UC San Diego

Human females have unique life history traits when compared to other apes including early weaning of infants and a shortened inter-birth interval (IBI). The current study explores the mechanisms that allow human females to successfully have more closely spaced births and support multiple dependents by testing the Cooperative Breeding hypothesis, which suggests that group members other than the biological mother (allomothers) assist in rearing offspring. Behavioral data on the frequency of holding children and ecological data on self-provisioning among juvenile foragers is used to explore aspects of cooperative caregiving and provisioning among the Hadza hunter-gatherers of Tanzania. The results indicate that although mothers spend the largest amount of time in direct care of their infants and young children, additional categories of caregivers routinely provide investment. Related caregivers spend the largest percentage of time holding children, which supports kin selection (or nepotism) as the strongest motivation to provide allomaternal care. In addition to behavioral investment, the current study determines the extent to which juveniles offset the high cost of their own care by provisioning themselves. The results suggest that Hadza children begin foraging for various types of plant and animal foods starting at the age of five years. A wide variation in returns is seen across all young foragers in the sample, however some individuals are able to collect 50 - 70% of their daily needs. The flexibility of the human foraging niche creates an opportunity for juvenile food collection that alleviates constraints on maternal investment. Taken together, the results of this study suggest that juvenile foraging and nepotistic investment are two mechanisms that allow mothers to maintain a short inter-birth interval while supporting multiple dependent children, lending support to the Cooperative Breeding hypothesis.