Read this about vampires that I didn't know.
Consider the vampire bat. At night, it ventures from its cave, along with thousands of other bats, and goes on the hunt. The vampire bat like some sort of comic-book villain has evolved a special brain region that enables the detection of hot spots on animals (usually goats, its favorite food). Its a sort of infrared vision.
The bat must feed every two nights, but doesnt always manage to find a goat. Instead, it often has to rely on the charity of other bats, who share blood after a successful hunt. The bat then pays this forward sharing blood with those bats when theyre hungry.
As it turns out, bats are pretty decent game theorists. The hunting-sharing cycle is sorta like an iterated prisoners dilemma. If everyone shares, were all well off, and if everyone is selfish, a lot of good bats will starve, but if everyone else shares except me, great, since I get gallons of blood. To enforce cooperation, the bats implement a tit-for-tat strategy if you share with me, Ill share with you next time. This, plus a friendliness clause sharing with unknown bats is tit for tat.
Such a strategy breaks down when the share-or-not-share decision is not iterated. In a single shot game, prisoners defect all the bats will be selfish. For blood sharing among bats to occur, they must frequently interact with bats they know.