The function of cooperative traits can change over time. For example, helping behaviors that originally evolved by kin selection can later yield direct fitness benefits and be stabilized by partner choice. In such cases, there may be multiple interacting factors that drive cooperation. Here, I review evidence that food sharing in vampire bats evolved as form of extended maternal care that was co-opted to yield reciprocal benefits, and that such reciprocal relationships may have led to investment strategies that balance the trade-offs between greater quality and quantity of cooperative relationships.
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