Parent-offspring conflict theory, signaling of need, and weight gain in early life.

Human growth in early life has major implications for fitness. During this period, the mother regulates the growth of her offspring through placental nutrition and lactation. However, parent-offspring conflict theory predicts that offspring are selected to demand more resources than the mother is selected to provide. This general issue has prompted the development of begging theory, which attempts to find the optimal levels of offspring demand and parental provisioning. Several models have been proposed to account for begging behavior, whether by biochemical or behavioral pathways, including: (1) blackmail of parents; (2) scramble competition between multiple offspring; (3) honest signaling of nutritional need; and (4) honest signaling of offspring worth. These models are all supported by data from nonhuman animals, with species varying according to which model is relevant. This paper examines the evidence that human suckling and crying signal nutritional demand, need, and worth to the mother. While suckling provides hormonal stimulation of breast milk production and signals hunger, crying fulfills a different role, with evidence suggesting that it signals both worth and need for resources (nutrition and thermoregulation). The role of signaling in nutritional demand is examined in the context of three common health problems that have traditionally been assumed to have physiological rather than behavioral causes: excess weight gain, failure to thrive, and colic. The value of such an evolutionary approach lies in its potential to enhance behavioral management of these conditions.