Food transfer between chimpanzee mothers and their infants.

Food sharing among chimpanzees is known to occur particularly between mothers and infants and has been proposed to be a form of parental investment. To explore the function of food sharing, it is essential to know how and what is transferred to an infant from its mother. We investigated details of interactions leading to food transfer and characteristics of items transferred in three mother-infant ( < 2 years old) pairs in captivity. We gave one kind of fruit or vegetable to a mother and observed interactions between the mother and her infant. Tested items consisted of familiar and novel foods for infants. Two patterns of direct food transfer, so-called sharing, were recognized: (1) "infant-initiated sharing" in which the infant attempted to take food and the mother did not resist, and (2) "mother-initiated sharing" in which the mother spontaneously offered a part of her food without the infant's attempts to take it. There were clear differences in the characteristics of items transferred in these two patterns of sharing. In infant-initiated sharing, palatable parts of the same food that the mother was eating were transferred. In contrast, in mother-initiated sharing, only unpalatable parts of food in the mother's possession were transferred. Mothers seemed to be reluctant to give nutritious foods to their infants during this study period. Infants, rather than mothers, were responsible for initiating and experiencing the diversity of adult foods in chimpanzees.