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The average fecundity per woman varies a lot from country to country. I call average fecundity per woman the average number of born children per woman. In Homo sapiens, what was the average fecundity per woman during the paleolithic age for example? Is it about 2 offspring per woman or about 12 offsprings per woman?
Infant mortality was high during the paleolithic, so the fecundity of females had to be quite a bit higher than 2 to sustain viable populations.
A recent paper by White (2014) that explores how the ratio of young and old individuals in a population relates to demographic rates compiles data on hunter-gatherer societies from previous studies that you will probably find useful. Among other statistics, that paper reports a range in total fertility of 2.6-8.0. One of the source papers (Hewlett, 1991, pdf here) has data for individual hunter-gatherer societies (along with data from other types of pre-industrial societies). Many populations lie in the range 4-7 offspring. Note however that most (all?) of these values are from current or recent hunter-gatherer societies and not paleolithic societies. However, to give a ballpark estimate they are probably useful and fairly accurate. Also note that the child mortality rate of these societies are often between 40-50% (infant mortality ~10-30%)
New research demands rethink on Darwin's theory of 'fecundity selection'
A key concept in Darwin's theory of evolution which suggests nature favours larger females that can produce greater numbers of off-spring must be redefined according to scientists behind new ground-breaking research.
The study, published in the scientific journal Biological Reviews, concludes that the theory of 'fecundity selection' -- one of Charles Darwin's three main evolutionary principles, also known as 'fertility selection' -- should be redefined so that it no longer rests on the idea that more fertile females are more successful in evolutionary terms. The research highlights that too many offspring can have severe implications for mothers and the success of their descendants, and that that males can also affect the evolutionary success of a brood.
Darwin's theory of fecundity selection was postulated in 1874 and, together with the principles of natural selection and sexual selection, remains a fundamental component of modern evolutionary theory. It describes the process of reproductive success among organisms, defined by the number of successful offspring which reach breeding age.
After years of research, an evolutionary biologist from the University of Lincoln, UK, has proposed a revised version of the theory of fecundity selection which recommends an updated definition, adjusts its traditional predictions and incorporates important new biological terms.
The research indicates that rather than aiding survival, too many offspring can be extremely costly, and can in fact reduce the lifetime reproductive success of females. It highlights that in many species, mothers who produce fewer offspring tend to raise them more efficiently, and that in some cases fathers can take the lead in nurturing young by evolving 'male pregnancy'.
The study also concludes that nature will favour all physical traits that influence 'optimal' fertility in either sex, and that climate and food availability also influence the evolution of reproductive processes -- factors which Darwin originally overlooked.
The research, led by Dr Daniel Pincheira-Donoso from the University of Lincoln's School of Life Sciences, reveals that phenomena such as climate change could therefore play a significant role in the fertility of species around the world.
Dr Pincheira-Donoso said: "Evolutionary theory is all about reproductive success, or the number of 'successful' offspring an individual can produce. The more successful offspring, the more genes encoding successful traits are passed on to the next generation.
"However, advances in fecundity selection theory reveal that a higher number of successful descendants can actually result from the production of fewer offspring which can be looked after more efficiently. We therefore need to acknowledge that fertility should be more efficient, not necessarily higher, and that males can have a substantial role in influencing the production of efficient broods.
"Also, a stream of evidence shows that climate and food availability play very important roles in the evolution of fecundity among species. This opens up opportunities for the development of theories involving major natural phenomena, such as rapid changes in the climate. We must explore how these climatic changes can affect the reproductive strategies which evolution has been shaping for thousands or millions of years"
Based on previous studies of the life-history, physical and ecological aspects of fecundity, Dr Pincheira-Donoso's work also concludes that the theory should distinguish between fertility during an animal's lifetime and during one particular breeding season, rather than grouping all time periods together. This is because some animals may have one extremely large brood per breeding season, while others produce one offspring on a more regular basis, which can have enormous implications for the overall reproductive success, and hence evolutionary potential, of species.
Les approches des démographes et des épidémiologistes dans l'étude de la fertilité des couples (cɾst-à-dire leur aptitude à concevoir), longtemps distinctes, se rapprochent maintenant. La démographie s'intéresse traditionnellement surtout à la fécondité, cɾst-à-dire aux circonstances dɺrrivée des naissances vivantes, en particulier pour pouvoir prévoir l'évolution d'une population. Elle sɾst aussi intéressée très tôt à la fertilité, posant des concepts comme la fécondabilité ou la stérilité définitive. La mesure de la fécondabilité repose sur celle du délai nécessaire pour concevoir (DNC), qui est aussi l'outil de base de l'épidémiologiste. Mais, tandis que les démographes développaient des méthodes permettant dɾstimer la distibution de la fécondabilité entre les couples à partir des DNC, les épidémiologistes se sont tournés vers des méthodes permettant dɺnalyser le rôle des divers facteurs pouvant influencer la fertilité au niveau individuel. Pour citer cet article : H. Leridon, C. R. Biologies 330 (2007).
