Rats in the lab learn to play best-responses in a repeated prisoner’s dilemma.  The rats were given rewards according to which of two compartments each walks into, and these rewards were structured as in a Prisoner’s dilemma.  First the rats were given a “training session” where they learned the payoff function.  Then the strategy of one rat was manipulated as the experimenters manually placed the rat into compartments before the other rat made his choice.

When the control rat played a random strategy, the experimental rat mostly “defected” but when the control rat played a reciprocating strategy (Tit-for-tat), the experimental rat not only learned to cooperate but also how to invite escape from a punishment phase.

It may not be entirely surprising that rats cooperated in the Prisoner’s Dilemma.  After all, animals often cooperate in nature to altruistically serve the group, whether that means hunting in packs to get more meat, or a surrogate mother animal adopting an abandoned baby to boost the pack’s numbers.  Still, there’s no direct evidence that shows rats grasp the concept of direct reciprocity.  Given that the rats in this study changed their strategy based on the game their opponent was playing, and cooperation rates were only high when the rats played against a tit-for-tat opponent, the authors showed, perhaps for the first time, that rats directly reciprocate.