LAUSR

OBJECTIVES Weaning of chimpanzees is considered to occur at 4-5-years-old with complete cessation of nipple contact and timing of reconception calculated by inter-birth interval minus gestation length. This is also the basis of "early weaning" in humans (i.e., approximately 2.5-years-old). However, recent studies of the survival of orphans and the first molar (M1) eruption in wild chimpanzees have predicted that infants move toward nutritional independence at 3-years-old. Therefore, this study aimed to investigate ontogeny of feeding behavior at around 3-years-old in wild infant chimpanzees. MATERIALS AND METHODS I studied 19 infants aged 1-60 months in the M group in Mahale Mountains National Park, Tanzania. The total observation time was 518 h, 25 min. RESULTS At around 3-years-old, infant chimpanzees spent more total feeding time, and time feeding on leaves, and food physically difficult to process without food transfer from other individuals. These results suggest that infant chimpanzees significantly reduced their dependence on milk for nutrition at around 3-years-old, that is, before cessation of nipple contact. DISCUSSION This study suggests that M1 eruption in wild Eastern Chimpanzees is an index of the period when infants move toward nutritional independence with a key dietary transition. This is the first study to provide behavioral evidence of the large temporal gap between nutritional independence of infants and reconception of mothers in great apes, and clarify the unique feature of human life history whereby mothers can reconceive before an infant reaches nutritional independence.