Breast size and asymmetry during pregnancy in dependence of a fetus's sex
Objectives: Breast size and fluctuating asymmetry (FA) are related to women's biological condition, as size correlates positively with fecundity, whereas FA correlates negatively with biological quality. We tested if breast volume, FA, and their changes during pregnancy are related to a fetus's sex. Women with bigger, symmetrical breasts, with a greater increase in size during pregnancy, should be more likely to carry a more ecologically sensitive and energetically demanding male fetus.
Methods: Ninety-three women participated in a 3-stage longitudinal study. 3D breast scans were performed in the first, second, and third trimester of pregnancy. As there was a small variation in pregnancy week at each research stage between the participants, the expected breast volume and FA values for the 12th, 22nd, and 32nd pregnancy week were calculated, basing on the obtained measurements. Those values were compared between mothers who carried a boy and mothers who carried a girl.
Results: Although women who carried a boy had somewhat larger breasts at each trimester than women who carried a girl, the difference was not significant. ANOVA for repeated measurements revealed a greater breast size increase in women carrying a boy (P = 0.039). FA decreased during pregnancy, but was not related to a fetus's sex.
Self-Deception about Fecundity in Women
Concealed fecundity and extended female sexual receptivity have evolved in some primates, including humans, conferring advantages both within primarily monogamous relationships (e.g., benefits from paternity assurance) and from extrapair liaisons (e.g., better access to good genes). As humans evolved the intellectual capacity for decision-making, women became capable of altering their own fertility. In some circumstances, they may choose to ameliorate risks and responsibilities associated with pregnancy by reducing sexual motivation near the perceived most fecund time of their menstrual cycle. But three findings—a general inability of women to accurately recognize their own intervals of fecundity, high variability in ovulation timing, and unconscious transmission and reception of cues associated with fecundity—constitute a physiological and behavioral syndrome that can be considered self-deception. In this study, I develop a descriptive model to determine implications of the hypothesis that these features of female and male physiology and behavior have been shaped by natural selection in response to female decision-making. My analysis shows that consensus motivation for coitus between partners influences both the importance of variable ovulation date and the probability of conception, under the influence of self-deception. It also identifies priorities for future empirical work.
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In conclusion, we constructed a global model of female fecundity using data from 182 countries. Our model captured known environmental and socioeconomic factors that have been implicated in reducing or increasing female fecundity. We included at the country-level measures of average annual temperature, average air pollution, GDP per capita and prevalence of BMI > = 25 for males and females (age-standardized). We also included interaction terms to capture the inherent inter-dependency among many of these factors. We found that GDP per capita was independently associated with fecundity (birth rate). In addition, air pollution (PM 2.5), and BMI were important contributory factors as interaction terms. BMI was found in significant interactions with GDP and air pollution. Annual temperature was only significant in the model that did not include BMI pointing to other mechanisms underlying the temperature to fecundity relationship.
Ethics approval and consent to participate
This study only includes publically available data. No patient-level data was used only aggregate statistics therefore no consent was required to conduct this study.
Human fecundity is an issue of considerable interest for both epidemiological and clinical audiences, and is dependent upon a couple’s biologic capacity for reproduction coupled with behaviors that place a couple at risk for pregnancy. Bayesian hierarchical models have been proposed to better model the conception probabilities by accounting for the acts of intercourse around the day of ovulation, i.e., during the fertile window. These models can be viewed in the framework of a generalized nonlinear model with an exponential link. However, a fixed choice of link function may not always provide the best fit, leading to potentially biased estimates for probability of conception. Motivated by this, we propose a general class of models for fecundity by relaxing the choice of the link function under the generalized nonlinear model framework. We use a sample from the Oxford Conception Study (OCS) to illustrate the utility and fit of this general class of models for estimating human conception. Our findings reinforce the need for attention to be paid to the choice of link function in modeling conception, as it may bias the estimation of conception probabilities. Various properties of the proposed models are examined and a Markov chain Monte Carlo sampling algorithm was developed for implementing the Bayesian computations. The deviance information criterion measure and logarithm of pseudo marginal likelihood are used for guiding the choice of links. The supplemental material section contains technical details of the proof of the theorem stated in the paper, and contains further simulation results and analysis.
