role of the brain and the complexity of the endocrine “code.” The same hormone may stimulate a response at some levels or in some pulsatile patterns and suppress that response at other levels. Many hormonal influences on sexual function are paralleled by influences on sexual desire and behavior.

For the biodemography of fertility, Cameron points to several priorities for interdisciplinary research. One involves effects of extensive artificial exposure to steroids and other endocrinologically active substances now occurring in developed societies through medication, self-medication, and consumer choice. Another follows from experiments documenting the effects of psychosocial stress on hormone secretion, plausibly with implications not only for individual health but also for social gradients. Finally, Cameron directs renewed attention to the question of nutrition and fertility. She notes that biomedical scientists drawing on laboratory data tend to regard energetic stress as “an important regulator of reproductive ecology,” posing a contrast, at least in emphasis, to the conclusions of demographers drawing on field data from populations.

In the abstract, one might have imagined that it could be an advantage in evolutionary terms for the reproductive systems of humans and other primates to turn off easily and automatically in the face of hard times. Gains in fitness might be thought to accrue from saving on large physiological and energetic investments in infants when chances for their survival were low and from concentrating resources on procreation when chances for infant survival were high. In fact, however, evolution seems to have honed our reproductive systems to turn on whenever they minimally can. Perhaps evolutionary environments were too unpredictable for automatic regulation to be worthwhile. It is tempting to wonder how different our overpopulated modern world might have been had we come to it endowed with built-in preventive checks on fertility.

INTEGRATIVE BIOLOGY

Happily, both the words “demography” and “population” have grown beyond their etymological roots in words for people and now apply to other species. The mathematical core of demography carries over directly. The species that have figured prominently up to now in the biodemography of longevity, including species of flies and worms, have little in the way of social structure. Now comprehensive demographic data on long-lived species with social structure are beginning to come into being. In Chapter 6, Jeanne Altmann and Susan Alberts treat us to an overview of the remarkable demography of populations of baboons from Amboseli, Kenya, based on observations collected over some 30 years. After setting the life course patterns of humans within the context of other mammals and other pri-



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