Heritability of age at first birth in captive olive baboons

The evolution of life history traits is a topic of growing interest in primatology. Traits associated with fertility, such as age at menarche and age at first birth, have great significance for natural selection, and knowing the genetic basis of such demographic traits may improve our understanding of population dynamics. Knowledge of the heritability of reproductive traits may also have practical implications for the management of captive breeding colonies. A maximum likelihood method was used to estimate heritability of age at first birth for a sample of female olive baboons resident at the Southwest Foundation for Biomedical Research. Only animals born at the Foundation that were caged in mixed sex groups and that had previously given birth were included in the sample (n = 316). There were 117 independent individuals and 199 individuals in 35 pedigrees composed of 2 to 26 members. Age at first birth ranged from 3.85 years to 13.11 years. Age at first birth is highly heritable (h² = 0.87) with no evidence for maternal effects or a dominance genetic component. This level of genetic variability in a fitness-related trait is contrary to evolutionary expectation and may reflect the uniform environment of a captive breeding situation. Thus, the heritability observed in this population may be taken as an upper bound for that in natural populations. © 1995 Wiley-Liss, Inc.     

An evolutionary model of stature, age at first birth and reproductive success in Gambian women

We have built a model to predict optimal age at first birth for women in a natural fertility population. The only existing fully evolutionary model, based on Ache hunter-gatherers, argues that as women gain weight, their fertility (rate of giving birth) increases-thus age at first birth represents a trade-off between time allocated to weight gain and greater fertility when mature. We identify the life-history implications of female age at first birth in a Gambian population, using uniquely detailed longitudinal data collected from 1950 to date. We use height rather than weight as an indicator of growth as it is more strongly correlated with age at first birth. Stature does not greatly influence fertility in this population but has a significant effect on offspring mortality. We model age at first reproduction as a trade-off between the time spent growing and reduced infant mortality after maturation. Parameters derived from this population are fitted to show that the predicted optimal mean age of first birth, which maximizes reproductive success, is 18 years, very close to that observed. The reaction norm associated with variation in growth rate during childhood also satisfactorily predicts the variation in age at first birth.     

Evolutionarily stable age at first reproduction in a density-dependent model.

We develop a new model of life history evolution to investigate the evolution of age at first reproduction. Density dependence is taken into account. For a given "species", age of maturity, offspring survival, immature survival, adult survival, fecundity, immature age-classes entering in competition with adults and immature competitive ability are traits adjustable by natural selection, and constitute a particular strategy. On the contrary, the type of intraspecific competition (scramble or contest), strength of competition and inherent net reproductive rate Ro(inh) are fixed (specific) characteristics. As a consequence of fixing Ro(inh), the evolution of any trait will affect trade-offs between others. Evolutionarily stable strategies are determined numerically by using the mathematical concept of Lyapunov exponents. Altogether, we consider 960 different hypothetical "species" (i.e. different combinations of fixed traits). Corresponding ESSs are analyzed with respect to their age at first reproduction, adult survival and immature competitive ability components. They appear to be gathered in three groups. One is intuitive and characterized by a reduction of immature competitive ability and a correlation of age of maturity with adult survival; populations reach mainly equilibria. The two other groups respectively include "species" with low age of maturity but high adult survival, and "species" close to semelparity with delayed maturity; immature competitive ability may not be minimized, and populations possibly exhibit complex dynamics. In conclusion, the hypothesis that the evolution of a demographic parameter modifies trade-offs between others turns out to have important consequences. We argue that life history theory cannot ignore the source and mode-of-operation of density dependence and must regard potential short-term instability as essential.     

Analysing cross-sectional data with time-dependent covariates: the case of age at first birth in South Africa

Analysing time-dependent independent variables requires the use of process-oriented statistical models. Yet social scientists--especially those in poor countries--have often had to use data collected at a single point in time, making their task difficult. Making several assumptions about the covariates, the present study uses survival analysis and other statistical techniques to analyse the 1996 South African population census data and examine the effects of selected independent variables on the timing of parenthood in the country. It was found that the onset of parenthood occurs late in South Africa compared with the pattern in most other African societies. While education plays a role in the postponement of parenthood within racial groups, it fails to explain the differences between African and Coloured women on the one hand, and White and Asian women on the other hand, a finding that suggests the existence of two regimes of family formation in South African society.     

Cross-country relationships between life expectancy,intertemporal choice and age at first birth

Humans, like other animals, typically discount the value of delayed rewards relative to those available in the present. From an evolutionary perspective, prioritising immediate rewards is a predictable response to high local mortality rates, as is an acceleration of reproductive scheduling. In a sample of 46 countries, we explored the cross-country relationships between average life expectancy, intertemporal choice, and women's age at first birth. We find that, across countries, lower life expectancy is associated with both a smaller percentage of people willing to wait for a larger but delayed reward, as well as a younger age at first birth. These results, which hold when controlling for region and economic pressure (GDP-per capita), dovetail with findings at the individual level to suggest that life expectancy is an important ecological predictor of both intertemporal and reproductive decision-making.     

