On the spurious occurrence of Tit for Tat in pairs of predator-approaching fish

An experimental analysis of the movements of predator-approaching fish is presented. The experiments evaluated two competing hypotheses. (1) Predator-approaching fish play the game-theoretical strategy Tit for Tat. Alternatively, (2) the movements of predator-approaching fish superficially resemble Tit for Tat, because fish independently orient to a predator and simultaneously attempt to stay close together. Experimental subjects were mosquito fish, Gambusia affinis, approaching a green sunfish. Lepomis cyanellus. Two experiments were performed. Experiment 1 replicated results of Milinski (1987) and Dugatkin (1991), showing that Gambusia come closer to a visible predator when a mirror is oriented parallel to their direction of travel. Experiment 2 attempted to separate the effects of common orientation and social cohesion in accounting for the frequency of Tit-for-Tat-like motions in pairs of predator-approaching Gambusia. Results of experiment 2 suggest that a simple additive combination of the effects of (1) social cohesion in the absence of a visible predator and (2) orientation to a visible predator in the absence of a visible companion can account for the observed frequency of Tit-for-Tat-like motions for pairs of predator-approaching Gambusia. It is concluded that predator approach in shoaling fishes is probably a simple by-product mutualism, rather than cooperation maintained by reciprocity in a Prisoner's Dilemma.