Human reproductive ecology is a subfield in evolutionary biology that is concerned with human reproductive processes and responses to ecological variables. It is based in the natural and social sciences, and is based on theory and models deriving from human and animal biology, evolutionary theory, and ecology. It is associated with fields such as evolutionary anthropology and seeks to explain human reproductive variation and adaptations. The theoretical orientation of reproductive ecology applies the theory of natural selection to reproductive behaviors, and has also been referred to as the evolutionary ecology of human reproduction. Theoretical foundations Multiple theoretical foundations from evolutionary biology and evolutionary anthropology are important to human reproductive ecology. Notably, reproductive ecology relies heavily on Life History Theory, energetics, fitness theories, kin selection, and theories based on the study of animal evolution. Life history theory Life history theory is a prominent analytical framework used in evolutionary anthropology, biology, and reproductive ecology that seeks to explain growth and development of an organism through various life history stages of the entire lifespan. The life history stages include early growth and development, puberty, sexual development, reproductive career, and post-reproductive stage. Life history theory is based in evolutionary theory and suggests that natural selection operates on the allocation of different types of resources (material and metabolic) to meet the competing demands of growth, maintenance, and reproduction at the various life stages. Life history theory is applied to reproductive ecology in the theoretical understandings of puberty, sexual growth and maturation, fertility, parenting, and senescence because at every life stage organisms are bound to encounter and cope with unconscious and conscious decisions that hold trade-offs. Reproductive ecologists have specifically impacted life history by improving on the energetic models because they are complicated in humans, and involve many causal factors. They draw on classical life history theory, behavioral ecology, and reproductive ecology to make predictions about reproductive behavior and growth Energetics Analytical frameworks that explore problems relevant to reproductive ecology, such as age at menarche, or lactational amenorrhea, often employ understandings of energetics to their hypotheses and models. Energetics in this context refers to energy allocation, under the assumption that natural selection favors optimal allocation and use of energy, but also that trade-offs often pose energetic constraints. Allocations of energy are evolved so they in turn, can be foreseeable but they are also variable depending on ecological constraints. Essential nutrient cost variability The assumption that energy