Subfield in evolutionary biology
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 measured in calories can be used as a universal measure of nutritional cost is criticized by a number of scientists on the basis of essential nutrients, nutrients that the body cannot produce regardless of calorie availability and the specific nutrients must be present in the diet. It is argued that since there are different dietary conditions in which different essential nutrients are the most scarce in different regions and the few foods that contain the scarcest nutrients that are needed to avoid deficit diseases are therefore the most expensive (the cost may be paid in the form of other goods and services in societies without money), and different functions in the body primarily consume different essential nutrients, no universal ranking of the costs of different aspects of reproduction can be made. For example, it is possible for the few micronutrients that men consume more of the more sperm they produce but the consumption of which does not increase in women during pregnancy or lactation to be the scarcest nutrients contained in the most expensive food in some societies, making sperm production effectively more expensive than pregnancy and lactation under local food prices in such societies. It is also argued that the variability of what food is the most valuable due to containing the rarest essential nutrients extend their effects to the economical significance ratio between hunting and gathering in the case of hunter-gatherer societies, and therefore that any attempt to circumvent the evolutionary psychology paradox of men not being able to be in two places at the same time to hunt and protect his family by reference to hiring guards by bartering meat would fail to make sex roles universal due to the difference between regions where the rarest essential nutrients were contained in one or more types of meat and regions where the rarest such nutrients were contained in some types of plants. It is cited in this context that humans evolved over relatively large parts of Africa with different food ecologies, making it impossible for humans to have specialized evolutionarily for one specific food cost ratio. This variability of food value ratios within Africa may have prepared humans evolutionarily to be able to leave Africa.
Biodemography and human reproduction.
The researchers involved in human reproductive ecology use the combined approach of demography and evolutionary biology to explain the reproductive phenomenon. Biodemography is the study of demography related to biology and evolutionary biology. Biodemographers do research on demographic outcomes such as conception, spontaneous abortion, births, marriage, divorce, menarche, menopause, aging, and mortality. Biodemographers use mathematical models, statistical estimates and biomarkers to analyze the demographic data. The field of biodemography often explores the scientific questions associated with fertility and mortality across cultures, the determinants of reproductive senescence, mortality and sex differences, low fertility in humans, and longer post-reproductive lifespan in women.
Key topics.
Gestation.
In human reproductive ecology, the study of pregnancy is primarily focused variation in pregnancy and on rates of pregnancy loss.
Variation in pregnancy.
Pregnancy varies person-to-person and across cultural and socioeconomic lines. Human gestation is between 30 and 40 weeks long. The dynamic between the mother and the fetus is one of conflict: it is in the best interest of the fetus to gestate as long as possible to continue receiving the nutritional and developmental benefits of being physically attached to the mother. For the mother, however, pregnancy is a highly demanding and risky time. Earlier births avoid complications in the birth of a too-large infant. The length of the pregnancy is a compromise between these two demands, and is influenced by factors such as socioeconomic status, health, and fetal development. Women of lower socioeconomic status have been shown to deliver their babies earlier on average than women of higher socioeconomic status. Research has also shown that stress, especially during early pregnancy, can cause shorter gestation length and increase premature births.
Pregnancy loss.
The rate of embryo loss changes throughout pregnancy. Pre-implantation in the uterine wall, rate of loss is undetectable as the hCG hormone is not secreted until implantation. There is no current way to detect pregnancy or pregnancy loss at this stage. Post-implantation, rate of loss is highest in the first trimester of a pregnancy. The chance of pregnancy loss lowers the further into gestation a woman is.
Pregnancies may be unsuccessful for multiple reasons. The maternal immune system, though suppressed during ovulation, views the fertilized egg as a foreign body and will attack it. Defective embryos may also be spontaneously aborted, or miscarried, whether due to chromosomal abnormality or developmental defects. Endometrial or placental development issues may also cause a pregnancy to fail. Additionally, the frequency of spontaneous abortion increases with the mother's age. Older mothers have a higher rate of genetic abnormalities that can trigger pregnancy loss.
Because human pregnancy is so costly, and human offspring so dependent on their mothers, early spontaneous abortion is high to ensure that energy of a pregnancy is spent on developing a fetus with a high chance of survival.
