Life History Framework
Life history theory is a useful framework to consider the optimal age to initiate childbearing under different circumstances. Women who start having children too young may compromise their own growth and potentially jeopardize their health as well as fetal development and offspring survival. Women who delay childbearing too long have shorter reproductive careers and potentially reduce the total number of offspring produced in a lifetime. As such, the timing of first birth represents a trade-off between the costs and benefits associated with current versus future reproduction (Clutton-Brock 1988; Stearns 1989). Age at first birth is influenced by extrinsic mortality (Charnov 1992; Ernande et al. 2004; Gadgil and Bossert 1970). In high-mortality environments, females may initiate reproduction early rather than risk not surviving to a later age. In contrast, a female is more likely to delay reproduction in low-mortality environments, especially if growing for longer periods and having a larger body size is beneficial. Empirical studies across a number of taxa confirm that age at first birth is sensitive to background mortality rates. For example, studies have shown that Northern cod start to mature at earlier ages and smaller body sizes in response to increased subadult mortality resulting from fishing practices (Olsen et al. 2004). Among adult Tasmanian devils, the development of a widespread fatal cancer has led to a dramatic increase in the proportion of early-maturing individuals (Jones et al. 2008). In humans, several studies indicate that girls growing up in high-mortality environments with low life expectancy tend to mature more quickly than girls growing up in safe, low-mortality environments (Chisholm 1999; Cooper et al. 1996; Ellis et al. 2009; Hill and Kaplan 1999; Ibanez et al. 2000; Kramer et al. 2009; Lancaster et al. 2000; Walker et al. 2006). For example, the Hiwi and Pume, two groups of foragers who live in a high-subadult mortality environment in western Venezuela, grow quickly during the juvenile period and reach menarche at a relatively young age: 12.6 years on average for the Hiwi (Walker et al. 2006) and 12.9 years for the Pume (Kramer et al. 2009). Thinking about age at first birth as a life history trade-off between current and future reproduction offers insight into how teen motherhood varies in relation to different socio-ecological conditions. The Pume foragers of South America are an ideal population to use a life history framework to investigate the biological costs and benefits associated with teen motherhood. Women in Pume society have no access to education or health care and are not stratified by variation in food availability or other social factors known to affect birth outcomes in developed societies. Furthermore, teenage marriage and pregnancy are not stigmatized or associated with negative social consequences. These factors allow the fitness consequences of teen motherhood to be more clearly observed and evaluated. As in many hunter-gatherer societies, Pume women marry young and usually initiate conjugal relations soon after reaching menarche, with 95 % of girls married by age 15 (Kramer 2008). Given their menarcheal and marriage pattern, the Pume not surprisingly have a relatively high teen birth rate (defined as the number of births to adolescent girls ages 15-19 per 1000 adolescent girls ages 15-19) of 195 births per 1000 teens ages 15-19. This compares to 110 births per 1000 teens for Bangladesh, which has one of the highest national teen birth rates, 90 for Venezuela (Population Reference Bureau 2007), and 23.6 for the USA in 2013 (U.S. Department of Health and Human Services 2015).
From a life history perspective, evaluating the adaptive value of teen motherhood for the Pume rests on addressing two central questions: (1) Does the risk of infant mortality vary with maternal age at first birth? (2) What are the lifetime fitness consequences of teen motherhood? In particular, if pregnancy at a young age poses significant consequences for lifetime reproductive success, then older first?time Pume mothers should have lower infant mortality and/or higher lifetime fertility than younger first-time Pume mothers.