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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 infertility. Infertility occurs in 1 in every 6 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 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.

Mate Choice
Mate choice in human reproductive ecology is the process by which individuals rationally partner with others. Mate choice practices, like many of the topics in human reproductive ecology, vary greatly between individuals and between cultures.

Culture heavily influences mate choice, but there are evolutionary concepts that underpin research into mate choice. Honest signals are characteristics of an individual that are assumed to be true indicators of health and fecundity. Honest signals guide sexual selection, the process by which certain traits are picked by the potential mate and then proliferate throughout a species. Human cultures vary on what is considered to be a desirable honest signal. Emphasis on wealth, aesthetics, religious affiliation, and lineage, to name a few examples, are all used in different cultures as ways to choose a mate.

Monogamy is the mating strategy of two individuals partnering exclusively with each other for a period or time or for life. Monogamy in humans is generally accompanied by selective mate-choice and mating, cohabitation, and bi-parental care for children. Humans may practice life-long monogamy, as well as serial monogamy. Serial monogamy is the mating strategy of having sequential, non-overlapping partners.

Polygamy is the umbrella term for the practice of having multiple partners at the same time. The composition of the relationship will determine which type of polygamy is being practiced. Polygyny is the practice of a male partnering with multiple females. It is a fairly common mating strategy in humans, as well as in many other animals. Polygyny often occurs in agricultural societies and is often paired with male wealth or land access. When males are able to disproportionately control resources, they may be able to support more than one female partner. Polyandry is the practice of a female partnering with multiple males. It is not as common in humans as polygyny, due in part to the constraints of female reproduction. While a female may only reproduce once at a time, a male may be able to contribute to multiple concurrent pregnancies. Polyandry is often seen in cases when there are more males in a society than females, or when males are considered to be unavailable.

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 occurs over, on average, forty-two weeks. 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. She is best served delivering a baby early, to avoid complications in the birth of a too-large infant. The balance struck between these two demands, and thereby the length of a pregnancy, 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 eggs 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.