Fecundity and fertility.
Human reproductive ecology considers fecundity and fertility from a demographic perspective. In this view, fecundity is the reproductive "potential" of an individual and fertility is the actual reproductive "output" of an individual.
Fecundity.
Fecundity is determined by the biological limitations of the individual and can be reduced when biological and ecological factors impact an individual's reproductive capabilities. The key components of fecundity are a person's reproductive maturation and the maintenance of their reproductive system. In humans, the timing of female reproductive maturation is particularly variable and is heavily influenced by ecological considerations. In addition, the age at menarche has decreased over time in many global populations. This phenomenon is referred to as the secular trend. Age at menarche is one measure of the fecundity of an individual female. Male reproductive maturity is less subject to environmental and ecological factors, and does not follow the secular trend that female puberty does.
In adults, fecundity is determined by the biological processes of reproduction. Female fecundity is heavily influenced by reproduction and energetics. The ovarian cycle limits the potential of conception to a brief period of fertility roughly once a month. Successful egg maturation, fertilization, and implantation must be able to occur for a reproductively mature female to be fecund. Changes in energy levels, diet, and hormones can all interfere in this process. During breastfeeding, a period of lactational infertility also reduces female fecundity. The metabolic load hypothesis in human reproductive ecology describes how the energetic expenditure of lactation acts to inhibit ovarian cycling. With the majority of available energy going towards milk production, energy is not expended on reproductive effort.
Male fecundity is primarily determined by the quality of sperm and the availability of fertile female mates. Individual variation in sperm load, pH, lifespan, and morphology creates varying fecundity in males. As males do not gestate, their contribution to fecundity is less well established post-reproduction.
A lack of fecundity in adults can be described as infecundity or infertility. Infertility occurs in about 10–15% of couples, with the causes of infertility shared equally between males and females.
Fertility.
Fertility is the measure of an individual's actual reproductive output, rather than just their potential for reproductive success. Fertility rates vary both inter- and intra-culturally.  Fertility for both males and females is dependent not just on biology but on cultural, religious, economic, and other sociological factors as well.
Natural fertility is emphasized in the study of human reproductive ecology. Natural fertility is the measure of human fertility in populations without birth control. Research on natural fertility populations seeks to understand the evolutionary context, ecological constraints, and predict outcomes for human fertility.
Fertility is influenced by fecundity, but has additional factors that can increase or decrease an individual's lifetime reproductive success. The inter-birth interval, the amount of time between a woman's births, impacts a woman's total fertility. This amount of time varies cross-culturally, as well as varies with different environmental constraints. Many cultures practice conscious birth spacing to adhere to the desired length of time between pregnancies, or desired number of children. Environmental concerns like fetal loss, lack of resource access, and disease may all impact fertility for females or males.
Fertility rates across the globe have steadily declined. This trend, known as the demographic transition, began in the 1700s and continues today. It is strongly correlated with increased industrialization in a society. This trend is now seen in almost all cultures, resulting in some societies with below replacement fertility. Below replacement fertility is when the rate of childbirth in a society is less than the amount needed for each woman to have at least one daughter. Since the chance of having a daughter is 50/50, there must be at least two children for every adult woman in the population.
Natural fertility populations.
In 1961, French demographer Louis Henry introduced the term "natural fertility". Natural fertility is defined as uncontrolled fertility when the couples do not control the number of children and the family size. Controlled fertility populations use controlled methods to stop having children after reaching a certain number of children. 
In natural fertility populations, the parity related controls of fertility are not influenced by modern birth controls. Therefore, studying and understanding the age-related changes in fecundity is easier in natural fertility populations in compare to controlled fertility populations. Natural fertility populations deliver an easier platform to study the reproductive behavior which may affect the levels of fertility such as pregnancy loss, time for conception, and length of breastfeeding. In Pennsylvania and Ohio states in the United States, the Amish settlements have been studied to understand the age of marriage, the age of first birth, birth intervals, the age at last birth, and total fertility rate as they are natural fertility population due to their religious belief. The Dogon population in Mali, West Africa are a natural fertility population with high fertility rate and they have been studied to understand the role of the age of wife, the age of husband, nutritional status, breastfeeding status, sex of last child, economic status, and polygyny on the waiting time