The phenomenon of reciprocity in nature is seen by evolutionary biologists as one way to begin to understand human morality. Its function is typically to ensure a reliable supply of essential resources, especially for animals living in a habitat where food quantity or quality fluctuates unpredictably. For example, some vampire bats fail to feed on prey some nights while others manage to consume a surplus. Bats that did eat will then regurgitate part of their blood meal to save a conspecific from starvation. Since these animals live in close-knit groups over many years, an individual can count on other group members to return the favor on nights when it goes hungry (Wilkinson, 1984)