[Show abstract][Hide abstract] ABSTRACT: Human life history contains a series of paradoxes not easily explained by classical life history theory. Although overall reproductive output is higher than in related primates, juvenile growth is slower and age-specific reproductive rates decline faster with age. A simple energetic model would predict that growth and reproductive rates should be positively correlated and that reproductive effort should not decelerate with age. The pattern of negative correlations in humans suggest the presence of trade-offs among peak reproductive rate, childhood growth, and reproductive rate at older ages. To address this puzzle, we propose a synthesis of reproductive ecology and behavioral ecology focused on intra- and inter-somatic energy transfers. This integration includes three concepts: the mother as final common pathway through which energy must pass to result in offspring; a distinction between direct and indirect reproductive effort, proposing the latter as a novel net energy allocation category relative to growth and direct reproductive effort; and a pooled energy budget representing the energetic contributions and withdrawals of all members of a breeding community. Individuals at all reproductive life stages are considered in light of their contributions to the pooled energy budget.
American Journal of Human Biology 07/2009; 21(4):421-9. DOI:10.1002/ajhb.20906 · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Compared to non-human primates, human life history is characterized by slow juvenile growth, late age at maturity, short interbirth intervals, and a decoupling of reproductive senescence and somatic senescence. Some of these traits represent a delay of reproductive effort while others represent an acceleration. To address this puzzle, we propose that humans in all three stages of the life span, pre-reproductives, adults of reproductive age, and post-reproductives, all contribute to a “pooled energy budget” (PEB) that is primarily utilized by the reproductively active female to carry out the energetically expensive tasks of gestation and lactation. Adults contribute to the pooled energy budget by providing food and care to children, thus freeing up the mother to provide more resources to a gestating fetus or a nursing infant and to resume postpartum ovulation more quickly. Pre-reproductives also contribute to the PEB. Older children, through participating in childcare and subsistence activities, can provide resources to partially meet their own energetic needs and to care for and provision younger siblings. The mother becomes the “final common pathway” through which energy flows in order to produce new offspring. While life history theory has traditionally considered growth, maintenance, and reproduction as the three main categories of energetic tradeoffs, we propose that energy allocated towards reproduction can be broken into two categories: direct and indirect reproductive effort. The contributions to the PEB made by both pre-reproductives and post-reproductives can be seen as indirect reproductive effort and can provide an explanation for slow childhood growth rates and menopause. Anthropology Human Evolutionary Biology
[Show abstract][Hide abstract] ABSTRACT: Life history models predict differential energy allocation patterns among growth, maintenance, and reproduction under varying ecological conditions. The assessment of energy expenditure is one important tool in quantifying energy allocation; activity is more immediately modifiable than endocrine or genetic factors. In a pilot project among subsistence agriculturalists in The Gambia, West Africa, we tested the efficacy of accelerometers against heart rate monitors for measuring energy expenditure in free-living Gambian women. While accelerometers are light, do not require contact with the skin, and may be programmed to start and stop at predetermined times, they record movement rather than exertion. Both devices were calibrated to oxygen consumption, and individual regression lines for energy produced in Watts were computed. Women wore heart rate monitors and hip and arm accelerometers for approximately twelve hours, during which they were observed for two hours, and activity and intensity level were recorded. Results indicate that, in this sample, an accelerometer worn on the hip is as effective as heart rate monitoring in measuring energy expenditure. Though activity counts spike more easily than heart rate, a rolling average of activity counts yields a qualitatively comparable energy expenditure curve to heart rate. Accelerometer activities recorded at the upper arm and the hip were generally concordant, but showed significant periods of divergence. Focal observations suggest that certain typical activities may be missed by one accelerometer location or the other. We conclude that differences in energy expenditure between individuals and between groups can be obtained in field conditions using hip accelerometers. Anthropology Human Evolutionary Biology
[Show abstract][Hide abstract] ABSTRACT: Leptin is thought to signal energy stores, thus helping the body balance energy intake and expenditure. However, the strong relationship between leptin and adiposity in populations with adequate nutrition or common obesity is not universal across ecologic contexts, and leptin often correlates only weakly, or not at all, with adiposity in populations of lean or marginally-nourished males. To clarify whether the relationship between adiposity and leptin changes during development, this study examines leptin and body fat among children and adolescents of lowland Bolivia. Anthropometric measures of body composition and dried blood spot samples were collected from 487 Tsimane' ranging from 2 to 15 years of age. Leptin was assayed using an enzyme immunoassay protocol validated for use with blood spot samples. In this population, leptin concentrations were among the lowest reported in a human population (mean +/- SD: 1.26 +/- 0.5 and 0.57 +/- 0.3 in females and males). In addition, the relationship between leptin and adiposity follows distinct developmental trajectories in males and females. In males, leptin is weakly correlated with most measures of body composition at all ages investigated. However, in females, the level of body fat and the strength of the correlation between body fat and leptin (a measure of its strength as a signal of energy stores) both increase markedly with age. These findings suggest a more important role of leptin as a signal of energy stores among females as they approach reproductive maturity, while raising questions about the function of this hormone in lean males.
American Journal of Human Biology 07/2008; 20(4):392-8. DOI:10.1002/ajhb.20765 · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Leptin is important in a wide range of physiological processes, but logistical constraints associated with venipuncture blood collection have limited research on leptin in diverse, community-based settings. The aim of this short report is to present and validate an enzyme immunoassay method for quantifying leptin in samples of capillary whole blood collected from a simple finger prick and dried on filter paper. The method was evaluated through analysis of precision, reliability, stability, and comparisons with matched plasma and blood spot samples. We report acceptable levels of assay precision and reliability, and good agreement between results obtained from matched plasma and blood spot samples (r = 0.976, P < 0.001). Leptin concentrations begin to deteriorate after only 3 days at room temperature. Thus, care should be taken to refrigerate or freeze samples promptly. The relative ease of blood spot sample collection may facilitate research on leptin in a wider range of cultural and ecological settings.
American Journal of Human Biology 11/2006; 18(6):857-60. DOI:10.1002/ajhb.20566 · 1.70 Impact Factor