Factors affecting reproduction in rehabilitant female orangutans: young age at first birth and short inter-birth interval

This study investigated the reproductive parameters of free-ranging rehabilitant female orangutans. We aimed to assess the factors that influence these parameters and provide information that could assist with the management of orangutan reintroduction programs. We analyzed the birth records of free-ranging female rehabilitants at Bukit Lawang, Bukit Tigapuluh, Sepilok, Camp Leakey, Kaja Island, Sungai Wain, and Meratus and compared them with reproductive parameters reported in wild and zoo populations. Females’ ages at first birth were 10.6–14.7 years, significantly earlier than those of wild and zoo orangutans. Computed inter-birth intervals (IBIs) calculated by the Kaplan–Meier method were 65.1–90.1 months; the values for Camp Leakey and Bukit Lawang rehabilitants were significantly shorter than those reported for wild Sumatran orangutans. Infant mortality rates were 18–61%; the values for Bukit Lawang and Sepilok were significantly higher than those reported for wild Sumatran and zoo orangutans. In rehabilitants, young ages at first birth and shorter IBIs may result from the high energy intake enabled by provisioning, although the possibility exists that they reflect underestimations of age on arrival at rehabilitation centers. The observed high infant mortality rate may reflect poor mothering skills due to human rearing and/or increased disease transmission. This study demonstrates that accelerated reproductive rates (younger age at first birth and shorter IBI) are common in provisioned rehabilitant females on both Sumatra and Borneo.     

Infertility and age at first coitus: a possible relationship

Reported age at first coitus was shown to be associated with incidence and degree of infertility. Infertility patients reported later first coital ages than gynecological patients in a sample of 792 women seen in a major fertility service. Among the infertility patients, those who had never conceived presented a later first coital age than those who had previously conceived. Routine gynecological patients reported the youngest first coital age of the groups studied.     

Maternal age at last birth in Egypt

To assess the effects of women's education, residence, and marital experience on their age at the birth of their last child, a proportional hazards regression model was applied to 1980 Egyptian Fertility Survey (EFS) data. The detailed data include the date of birth of each child for every women interviewed, and the woman's date of birth and age at interview. Age at last birth was examined by regression analysis on birth history and socioeconomic information. 4 hypotheses were tested: women who are well educated have a greater probability of ending childbearing earlier than women with less education; women in rural areas have a higher probability of having their last child at older ages than urban women; marital disruption without remarriage lowers the probability of older maternal age at last birth; and marital disruption with remarriage increases the probability that a woman stops reproducing at an older age. The overall chi-square indicates a significant regression. All coefficients were significant, except the coefficient for women with intact 1st marriages. Women with more education had a greater probability of ending childbearing earlier than women with less education. Rural women tended to have their last children at ages significantly older than overall age at last birth. Current residence in urban areas had the opposite effect. The coefficient for those with intact 1st marriages was insignificant, meaning that the mean age at last birth for this group of women was not much different from the overall mean. Remarried women tended to end childbearing at ages significantly older than the overall average age at last birth, suggesting that these women tended to have children by their new husbands. Those with dissolved 1st marriages who had not remarried had a higher probability of ending childbearing earlier than did older women. Marriage age and final parity had highly significant negative coefficients; as marriage age and number of children born increased, so did the "survival" time or the age at last birth. Results from the hazards model indicate that the effects were as anticipated. The median age at last birth for the total sample of women aged 45-49 was 45-49 years. The median age at last birth was about 2 years older for rural compared to urban women. Illiterate women had the oldest median age at last birth of the education groups. There was little differences between median ages at last birth for women with intact 1st marriages and those whose 1st unions were dissolved and who had remarried. The median age at last birth increased with final parity.     

Age at first birth and family size: Evidence from a longitudinal study

Abstract

This paper examines the correlates of the relationship of age at first birth to completed parity, using data from a fifteen‐year longitudinal study. Particular emphasis is given to measures of orientation toward family roles which have been singled out in previous studies as a likely causal factor not yet examined. Women who had an early first birth went on to have larger families than women who postponed childbearing longer. Demographic factors, including premarital pregnancies, unwanted births, and fecundity differentials, did not appear to account for the observed relationship. The hypothesis that early socialization toward traditional female roles might account for the higher fertility of women who began childbearing early was not confirmed. Women with a first birth at a young age were not characterized by more traditional sex‐role behavior or attitudes nor did they express higher initial fertility preferences. They did more often increase their fertility preferences over the inter‐survey period.     

