Editor's choice Early conditions and fitness: effect of maternal parity on human life-history traits

Stable pair bonds are widespread in humans and reproductive success varies greatly among pairs. Determinants of fitness may be based on testosterone-dependent sexually dimorphic traits. Prenatal condition is a critical period of physical development, and fetal testosterone has a profound effect on adult behavior and reproduction. Maternal parity (the number of previous births) might be a proxy for fetal testosterone because the prenatal level of male sex hormone, testosterone, is previously shown to decrease across parities. Offspring reproductive potential may, therefore, be positively associated with low parity in sons and high in daughters, by different masculinization by testosterone. In this study, we explored the effects of maternal parity on offspring’s subsequent (individual and pair-specific) life histories including number of grandchildren (controlling for age at marriage, number of siblings, social status, and year of birth) in a Norwegian preindustrial society. We found that parity was a positive predictor of daughters’ and negative (but not significantly; P = 0.058) predictor of sons’ subsequent fitness. Furthermore, within the mated pairs, the difference between the parity of their 2 mothers was a strong predictor of each couple’s fitness. Sons born to mothers of low parity married to daughters from mothers with high parity had much higher number of grandchildren than, for example, sons of mothers with high parity married to daughters of mothers with low parity. The result highlights the importance of early prenatal condition on human life histories, particularly pair-bond duration and fitness, and that parity may have an impact on offspring pair-specific fitness in the studied population.