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Cooperative Behavior

Predator Inspection
Cooperative behavior among three-spined sticklebacks has been proven to exist. The main cooperative behaviors of the three-spined stickleback is predator inspection. Predator inspection appears to allow acquisition of information about the risk that a potential predator presents, and may deter attack, with the cost being an increased chance of being attacked if the predator proves to be hungry.

Tit-For-Tat Strategy
Sticklebacks are known to cooperate in a tit-for-tat (TFT) strategy when doing predator inspection. This is how TFT works: cooperate on the first move and thereafter do whatever your opponent did on the previous move. This allows for a combination of collaborative (it starts by cooperating), retaliatory (punishes defection), and forgiving (respond to cooperation of others, even if they had defected previously). . In one experiment that used a system of mirrors, single three-spined sticklebacks approaching a live predator were provided with either a stimulated cooperating companion or a stimulated defecting one. In both cases, the test fish behaved according to tit-for-tat strategy, supporting the hypothesis that cooperation can evolve among egoists.

Typically, sticklebacks operate in pairs. In one study involving a shoal of four sticklebacks, individual sticklebacks had partners with whom they repeatedly performed pairwise predator inspection visits. In six different trials for that study, the researchers found two reciprocal pairs per trial significantly more often than what was expected due to chance. These results provide further evidence for a tit-for-tat cooperation strategy in sticklebacks.

Stickleback behavior is often cited as a great example of cooperative behavior during predator inspection. In one study, there were two experiments that further substantiated the claim that sticklebacks use the tit-for-tat strategy when doing predator inspection. In experiment 1, fish from three sites differing in predation risk inspected a model predator in pairs and reciprocated both cooperative moves and defections by the partner, but not on every opportunity. Sticklebacks that originated in the two sites that contained piscivorous fish were more likely to reciprocate following a cooperative move than following a defection. Individuals from higher risk sites were generally more cooperative. In experiment 2, individuals were accompanied by a model companion and showed reciprocal moves of cooperation and defection in response to the model's movements about a third of the time. The conclusion is that these results demonstrate the elements of a strategy of cooperation that may resemble tit-for-tat.

Partner-Dependence
While the tit-for-tat cooperation strategy has been shown to be evident in sticklebacks, one study suggests that the size of a stickleback's partner fish also was a factor in determining what a stickleback would do when both were faced with a predator. When two sticklebacks were simultaneously presented to a rainbow trout, a predator much larger in size, then the larger of the two sticklebacks had a higher risk of being attacked. Individual sticklebacks (test fish) in the present study moved closer to a trout when a larger potential partner, which the test fish could only see through a one-way mirror, moved close to the trout than when a smaller partner approached the trout. Although both the large and small partners behaved similarly, the small partner's behavior affected the strategy of the test fish more than that of the large partner. Both when alone and when the partner cooperated, larger fish approached the predator more closely than did smaller fish. If a partner defected, then the test fish's condition-factor (i.e. it's ability to flee) determined how closely it approached the predator rather than the test fish's size. Both the strategy and reaction to different-sized partners seemed to be dependent on whether the partner cooperated or defected.