Introduction to special issue on “Trade‐Offs in Female Life Histories: Integrating Evolutionary Frameworks”

Trade-offs form a cornerstone of life history theory (Roff and Fairbairn, 2007; Stearns, 1992). Assuming a limited energy budget, individuals must trade-off energetically expensive functions such as growth, somatic maintenance, and reproduction, to maximize their fitness. An understanding of trade-offs is, therefore, vital in explaining the shape of human life histories, and how life histories vary between individuals and between populations (Hill, 1993; Hill and Kaplan, 1999). This special issue is based on a workshop which brought together researchers working on a range of different female life history trade-offs, across a range of populations, including historical, contemporary developed, and contemporary developing, and from a range of perspectives, including anthropology, reproductive ecology, behavioral ecology, and demography. It follows on from a similar interdisciplinary workshop on male life histories, which also brought together apparently disparate work, unified under the common theoretical framework of evolutionary life history theory (Mueller and Bogerhoff Mulder, 2006). A key theme to emerge from the workshop was the importance of sociality in understanding human life history trade-offs. Many life history models are based on the assumption that the only energy available to individuals is that acquired by the individual’s own efforts. However, the cooperative nature of human child-rearing means that human females can rely on the energetic contributions of other individuals to offset some of the costs of reproducing (Hrdy, 2005). Direct food transfers may come from the fathers of children, other men, or other female kin less burdened with children (Hawkes et al., 1989; Hill et al., 1985). Kin may also contribute by providing childcare or other labor necessary for subsistence or domestic tasks (Kramer, 2005). A number of papers in this volume address the implications of this ‘‘pooled energy budget’’ for female life history trade-offs. Reiches et al. open the issue by introducing their concept of the ‘‘pooled energy budget’’ and how it affects the investigation of human female life histories (Reiches et al., 2009). This approach blends reproductive ecology and behavioral ecology, by incorporating aspects of both the physiological focus of reproductive ecology and the social framework of cooperative breeding. Kramer et al. then apply this concept to one of the key trade-offs in life history, that between growth and reproduction (Kramer et al., 2009). In a species such as our own, growth and reproduction are largely mutually exclusive, so that individuals have a difficult decision to make: when to stop growing and start reproducing. Kramer et al.’s analysis of a South American forager population, the Pumé, suggests that pooling energy budgets buffers girls against environmental stresses, and allows them to mature faster and reproduce earlier than if they had to rely solely on their own foraging ability. Leonetti and Nath also consider the transition to first birth in the context of kin relationships, but present data for populations where a market economy is beginning to penetrate (Leonetti and Nath, 2009). They suggest that later first births become beneficial as ecological conditions shift in this way, since later births allow the accumulation of physiological and educational capital. By comparing two kinships systems, one matrilineal and one patrilineal, they further find that women living normatively (i.e., with matrilineal or patrilineal kin, respectively) in each population have the most favorable reproductive outcomes, suggesting patterns of kin help vary adaptively between ecologies. While women clearly receive energetic contributions from a variety of relatives and affinal kin, the role of grandmothers in helping mothers has attracted particular attention in recent years (Hawkes et al., 1998). Other cooperatively breeding species tend to obtain help from prereproductive juveniles or subordinate, reproductively inactive adult helpers (Cockburn, 1998; Jennions and Macdonald, 1994). Human females, however, can make use of the productivity of older women, who form a convenient pool of postreproductive labor (though this grandmaternal help does have antecedents in some primate species (Paul, 2005), and grandparental help has also just been observed for the first time in a cooperatively breeding bird, the Seychelles’ warbler (Richardson et al., 2007)). While grandmaternal help appears to be widespread across populations, its nature does vary ecologically (Sear and Mace, 2008). In this issue, Scelza and Coall et al. both focus on aspects of grandmaternal help. In contrast to the majority of previous studies, which have looked for indirect evidence that grandmothers help, Scelza provides evidence from an observational study of Martu Aborigines that grandmothers provide the most hands-on care for babies, after mothers (Scelza, 2009). Coall et al. investigate grandparental help in a very different population: contemporary Switzerland (Coall et al., 2009). They focus on grandparental care from the grandparents’ perspective and find that grandparental care is affected by the grandparent’s reproductive history: grandparents with greater numbers of children and grandchildren and who began reproducing early exhibit reduced grandparental investment, similar to trade-offs observed in parental investment. While kin can provide help by pooling energy budgets, kin, as well as other members of a social group, may also influence life history through other means. In this issue, Newson, Mace and Colleran, and Borgerhoff Mulder all consider the role social information transmission plays in life history decisions, specifically decisions on whether and when to have children. On the basis of the assumption that kin are pronatal and will promote childbearing by the transmission of pronatal norms, Newson has previously proposed that the almost universal fertility decline observed in recent decades can be partly explained by a loosening of kin ties (Newson et al., 2005). Here, she uses cross-cultural data from a survey of beliefs and values, to provide evidence in favor of one prediction from this model: that people in countries with a long history of low fertility will perceive the costs of children to be higher and their benefits lower than those countries with more recent