The Prisoner's Dilemma is framework in which to study the evolution animal cooperation, altruism, and mutualism. If both players cooperate, they both receive a payoff. If one cooperates and the other does not, the cooperating player receives the smallest possible payoff, and the defecting player the largest. If both players do not cooperate, they receive a payoff, but it is less than what they would gain if both had cooperated. In other words, it pays to cooperate, but it can pay even more to be selfish.
For example, let’s say a pair of male lions have formed a coalition and control a pride of females. They are challenged for control of the pride by two larger, stronger lions. If they cooperate and win the fight, they both receive the payoff of maintaining control of breeding females and any cubs they may have sired. If one lion decides to defect and not fight, the other lion may be killed or ejected in the two-against-one fight for control while the defecting lion is not killed or injured. Finally, if both males decided not to fight and leave willingly they won’t be killed or injured, but they have also lost control of the pride and any cubs they may have sired may be killed by the incoming coalition.