[5] Reproductive ecologists have specifically impacted life history by improving on the energetic models because they are complicated in humans, and involve many causal factors.
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.
[10] Biodemographers do research on demographic outcomes such as conception, spontaneous abortion, births, marriage, divorce, menarche, menopause, aging, and mortality.
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.
[14] Research has also shown that stress, especially during early pregnancy, can cause shorter gestation length and increase premature births.
The maternal immune system, though suppressed during ovulation, views the fertilized egg as a foreign body and will attack it.
Male reproductive maturity is less subject to environmental and ecological factors, and does not follow the secular trend that female puberty does.
The metabolic load hypothesis in human reproductive ecology describes how the energetic expenditure of lactation acts to inhibit ovarian cycling.
Individual variation in sperm load, pH, lifespan, and morphology creates varying fecundity in males.
Fertility is influenced by fecundity, but has additional factors that can increase or decrease an individual's lifetime reproductive success.
Environmental concerns like fetal loss, lack of resource access, and disease may all impact fertility for females or males.
[26] Natural fertility population in rural Bangladesh have been studied to predict the role of parity, pregnancy loss, mother's age, economic status, child's sex, and husband's migration on the distribution of postpartum amenorrhea.
The hunter-gatherer !Kung mothers require to carry a greater amount of food and baby on foraging trips and shorter birth interval length results higher infant mortality among them.
In evolutionary context, it is assumed that human physiology has been modeled through natural selection to maximize reproductive success by allotting energy and resources through trade-offs.
Secondary sexual characteristics include adolescent growth spurt, pubic and axillary hair, genital enlargement, breast development in girls, beard growth in boys, increase in subcutaneous fat, increase in muscle mass, and widening of the pelvis in girls.
[42] This period is also a time of cognitive and psychosocial development where social relationships, skills, and experiences outside of the core family are explored.
[43][44][45] Dietary composition, disease, psyschosocial circumstances, developmental conditions, genetics and epigenetics, and other environmental factors can all affect the age of the onset of puberty.
[46] These factors can come together and in terms of evolutionary trade offs, alter the allocation of energy into growth, maintenance, or reproduction, as best needed for survival.
[47][49][50][51][52] Chemicals and hormones found in the environment[53] and plastics such as Bisphenol A (BPA)[54] have been thought to affect sexual development in humans at the prenatal or postnatal stage.
Mate choice practices, like many of the topics in human reproductive ecology, vary greatly between individuals and between cultures.
Honest signals guide sexual selection, the process by which certain traits are picked by the potential mate and then proliferate throughout a species.
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 in humans is generally accompanied by selective mate-choice and mating, cohabitation, and bi-parental care for children.
The concept of parental investment defined by Trivers and Willard[55] in the 1970s is used widely in reproductive ecology to analyze and understand provisioning strategies and how they relate to life history trade-offs.
[58] In the field of reproductive ecology, it has been a recent interest to explore the endocrinology of social relationships, including the relation of paternal investment and endocrine function.
Based on kin selection theory, it is usually assumed that mothers have been ancestrally necessary to ensure offsprings survival and reproduction.
Traits in our species that favor cooperative breeding evolve over time due to altruism, and within the context of kin selection and reciprocity.
Lactation is one of the costliest forms of parental investment because it is taxing at a metabolic and physiological level, but also in terms of time and emotion as well.
[16] With breastfeeding, the resumption of normal menses occurs many months later, and the overall effect of lactational amenorrhea is influenced by the intensity of infant suckling.