Ilona Nenko

Community-level education accelerates the cultural evolution of fertility decline

Explaining why fertility declines as populations modernize is a profound theoretical challenge. It remains unclear whether the fundamental drivers are economic or cultural in nature. Cultural evolutionary theory suggests that community-level characteristics, for example average education, can alter how low-fertility preferences are transmitted and adopted. These assumptions have not been empirically tested. Here, we show that community-level education accelerates fertility decline in a way that is neither predicted by individual characteristics, nor by the level of economic modernization in a population. In 22 high-fertility communities in Poland, fertility converged on a smaller family size as average education in the community increased—indeed community-level education had a larger impact on fertility decline than did individual education. This convergence was not driven by educational levels being more homogeneous, but by less educated women having fewer children than expected, and more highly educated social networks, when living among more highly educated neighbours. The average level of education in a community may influence the social partners women interact with, both within and beyond their immediate social environments, altering the reproductive norms they are exposed to. Given a critical mass of highly educated women, less educated neighbours may adopt their reproductive behaviour, accelerating the pace of demographic transition. Individual characteristics alone cannot capture these dynamics and studies relying solely on them may systematically underestimate the importance of cultural transmission in driving fertility declines. Our results are inconsistent with a purely individualistic, rational-actor model of fertility decline and suggest that optimization of reproduction is partly driven by cultural dynamics beyond the individual.

Relationships between biomarkers of inflammation, ovarian steroids, and age at menarche in a rural polish sample

To test the hypothesis that life history trade‐offs between maintenance and reproductive effort would be evident through inverse associations between levels of a biomarker of inflammation [C‐reactive protein (CRP)], and ovarian hormones. Associations between CRP and age at menarche were also explored.

CYP17 genotypes differ in salivary 17-β estradiol levels: A study based on hormonal profiles from entire menstrual cycles

Variation in the levels of sex-steroid hormones results from differences in developmental conditions, adult lifestyle, and genetic polymorphism. Genes involved in sex-steroid biosynthesis have been implicated to influence levels of hormones in premenopausal women, but the results were inconclusive. We tested variation among women in levels of salivary estradiol (E2) corresponding to CYP17 genotypes. CYP17 encodes cytochrome P450c17α, which mediates two enzymes important in E2 synthesis. In contrast to the earlier studies that relied on one or a few samples for assessing the E2 levels of an individual woman, our study is based on daily collected saliva samples for one entire menstrual cycle. Sixty Polish women, ages 24 to 36 years, with regular menstrual cycles and no reported fertility problems participated in the study. Women with A2/A2 genotype had 54% higher mean E2 levels than women with A1/A1 genotype (P = 0.0001) and 37% higher than women with A1/A2 genotype (P = 0.0008). Heterozygous A1/A2 women had 13 % higher E2 levels than homozygous A1/A1 women (but this difference was significant only in a nonparametric test). Levels of E2 during the day with highest E2 (day −1) were 72% higher in A2/A2 compared with A1/A1 (P = 0.01) and 52 % higher compared with A1/A2 (P = 0.03). Our results suggest that CYP17 genotype may serve as a biomarker of endocrine function in women of reproductive age. (Cancer Epidemiol Biomarkers Prev 2006;15(11):2131–5)

Digit ratio (2D:4D) as an indicator of body size, testosterone concentration and number of children in human males

Abstract Objectives: The 2nd to 4th digit ratio (2D:4D) is thought to reflect exposure to androgens during foetal development. This study examined the relationship between low (more masculine) and high (more feminine) 2D:4D and body size at different stages of the life course, adult testosterone levels and number of children among males. Methods: Five hundred and fifty-eight men from rural Poland at the Mogielica Human Ecology Study Site participated in this study. Life history data and anthropometric measurements were collected. Salivary morning and evening testosterone levels among 110 men from the same population were measured. Results: Low 2D:4D was related to higher birth weight (p = 0.04), higher birth length (p = 0.01), higher body mass during childhood and adolescence (p = 0.01), higher BMI (borderline significance, p = 0.06), higher number of children among fathers (p = 0.04) and higher testosterone levels during adulthood (p = 0.04). Conclusions: This study shows, for the first time in a single population, that digit ratio is related to sub-adult body size at different stages of the life course, adult testosterone levels and number of children. The observed results suggest that digit ratio might be a valuable predictor of male body size and reproductive characteristics.

Fertility, body size, and shape: An empirical test of the covert maternal depletion hypothesis

In populations with limited resources, high‐reproductive effort may lead to poor nutritional status of the mother (the maternal depletion syndrome), whereas in well‐nourished populations womans body weight tends to increase after each pregnancy. However, in affluent populations, womens body shape may change due to mobilization of polyunsaturated fatty acids (PUFAs) from the lower parts of their bodies to meet the needs of the developing child (the “covert maternal depletion”). We studied relationships between reproductive history traits and body size and shape for 296 rural, parous women in good nutritional status (mean body mass index, BMI = 27.9, SD = 5.94), aged 22–85 (mean 47.8, SD = 16.34) from southern Poland. Body mass adjusted for age, age of menarche, body height, and similarly adjusted BMI were each positively related to the number of children born by a woman (R = 0.13, P = 0.02 and R = 0.13, P = 0.02, respectively). Waist and hip circumferences, adjusted for confounders, did not show statistically significant relationships with the number of children. Moreover, groups with low and high parity did not significantly differ in hip/BMI and waist/BMI ratios, which were proposed to be indicators of covert form of maternal depletion (after controlling for overall body fatness and age). In conclusion, parity caused a slightly higher body mass and BMI later in life. However, parity did not lead to covert maternal depletion, perhaps because women in this population have relatively high‐dietary intake of PUFAs. Am. J. Hum. Biol., 2009.

