Simultaneous hermaphroditism, egg trading, and the Prisoner’s Dilemma

Researchers utilize models to simulate interactions in complex systems. Game theory is the application of mathematical models to predict the behavior of individuals or groups in real world scenarios. One such game is the Prisoner’s Dilemma. In the Prisoner’s Dilemma game, each player may cooperate with the preceding move or not cooperate. Not cooperating is termed defecting. A successful strategy in this game was determined to have four properties: niceness (the strategy begins by cooperating), provokability (if the player cooperated and the other player defects, the first player will defect in the next round), forgiveness (resumed cooperation when the opponent cooperates), and a clear and predictable strategy.

There are many strategies, which are employed by players in the Prisoner’s Dilemma game. Tit-for-Tat begins by cooperating and then responds in each subsequent round with the opposing player’s previous move. Grim Trigger also begins with cooperation; however, once the opponent has defected, the player defects the rest of the game. A pure strategy, such as cooperate unconditionally or defect unconditionally, occurs when a player performs the same move regardless of the move of the other player.

One application of game theory is mating behavior. Fish of the species Serranus tortugarum, the chalk bass, are simultaneous hermaphrodites, producing both male and female gametes. The mating behavior, egg trading, can be modeled as an iterated two-person Prisoner’s Dilemma game. Although the player moving first is random, in nature the last fish to make a courtship display releases eggs. Production of eggs requires more resources; however, fertilization is external and planktonic (dispersed widely). This drastically reduces the impact of kin selection. Cooperation is a release of eggs or sperm, while defection is the failure to release eggs or sperm. The synchronization of mating based on a common external reference rather than the active decisions of two individuals, or the church-clock fallacy, does not adequately describe the mating behavior displayed. Desertion, as a retaliatory action for defection, is not permitted, since this would end the game.