Equality and polyamory: why early humans weren't The Flintstones
A study released last week presented evidence that prehistoric men and women lived in relative equality. But is the truth even further from the nuclear narrative?
The “standard narrative of prehistory” presents the idea that, like Fred and Wilma, men have always gone out to hunt/work and women care for home and children. Photograph: Everett Collection/Rex Features
The “standard narrative of prehistory” presents the idea that, like Fred and Wilma, men have always gone out to hunt/work and women care for home and children. Photograph: Everett Collection/Rex Features
Last modified on Wed 14 Feb 2018 21.30 GMT
Last week, scientists from University College London released a paper presenting evidence that men and women in early society lived in relative equality. The paper challenges much of our understanding of human history, a fact not lost on the scientists. Mark Dyble, the study’s lead author, stated “sexual equality is one of the important changes that distinguishes humans. It hasn’t really been highlighted before.”
Despite Dyble’s comments, however, this paper isn’t the first foray into the issue. In fact, it represents another shot fired in a debate between scientific and anthropological communities that has been raging for centuries. It’s a debate that asks some fundamental questions: who are we, and how did we become the society we are today?
Our modern picture of prehistoric societies, or what we can call the “standard narrative of prehistory” looks a lot like The Flintstones. The narrative goes that we have always lived in nuclear families. Men have always gone out to work or hunt, while women stayed at home to look after the house and the children. The nuclear family and the patriarchy are as old as society itself.
The narrative is multifaceted, but has strong roots in biological science, which can probably be traced back to Charles Darwin’s theory of sexual selection. Darwin’s premise was that due to their need to carry and nurture a child women have a greater investment in offspring than men. Women are therefore significantly more hesitant to participate in sexual activity, creating conflicting sexual agendas between the two genders.
This creates a rather awkward situation. With women producing such “unusually helpless and dependent offspring”, they require a mate who not only has good genes, but is able to provide goods and services (i.e. shelter, meat and protection) to the woman and her child. However, men are unwilling to provide women with the support they require unless they have certainty the children are theirs — otherwise they are providing support to the genes of another man. In turn men demand fidelity an assurance their genetic line is being maintained.
Helen Fisher calls this ‘The Sex Contract’, but the authors of Sex at Dawn, Christopher Ryan and Cacilda Jethá, are a little more cutting in their analysis: “the standard narrative of heterosexual interaction boils down to prostitution: a woman exchanges her sexual services for access to resources … Darwin says your mother’s a whore. Simple as that.”
Herein, so some scientists say, lie the roots of our nuclear family and the patriarchy. Our gendered hierarchy is based on an innate biological need for women to be supported by men. The very capacity for women to give birth to children places them in a lower position within society.
Scientists use a whole range of other evidence to support this narrative. Many for example point our closest relatives. Scientists have researched monogamy of gibbons and the sexual hierarchies of chimpanzees to point to a “natural” expression of our innate desires.
Other scientists use human biology. A common example is women’s apparently weak libido. Discussing his book Why Can’t a Woman be More Like a Man? released last year, for example, Lewis Wolpert states: “About half of men think about sex every day or several times a day, which fits with my own experience, while only 20 per cent of women think about sex equally often. Men are far more likely to be sexually promiscuous, a throwback to evolution where procreation was all-important.”
If you subscribe to the theory of a sex contract this is logical. A lower sex drive ensures women are more selective in their sexual decisions, making certain that they only mate with high-quality men. Women, so some scientists say, are evolutionarily designed to be selective in their mates.
Yet, for centuries many have questioned the logic, and the biology, of the standard narrative.
The first real splash in this arena came from the anthropologist Lewis Morgan, and his book Ancient Society. In the book Morgan presented the results of his study of the Iroquois, a Native American hunter-gatherer society in upstate New York. The Iroquois, Morgan observed, lived in large family units based on polyamorous relationships, in which men and women lived in general equality.
Morgan’s work hit a broader audience when it was taken up by Friedrich Engels (most famous for being the co-author of the The Communist Manifesto) in his book The Origin of Family, Private Property and the State. Engels drew on Morgan’s data, as well as evidence from around the world to argue that prehistoric societies lived in what he called “primitive communism”. Other anthropologists now call this “fierce egalitarianism”: societies where families were based on polyamory and in which people lived in active equality (i.e. equality is enforced).
