In other scenarios, cooperation is trivially the best choice: if Alice and Bob want to move a heavy object from point A to point B and neither is strong enough to move it alone, but they can move with their combined strength, then they have an incentive to cooperate, neither has an incentive to defect since if one of them defects then the heavy object doesn’t reach point B.
Unless the object weighs exactly enough that it requires both of their full strength to move, then they both have an incentive to defect (not to put their full effort in, and let the other work harder). Mutual defection then results in the object not reaching point A.
Most scenarios involve some variation. Even the hunter-prey scenario; the herd or hunters could deliberately choose a sacrifice, saving both hunters and prey from running and expending additional calories on all sides, and reducing the number of prey animals, overall, that the hunters would need to eat. (Consider a real-life example of this—human herders, and their herds. Human-herd relationships are more complex than that, but it could be modeled that way.)
Unless the object weighs exactly enough that it requires both of their full strength to move, then they both have an incentive to defect (not to put their full effort in, and let the other work harder). Mutual defection then results in the object not reaching point A.
Most scenarios involve some variation. Even the hunter-prey scenario; the herd or hunters could deliberately choose a sacrifice, saving both hunters and prey from running and expending additional calories on all sides, and reducing the number of prey animals, overall, that the hunters would need to eat. (Consider a real-life example of this—human herders, and their herds. Human-herd relationships are more complex than that, but it could be modeled that way.)