Reciprocal food sharing is a form of reciprocal altruism

Reciprocal altruism

In evolutionary biology, reciprocal altruism is a behaviour whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time...

where one individual gives up the food it has forged to another individual. Food sharing has been observed in a wide range of animals, including insects, birds, cetaceans, vampire bats, and primates . It is not always an active behavior; tolerance of theft may also be considered a form of food sharing (Stevens and Gilby). Not only does food sharing occur among members of the same family, but also among non-kin individuals as well, creating an interesting evolutionary question.

Vampire bats

Vampire bats, which primarily feed on livestock or other vertebrates, must obtain a meal every 48-72 hours or face starvation. On a given night, there are individuals that do not successfully feed. Fortunately for them, a successful individual may regurgitate their meal for the unsuccessful individual.
In order for this trait to have persisted through evolutionary time, a level of recognition is necessary among individuals. An altruistic bat may refuse to regurgitate blood for another bat that has not given blood to others in the past. The mechanism for this reaction is not known.

Grooming

Wilkinson suggests that repayment of blood-giving may not necessarily be in the form of blood, but perhaps grooming.

Meat-for-sex hypothesis

Males of a certain insect, bird, or mammal species may give food to a female in order to increase their own reproductive success. The food provision may occur before copulation, in order to attract females, or after in order to nourish the female and increase her fecundity. Among some cannibalistic insect species, the male may provision food to the female in order to prevent her from eating him after copulation.