ARTICLES Aping Our Ancestors: Comparative Aspects of Reproductive Ecology GILLIAN R. BENTLEY Energetic status, in fact, can be em- pirically shown to impact in a variety of ways on human life-history traits and reproductive physiology, affecting the tempo of growth, hormonal levels during the menstrual cycle, the prob- ability of conception, the timing of births, and even birth outcome in some cases. For example, chronic nutri- tional deficits during childhood dis- rupt normal growth patterns and of- ten lead to smaller adult body sizes. 10–12 The age at reproductive maturation is similarly affected by these deficits, pri- marily as a corollary of slower growth rates, because populations living in environments of different quality ex- hibit a wide range of pubertal ages. 11 On the other hand, short-term sea- sonal perturbations in nutrition or energy output have more temporary effects on human reproductive capa- bilities,graduallyreducingfecundabil- ity(the monthlyprobabilityofconcep- tion). This reduction can range from a lowering of reproductive hormone lev- els to anovulation and the complete cessation of menstrual function. 13,14 The field of reproductive ecology has been at the forefront in outlining the importance of such environmental constraints on human reproduc- tion. 13,15–19 This is especially true among traditional groups whose life- styles are much closer than those of modern, well-fed Westerners to the ecologically constrained circum- stances in which most human popula- tions probably lived in our evolution- ary past. 20–24 Reproductive ecologists assume that reproductive responses to environmental constraints are adap- tive. Economizing on reproductive ef- fort, it is argued, diminishes the possi- bility of a conception, gestation, and long lactational period when maternal condition and, hence, reproductive outcome looks poor. 13,16–18,25–27 That response probably evolved under eco- logical conditions in which resources were unstable and fluctuatingand indi- viduals with the abilityto curtailrepro- ductive investment during periods of stress would have had longer life spans and greater lifetime fertility. However, reproductive ecologists have yet to inves- tigate whether the adaptive functions hypothesized to exist in humans have phylogenetic origins—that is, whether similar kinds of physiological responses to environmental stressors can be ob- served in our close primate cousins. In contrast,animalbehaviorists and prima- tologists studying the effects of energetic stress on the subjects of their investiga- tions often refer to the admittedly larger human literature for comparison. 25,26,28–31 The lack of comparative discussion in the literature on human reproduc- tive ecology partly stems from the early influences on this field, which came primarily from clinical studies in sports medicine, 17,32 developments in reproductive endocrinology and its associated technologies, 33 and impor- tant studies like the long-term field research in Gambia that looked at the relationship between energetics and lactation. 34,35 The contrasts in ap- proach with most clinical studies is, however, explicitly acknowledged: Cli- nicians tend to view reproductive dys- function in reaction to energetic con- straints as pathological rather than adaptive. 26,36 Another reason why a better comparative perspective has not been forthcoming is a lack of equiva- lent data, particularly hormonal data, from free-rangingprimate groups.This situation is only now being remedied, facilitated by new field techniques for collecting suitable samples for the analysis of hormones in both the field and the laboratory. 37 The purpose of this paper is, therefore, to shed greater light on some comparative aspects of reproductive ecology among primates Gillian Bentley is a Royal Society Re- search Fellow in the Department of Biologi- cal Anthropology, University of Cambridge, England. Her research centers on human reproductive ecology, and particularly hor- monal responses to environmental stress. At present, she is conducting a collabora- tive project to investigate the causes of interpopulational differences in reproduc- tive hormone levels, and the effect of this variation on fecundity. Key words: reproductive ecology, primates, hor- mones, nutrition, development There are many similarities among mammalian species in how ecological factors affect their reproductive potential and individual life histories. One of the most important limiting factors is the availability of sufficient resources to partition among essential growth, maintenance, and eventual reproduction. 1–3 How each mammal juggles these constraints constitutes its unique life history and determines its success as a species. Yet, as Hill and Hurtado 4 recently argued, life-history analyses are rarely applied to human reproduction. In fact, despite the demonstrated significance of adequate nutrition for reproductive performance among most mammalian species that have been studied, 1,3,5 many demographers, and even some biological anthropologists, have resisted the idea that humans, except under extreme famine conditions, might be subject to the same kinds of nutritional constraints that affect other mammals. 6–9 Evolutionary Anthropology 175