Early stress predicts age at menarche and first birth,adult attachment,and expected lifespan

Life history theory suggests that in risky and uncertain environments the optimal reproductive strategy is to reproduce early in order to maximize the probability of leaving any descendants at all. The fact that early menarche facilitates early reproduction provides an adaptationist rationale for our first two hypotheses: that women who experience more risky and uncertain environments early in life would have (1) earlier menarche and (2) earlier first births than women who experience less stress at an early age. Attachment theory and research provide the rationale for our second two hypotheses: that the subjective early experience of risky and uncertain environments (insecurity) is (3) part of an evolved mechanism for entraining alternative reproductive strategies contingent on environmental risk and uncertainty and (4) reflected in expected lifespan. Evidence from our pilot study of 100 women attending antenatal clinics at a large metropolitan hospital is consistent with all four hypotheses: Women reporting more troubled family relations early in life had earlier menarche, earlier first birth, were more likely to identify with insecure adult attachment styles, and expected shorter lifespans. Multivariate analyses show that early stress directly affected age at menarche and first birth, affected adult attachment in interaction with expected lifespan, but had no effect on expected lifespan, where its original effect was taken over by interactions between age at menarche and adult attachment as well as age at first birth and adult attachment. We discuss our results in terms of the need to combine evolutionary and developmental perspectives and the relation between early stress in general and father absence in particular. This work was supported by The University of Melbourne Department of Obstetrics and Gynaecology. James S. Chisholm is Professor in the School of Anatomy and Human Biology at the University of Western Australia. He is an anthropologist whose interests lie in the fields of human behavioral biology, evolutionary ecology, life history theory, and parental investment theory, where he focuses on infant social-emotional development, the development of reproductive strategies, and the integration of evolutionary, developmental, and cultural psychology and public health. Julie A. Quinlivan is Associate Professor in Obstetrics and Gynaecology at the University of Melbourne and Head of the Maternity Care Program at the Royal Women’s Hospital, Melbourne. Her interests are teenage pregnancy, domestic violence, child abuse prevention, and high-risk pregnancy. Rodney W. Petersen is Senior Lecturer in Obstetrics and Gynaecology at the University of Melbourne and Senior Consultant in Obstetrics and Gynaecology at the Royal Women’s Hospital and Sunshine Hospital in Melbourne. His interests are in psychosocial aspects of women’s health and cancer. David A. Coall is a Ph.D. student in the School of Anatomy and Human Biology at the University of Western Australia. His main interest lies in the application of evolutionary theory within an epidemiological framework. He is currently working on the synthesis of life history theory, parental investment theory, and parent-offspring conflict theory in exploring factors that influence variation in human birth weight and placental weight.     

Size at birth,maternal nutritional status in pregnancy,and blood pressure at age 17: population based analysis.

OBJECTIVE: To assess the effect of size at birth, maternal nutrition, and body mass index on blood pressure in late adolescence. DESIGN: Population based analysis of birth weight corrected for gestational age, mother''s weight before pregnancy and weight gain in pregnancy, obtained from the Jerusalem perinatal study, and blood pressure and body mass index at age 17, available from military draft records. SETTING: Jerusalem, Israel. SUBJECTS: 10,883 subjects (6684 men and 4199 women) born in Jerusalem during 1974-6 and subsequently drafted to the army. MAIN OUTCOME MEASURES: Systolic and diastolic blood pressures measured at age 17 and their correlation with birth weight, size at birth, mother''s body mass index and weight gain during pregnancy, and height and weight at age 17. RESULTS: Systolic and diastolic blood pressures were significantly and positively correlated with body weight, height, body mass index at age 17, and with mother''s body weight and body mass index before pregnancy, but not with birth weight or mother''s weight gain in pregnancy. CONCLUSION: Variables reflecting poor intrauterine nutrition, including low maternal body mass index before pregnancy, poor maternal weight gain in pregnancy, and being born small for gestational age, were not associated with a higher blood pressure in late adolescence.     

Socioeconomic determinants of age at first marriage in Bangladesh

Using data from the 1976 Bangladesh Fertility Survey, multiple classification analysis was used to evaluate the effect of socioeconomic factors on age at 1st marriage. The independent variables considered were education, childhood and current residence, religion, work status before marriage, and husband's childhood residence, education, and occupation. Analysis was carried out for the total sample as well as for 3 birth cohorts of approximately equal size: 1) those born before 1940, 2) those born between 1940-50, and 3) those born after 1950. Of all the included variables, women's education has the strongest influence on the variation of age at 1st marriage. For all ever-married women, the mean age at marriage for women with primary education is 13.4 years, 0.9 years higher than for women with no education (12.5 years), and 1.2 years lower than for women with a high school education or beyond (14.6 years). Difference in means for cohorts indicate a gradually increasing influence of education on people's decision in marriage. Husband's education does not appear to be as important. Childhood residence has, directly and indirectly, a strong influence in marriage age. Among other factors, women's premarital work participation, as well as region and husband's occupation, are important. Since women's education, childhood residence, and work participation are the strongest socioeconomic variables affecting marriage age, the modernizing influences of education, urbanization, and female work participation should have an effect on the marriage pattern; this effect is consistent with that observed in other societies.