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 sup erficially resemble Tit for Tat, because fish independently orient to a predator and simultaneously attempt to stay close together. Experime ntal subjects were mosquito fish, Gambusia affinis, approaching a gree n sunfish, Lepomis cyanellus. Two experiments were performed. Experime nt 1 replicated results of Milinski (1987) and Dugatkin (1991), showin g that Gambusia come closer to a visible predator when a mirror is ori ented parallel to their direction of travel. Experiment 2 attempted to separate the effects of common orientation and social cohesion in acc ounting for the frequency of Tit-for-Tat-like motions in pairs of pred ator-approaching Gambusia. Results of experiment 2 suggest that a simp le additive combination of the effects of (1) social cohesion in the a bsence of a visible predator and (2) orientation to a visible predator in the absence of a visible companion can account for the observed fr equency 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 maint ained by reciprocity in a Prisoner's Dilemma. (C) 1997 The Association for the Study of Animal Behaviour.