Morgan and Engels were not painting a picture of a “noble savage”. Humans were not egalitarian nor polyamorous because of their social conscience, but because of need. Hunter-gather societies were based largely on small roaming clans where men engaged in hunting, while women’s roles focused around gathering roots, fruit and berries, as well as looking after the “home”. In these societies community was everything. People survived through the support of their clan and therefore sharing and working within their clan was essential. This crossed over into sex as well.
Polyamory helped foster strong networks, where it became everyone’s responsibility to look after children. As Christopher Ryan states: “These overlapping, intersecting sexual relationships strengthened group cohesion and could offer a measure of security in an uncertain world.” The same can be said for our other social hierarchies. As Jared Diamond explains, with no ability or need to store or hoard resources, “there can be no kings, no class of social parasites who grow fat on food seized from others”. Hunting and gathering enforced social equality. It was the only way people could survive.
While initially developed in the 1800s, these theories died down somewhat in the early 20th century. With Engels’ connection to Marx, many of these ideas were lost in the great philosophical debate of the Cold War. Many second wave feminists, led primarily by Simone de Beauvoir in her book The Second Sex, also argued against Engels’ ideas.
Recently however, these theories have had something of a renaissance. On top of Dyble’s study last week, new anthropological and scientific evidence backs up this challenge to the standard narrative. In 2012 Katherine Starkweather and Raymond Hames conducted a survey of examples on ‘non-classical polyandry’, discovering the phenomenon existed in many more societies than previously thought.
In another example Stephen Beckman and Paul Valentine examined the phenomenon of ‘partible paternity’ in tribes in South America: the belief that babies are made up from the culmination of the spermatozoa of multiple males. This belief, which is common in tribes in the Amazon requires polyamorous sexual activity by women, and that men share the load of supporting children.
And then there is the example of the Mosua in China, a society in which people are highly promiscuous and where there is no shame associated with this. Mosua women have a high level of authority, with children being looked after by a child’s mother and her relatives. Fathers have no role in the upbringing of a child — in fact the Mosua have no word to express the concept of “father”.
In Sex at Dawn, released in 2010, Ryan and Jethá provided a range of biological evidence to back up this anthropological data. Let’s take a look at their counteractions to the two examples produced earlier: the behaviour of our closest relatives and women’s apparently low libido.
Ryan and Jethá argue that while yes, gibbons and chimpanzees are close relatives, our closest relatives are in fact bonobos. Bonobos live in female-centered societies, where war is rare and sex serves an important social function. They are polyamorous, with both male and female apes having regular sex with multiple partners. This looks more like the societies Morgan and Engels were describing.
When it comes to women’s “low libido”, Ryan and Jethá simply disagree, arguing in fact that women have evolved for sex with multiple partners. They look, for example, at women’s ability to have multiple orgasms in a sexual session, to have sex at any time during their menstrual cycle and their propensity to make a lot of noise during sex — which they argue is a prehistoric mating call to encourage more men to come and join in. These evolutionary traits have occurred, they argue, to ensure breeding is successful.
In short, Dyble’s paper is unlikely to provide the conclusion to a battle that has been raging for at least two centuries. The paper, however, certainly is another nail in the coffin of the standard narrative of prehistory. One this seems clear: our history is much more complex than previously thought. How complex, we may never know. Without a time machine it is impossible to confirm. But we now can be certain that things in the past were very different to the standard narrative. We are not all just versions of the modern stone age family.
Mean values, ranges, and standard deviations of facial attractiveness assessments, AMH, E2, tT, and potential covariates (age, age at menarche, and BMI) are presented in Table 1. We found no relationship between age at menarche and AMH level (r = -0.01 p = 0.87 95% CI [-0.15 0.13]) or facial attractiveness (r = 0.04 p = 0.55 95%CI [-0.10 0.18]), thus we did not control for age at menarche in the analyses. We found no correlation between BMI and AMH level (r = 0.02 p = 0.78 95%CI [-0.12 0.16]) and a negative correlation between BMI and E2 level (r = -0.17 p = 0.02 95%CI [-0.31 -0.02]) and positive with tT (r = 0.12 p = 0.06 95%CI [-0.02 0.26]). BMI was also negatively correlated with facial attractiveness (r = -0.31 p < 0.001 95%CI [-0.43 -0.17]). AMH was negatively related with E2 level (r = -0.15, p = 0.04 95% CI [-0.29 -0.01]).