Apolipoprotein E (ApoE) polymorphism is related to differences in potential fertility in women: a case of antagonistic pleiotropy?

The alleles that are detrimental to health, especially in older age, are thought to persist in populations because they also confer some benefits for individuals (through antagonistic pleiotropy). The ApoE4 allele at the ApoE locus, encoding apolipoprotein E (ApoE), significantly increases risk of poor health, and yet it is present in many populations at relatively high frequencies. Why has it not been replaced by natural selection with the health-beneficial ApoE3 allele? ApoE is a major supplier of cholesterol precursor for the production of ovarian oestrogen and progesterone, thus ApoE has been suggested as the potential candidate gene that may cause variation in reproductive performance. Our results support this hypothesis showing that in 117 regularly menstruating women those with genotypes with at least one ApoE4 allele had significantly higher levels of mean luteal progesterone (144.21 pmol l−1) than women with genotypes without ApoE4 (120.49 pmol l−1), which indicates higher potential fertility. The hormonal profiles were based on daily data for entire menstrual cycles. We suggest that the finding of higher progesterone in women with ApoE4 allele could provide first strong evidence for an evolutionary mechanism of maintaining the ancestral and health-worsening ApoE4 allele in human populations.

Early-life reproduction is associated with increased mortality risk but enhanced lifetime fitness in pre-industrial humans

The physiology of reproductive senescence in women is well understood, but the drivers of variation in senescence rates are less so. Evolutionary theory predicts that early-life investment in reproduction should be favoured by selection at the cost of reduced survival and faster reproductive senescence. We tested this hypothesis using data collected from preindustrial Finnish church records. Reproductive success increased up to age 25 and was relatively stable until a decline from age 41. Women with higher early-life fecundity (ELF; producing more children before age 25) subsequently had higher mortality risk, but high ELF was not associated with accelerated senescence in annual breeding success. However, women with higher ELF experienced faster senescence in offspring survival. Despite these apparent costs, ELF was under positive selection: individuals with higher ELF had higher lifetime reproductive success. These results are consistent with previous observations in both humans and wild vertebrates that more births and earlier onset of reproduction are associated with reduced survival, and with evolutionary theory predicting trade-offs between early reproduction and later-life survival. The results are particularly significant given recent increases in maternal ages in many societies and the potential consequences for offspring health and fitness.

Why do women stop reproducing before menopause? A life-history approach to age at last birth

Evolutionary biologists have long considered menopause to be a fundamental puzzle in understanding human fertility behaviour, as post-menopausal women are no longer physiologically capable of direct reproduction. Menopause typically occurs between 45 and 55 years of age, but across cultures and history, women often stop reproducing many years before menopause. Unlike age at first reproduction or even birth spacing, a woman nearing the end of her reproductive cycle is able to reflect upon the offspring she already has—their numbers and phenotypic qualities, including sexes. This paper reviews demographic data on age at last birth both across and within societies, and also presents a case study of age at last birth in rural Bangladeshi women. In this Bangladeshi sample, age at last birth preceded age at menopause by an average of 11 years, with marked variation around that mean, even during a period of high fertility. Moreover, age at last birth was not strongly related to age at menopause. Our literature review and case study provide evidence that stopping behaviour needs to be more closely examined as an important part of human reproductive strategies and life-history theory. Menopause may be a final marker of permanent reproductive cessation, but it is only one piece of the evolutionary puzzle.

Women with more feminine digit ratio (2D:4D) have higher reproductive success

OBJECTIVES Prenatal development has a long-lasting influence on biological condition and health. Second-to-fourth digit ratio (2D:4D) is suggested as an indicator of sex hormone exposure during fetal development, and it is likely that women with a more feminine digit ratio were exposed to higher prenatal estrogen levels. Therefore, we tested if 2D:4D was related to a womans reproductive characteristics. METHODS We studied 319 women aged 46-92, who went through a natural menopause and whose husbands were alive at least until their menopause were studied. Women were recruited at the Mogielica Human Ecology Study Site located in rural Poland. RESULTS Women with more feminine 2D:4D had a higher number of children (P = 0.002), gave birth to their last child at a later age (P = 0.02), and had a longer reproductive lifespan (P = 0.04) than women with more masculine 2D:4D. Age and number of years of education were included as potential confounders in the analyses. CONCLUSIONS The results indicated that women with more feminine 2D:4D had higher reproductive success. While the exact mechanisms were not known, and the relationship between 2D:4D and adult sex hormone levels was questioned by previous studies, there might be other biological pathways explaining the observed results, that is, via behaviors that were indirectly related to fertility. Am J Phys Anthropol 160:549-553, 2016.

First birth interval, an indicator of energetic status, is a predictor of lifetime reproductive strategy.

Women should differ in their reproductive strategies according to their nutritional status. We tested a hypothesis that women who have a good nutritional status early in life, as indicated by a shorter waiting time to the first birth (first birth interval, FBI), are able to afford higher costs of reproduction than women who have worse nutritional condition.