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Brain growth in rhesus monkeys, chimpanzees, and humans. SOURCE: Lancaster (1986). Reprinted by permission
The storage of fat during pregnancy as a preparation for lactation may be a direct result of our feeding ecology. Human brains grow more than twice as fast and twice as long as chimpanzee brains (see Figure 10-7). In fact, during the first year of life, as much as 65 percent of all resting energetic expenditure is used to support the maintenance and growth of the brain (Holliday, 1978). It is highly likely that our large, flexible brain is a necessary component of the feeding strategy focusing on high-quality, difficult-to-acquire foods. It is also possible that the fat storage system for lactation evolved to produce a nonfluctuating, uninterrupted flow of energy to sensitive, fast-growing brains (this set of hypotheses is due to Jane Lancaster, personal communication; see also Lancaster, 1986).
The third important difference between apes and humans is that among humans, individuals other than the child's mother also provide the energy used to nurse and provision children. In fact, nursing women in the hunting and gathering groups for which the information is available work fewer hours than women who are not nursing an infant; this is true of the Aché (Hurtado, 1985), the Hiwi (Hurtado et al., 1992), the Efe (Peacock, 1985), and for women with young infants among the Hadza (Hawkes et al., 1996).
Support of reproduction in humans is derived from many sources. Most obvious is the provisioning of women and children by men, particularly the