikimedia.

It wasn’t too long ago that we (Homo sapiens) thought we were pretty special, far above and beyond the other animals. Then, one, by one, by one, the boundaries that supposedly kept us separated from the rest of the animal kingdom revealed themselves to be less than solid. We can’t even claim a monopoly on mourning anymore, and if this photo wasn’t enough to convince you of that, then two studies in the new issue of Current Biology provide close-up looks at the ways chimpanzees deal with death.

In the first study, researchers from the University of Stirling, witness and describe the final hours of Pansy, a 50-year-old female at the Blair Drummond Safari and Adventure Park in Stirlingshire, England, and the response of group-mates at the moment of her death. While the traumatic death of an adult chimp is usually with noisy, sometimes aggressive responses, the dying chimp’s group-mates remained mostly calm before and after her death. For several days before her death, the other chimps stayed very quiet and groomed and caressed her. At what the researchers presume was the moment of death, they closely inspected her face and tested her for signs of life. After she died they left the body alone, except for her adult daughter, who remained by her mother’s body through the night. When park staff removed the body the following morning, the group remained calm and for several days after avoided in the area where female had died (though it was normally a favored spot).

In this video from the study (tilt your head, as it’s more or less upside-down), three adult chimps gather around the dying female, nudge her, inspect her face and manipulate her head and shoulders. After not seeing any signs of life, two chimps leave and the third follows not long after.

In this video, taken the day after the chimp’s death, an adult male acts aggressively toward the body (the third display by that male observed by the researchers), but proceeds to remove straw from the body and grooms it. He is joined by an adult female, who also removes straw from the deceased’s face.

In the second study, researchers observed two infants in a semi-isolated chimpanzee community near Bossou, Guinea die from a flu-like respiratory ailment. For weeks afterward, the mothers of the dead infants continued to carry their children’s bodies around, groom them and take them to their nests during periods of rest. During this time, as the bodies mummified completely, the mothers began to “let go” of their babies. They gradually tolerated longer periods of separation from the bodies and eventually allowed other chimps in the group to handle them. Group members showed interest in the bodies and other infants and juveniles attempted to play with them.

In this video, one of the grieving mothers, Vuavua, keeps flies away from the body of her dead infant, Veve. Vuavua carried, groomed and cared for Veve’s body for a total of 19 days after death.

In this video, a juvenile chimp in the group, Fokayé, plays with the body of one of the dead infants, Jimato. Fokayé’s mother, Fotaiu (middle), appears to be a little uneasy about being touched by the corpse. This was the only time the researchers observed any of the chimps reacting to the bodies in that way. Eventually, Jimato’s mother, Jire (left), takes the Jimato’s body away. Jire carried the body for 68 days after death.

Dora Biro, lead author of the second study, says that her team’s observations confirm a mother-child bond powerful enough to persist even after death and provide all the more reason to learn more about the extent to which chimpanzees understand death in order to better understand both how chimps interpret their world and the evolutionary origins of our own perception of death.

Reference:

Anderson et al. (2010). “Pan thanatology.” Current Biology 20: 8

Biro et al. (2010). “Chimpanzee mothers at Bossou, Guinea carry the mummified remains of their dead infants.” Current Biology 20: 8

Researchers at the University of Illinois recently published the results of an experiment that spanned 20 years and involved several generations of largemouth bass and an untold of amount of bait. Their conclusion: The apple doesn’t fall far from the tree, even, apparently, if you’re a fish.

The study started in 1975 at Ridge Lake, an experimental study lake in Fox Ridge State Park in Charleston, Illinois. Over the course of four years of controlled fishing, the bass from the resident population of the lake were caught, measured and tagged to keep track of how many times each fish had been caught, and then released.

The researchers recorded thousands of catches and found that some fish went for the bait more often than others, a lot more. One fish was caught three times in the first two days of the experiment, and another was caught 16 times in one year. When the lake was drained, the researchers also found some 200 fish that had never been caught during the study.

A total of 1,700 fish were collected from the drained lake. Male and female fish that had been caught four or more times in the study were designated High Vulnerability (HV) parents, and those that had never been caught were designated Low Vulnerability (LV) parents. The HV and LV groups were placed in separate university research ponds, where they spawned and produced lines of HV and LV offspring. These two lines were marked, raised in common ponds until they were big enough to be fished and then the anglers were let loose, starting the process over again.

Through three generations, the fish in each group followed closely in their parents footsteps (finsteps? finswims?) of either getting caught, or not (the difference in vulnerability between the HV and LV lines grew even larger with each generation), confirming that vulnerability to being caught by fishermen is a heritable trait in largemouth bass.

While that fact might make for great trivia, the study gives us more than just gee-whiz science. It suggests that recreational fishing can cause evolutionary changes the same way commercial fishing can.

The researchers found that most of the selective pressure is occurring on the LV fish, making fish that are already unlikely to be caught even less vulnerable. On the other hand, there was only a small increase in vulnerability to being caught in the HV group[1].

The researchers aren’t sure which inherited behavior causes these differences (it may be a wariness of anglers’ hooks and general lack of aggression that are passed on to offspring), but both these changes, they suspect, have implications for the bass’ reproductive success. Female largemouth bass swim away from their eggs after laying them, while the males stay with the eggs and until they hatch and guard the fry for the first month of their lives. The LV males may go after anglers’ hooks less often, or not at all, but their lack of aggression may also mean that they provide less protection from predators for their young. More aggressive HV males likely have higher mating success and are good protecting their fry from predators, but that aggression also makes them more likely to go after lures, get caught and leave their offspring vulnerable to predators.

During spawning season (in Illinois, this is from about April 1-June 15), males are caught the most, which causes concern for the HV males. Most bass anglers practice catch-and-release fishing, and the research team says that perception is that this has no negative impact on the fish, but during spawning season, if a male bass caught and kept away from their nests for more than even a few minutes, that may be enough time for predators to find the nest and eat the eggs or fry (a previous study by other researchers showed that, if a smallmouth bass is away from the nest for 1.4 minutes, as many as 1,100 eggs can be eaten).

The researchers suggest that wildlife management agencies set aside portions of lakes as bass spawning sanctuaries, where all fishing would be prohibited, and makes catch-and-release mandatory in the rest of the lake during the spawning season. They also recommend immediate catch-and-release regulations in fishing tournaments held during the bass’ reproductive period.

Reference: Philipp, David P., Cooke, Steven J., Claussen, Julie E., Koppelman, Jeffrey B., Suski, Cory D., Burkett, Dale P. Selection for Vulnerability to Angling in Largemouth Bass. Transactions of the American Fisheries Society 2009;138:189–199. DOI: 10.1577/T06-243.1

Image:”Largemouth Bass – Micropterus salmoides.” Trisha M Shears.