Over the past two decades, researchers have been building an elaborate theoretical framework to explain the evolution of cooperative behavior among animals. Much of it is based on the findings of animal studies involving a classic experiment known as the "Prisoner's Dilemma."
A scenario commonly employed in game theory, Prisoner's Dilemma forces two players to choose individually whether to cooperate with one another for a shared reward or to "defect" by making a selfish bid to secure the reward only for themselves.
Each player stands to benefit handsomely by defecting. If both choose to defect, however, they risk losing out on the payoff because they could be outfoxed by the partner.
This situation leads to an unstable level of cooperation. Different strategies must be employed, then, to move toward greater equilibrium.
As theories about cooperation have evolved, many scientists have come to believe that the possibility of mutual cooperation is likely to be higher when partners interact repeatedly and engage in a give-and-get strategy. Yet that hasn't been consistently borne out in lab experiments.
In previous work with blue jays, for example, Stephens and a colleague found that even when the birds were trained to cooperate with one another, they eventually switched strategies and regularly chose to put their own interests first by defecting.
In the new experiments, Stephens said he and his present colleagues "wanted to look at why it's so hard to get animals in this kind of situation to cooperate." They modified the Prisoner's Dilemma testing scenario to investigate the potential effect of temporal discounting.
Stephens said it's important to keep in mind that the new study was set up to examine behavior in a very particular type of situation, known as a social dilemma. "It's like the 'tragedy of the commons,'" he noted. The tragedy of the commons is a classic situation in which some individuals selfishly use shared resources for their own needs to the detriment of the larger community.
Toward Greater Cooperation
In their experiments with blue jays, the researchers studied 16 unrelated birds whose sex and past behavior was unknown; the birds were arranged in pairs and placed side by side in individual compartments.
The experiments were rigged so that the payoff was delayed. All the birds had to complete a sequence of four interactions with their partners before the desired food was accessible to them. The food was visible in transparent boxes, enabling the blue jays to see the seeds but not consume them until after the sequence of cooperative behaviors was finished.
"Accumulation forced the birds to care about the long term," Stephens explained.
In a parallel set of experiments, the birds underwent the same sequence of interactive moves but the food did not accumulate; instead, dispensed seeds were dispensed after every play.
One bird in each pair was randomly assigned to be a subject and the other a "stooge." The subject could choose to cooperate or defect in a Prisoner's Dilemma situation. The stooge was preprogrammed to respond to stimuli in one of two ways: the bird could either defect or it could cooperate initially but then switch to copying the opponent's previous move on all subsequent plays (a strategy known as tit-for-tat).
Thus, there were four possible outcomes for each pair of blue jays that underwent the random sequence of interactions.
When the researchers assembled the data into a matrix, it showed that whenever a subject's opponent defected, there was no cooperation between the jays. When the opponent reciprocated, there were varying levels of cooperation.
What really made a strong difference was whether the seeds accumulated during the sequence of plays: When the food accumulated, the levels of cooperation between the bird pairs were high.
The researchers concluded that "stable cooperation requires both reduced discounting and reciprocity."
In a companion article in Science, Michael Mesterton-Gibbons of Florida State University and Eldridge Adams of the University of Connecticut at Storrs wrote: "The study is timely because it forces behavioral ecologists not only to rethink the potential importance of temporal discounting, but also to address a number of other issues," such as why some individuals of a species appear to have a greater propensity than others to cooperate.
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