Milinski noticed that when the sticklebacks approached the pike, one would advance a bit and then check whether the other had followed. "Each step was one round of the game, Milinski says.

To test whether the sticklebacks were using a tit-for-tat policy, Milinski placed a lone stickleback in a tank with a pike and positioned a mirror to give the stickleback the illusion that there was a second stickleback nearby. Depending on the angle of the mirror, this imaginary fish appeared to be either swimming apace or dawdling.

Milinski found that when the imaginary stickleback seemed to be defecting, the. real stickleback hung back. "What the experimental fish did corresponded very well to the predicted strategy," Milinski says.