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.

We examine the relationship between height and reproductive success (RS) in women from a natural fertility population in the Gambia. We observe the predicted trade-off between adult height and age at first birth: women who are tall in adulthood have later first births than short women do. However, tall women have reproductive advantages during the rest of their reproductive careers, primarily in the lower mortality rates of their children. This ultimately leads to higher fitness for taller women, despite their delayed start to reproduction. The higher RS of tall women appears to be entirely due to the physiological benefits of being tall. There is no evidence that female height is related to patterns of marriage or divorce in this population.

No abstract

In this study we examine first use of modern contraception in four Gambian villages over 25 years. This is the first such study showing micro-level change over time from the first availability of this new technological innovation. In 1975, a medical centre was opened in one village providing contraceptive services free of charge to those who wished to use it. We examined determinants of women's age at first use of modern contraceptives, from 1975 or from age 15 if younger than that in 1975. The ideal of large family size remains strong, and those at low parity are significantly less likely to start using contraception than those at high parity for their age. Wealth was also significantly related to the probability of contraceptive use, but negatively, with the wealthiest ranked women being the least likely to adopt the innovation. But we find that the largest effects on the probability of uptake were village and calendar year. Over the last 25 years, there is a doubling time of about 10 years in the risk of progressing to first use of contraception. Villages with strong social ties proceed at a similar rate, whereas one village that had fewer social ties with the others proceeded at a much faster rate. These patterns of uptake suggest that cultural transmission has an important effect on the spread of this technological innovation. We also compare the reproductive success (i.e. completed fertility) of users and non-users, and find that women using contraception actually have higher reproductive success than those that do not. The dynamics of uptake are discussed in the light of both evolutionary and social network models of cultural diffusion.