Energetics , Reproductive Ecology , and Human

Human reproductive ecology is a relatively new subfield of human evolutionary biology focusing on the responsiveness of the human reproductive system to ecological variables. Many of the advances in human, and more recently primate, reproductive ecology concern the influence of energetics on the allocation of reproductive effort. This paper reviews eleven working hypotheses that have emerged from recent work in reproductive ecology that have potential bearing on the role of energetics in human evolution. Suggestions are made about the inferences that may connect this body of work to our efforts to reconstruct the forces that have shaped human biology over the course of our evolutionary history. T here is increasing interest in the role that energetics may have played in shaping important aspects of human evolution (Aiello and Key 2002; Leonard and Ulijaszek 2002). By energetics I mean those processes by which energy is captured from the environment for metabolic purposes and allocated to those purposes. Such a broad definition captures a great deal that has traditionally been a part of theorizing about human evolution, including diet and foraging behavior (Sorensen and Leonard 2001), digestive physiology (Milton 1987), and the energetic efficiency of locomotion (Hunt 1994). But it also includes the consequences of necessary trade-offs in energy allocation among competing physiological domains, such as physical growth, immune function, thermoregulation, subsistence work, and reproduction. From the perspective of life history theory, trade-offs between reproduction and survival (under which many of the other categories of metabolic allocation can be placed) are central to understanding evolutionary dynamics (Stearns 1992). Differential allocation to reproduction is known in this branch of evolutionary ecology as " reproductive effort. " The central premise of life history theory is that natural selection has acted on organisms to optimize the allocation of reproductive effort over the organism's life span. This perspective has been remarkably successful in explaining the diversity of life histories observed in nature. Of late there has been particular attention paid to the energetic aspects of human brain evolution (Aiello and Wheeler 1995; Leonard et al. 2003; 2007). The human brain is energetically very expensive, both to grow and to maintain. Meeting the energetic cost of such a large brain is a challenge that has both phylogenetic and ontogenetic implications. Phylogenetically, the question might be posed as, " How was the energy for such a significant new allocation made available in the budget of an organism that …