Elder brothers affect the life history of younger siblings in preindustrial humans: social consequence or biological cost?

Sibling composition and household room sharing are associated with menarcheal status among rural Bengalee girls of West Bengal, India

Menarche, the first menstruation, is one of the most important events in a woman's reproductive life. The timing of menarche varies across populations and depends upon social interaction and family environment. It is also associated with several biological as well as social factors. The objective of this study was to explore the relationship between sibling composition and age at menarche (AAM) and to assess the association between the pattern of room sharing with family members of different sexes and menarcheal occurrence among rural Bengalee girls from West Bengal, India. The total sample comprised 577 Bengalee girls, 6-17 years of age, from various schools and madrasas in two blocks of the Nadia District of West Bengal State in India. The effects of room sharing on the occurrence of menarche, and of sibling composition on the menarcheal age, were assessed by analyses of covariance. The room-sharing pattern had a significant effect on menarcheal status (yes÷no): a significantly higher percentage of girls who shared a room with the mother and÷or sisters were postmenarcheal compared with those who shared a room with male family members. AAM did not differ significantly between girls having brothers or sisters. However, sibling order had a significant impact on AAM. Girls who had a younger sibling only (brother or sister) had a higher mean AAM, and girls who had both younger brothers and younger sisters had significantly higher mean AAM, than did the girls who had no younger sibling (singletons or having only elder siblings). There was no difference in AAM between the girls who had younger sister(s) and those who had younger brother(s). These differences were also independent of body mass index. In conclusion, the room sharing characteristics and the sibling sex composition, particularly their order, had significant effect on menarche in adolescent rural Bengalee girls.

The effect of maternal age and reproductive history on offspring survival and lifetime reproduction in preindustrial humans

Senescence is one of the least understood aspects of organism life history. In part, this stems from the relatively late advent of complete individual-level datasets and appropriate statistical tools. In addition, selection against senescence should depend on the contribution to population growth arising from physiological investment in offspring at given ages, but offspring are rarely tracked over their entire lives. Here, we use a multigenerational dataset of preindustrial (1732-1860) Finns to describe the association of maternal age at offspring birth with offspring survival and lifetime reproduction. We then conduct longitudinal analyses to understand the drivers of this association. At the population level, offspring lifetime reproductive success (LRS) declined by 22% and individual λ, which falls with delays to reproduction, declined by 45% as maternal age at offspring birth increased from 16 to 50 years. These results were mediated by within-mother declines in offspring survival and lifetime reproduction. We also found evidence for modifying effects of offspring sex and maternal socioeconomic status. We suggest that our results emerge from the interaction of physiological with social drivers of offspring LRS, which further weakens selection on late-age reproduction and potentially molds the rate of senescence in humans.

Are elder siblings helpers or competitors? Antagonistic fitness effects of sibling interactions in humans

Determining the fitness consequences of sibling interactions is pivotal for understanding the evolution of family living, but studies investigating them across lifetime are lacking. We used a large demographic dataset on preindustrial humans from Finland to study the effect of elder siblings on key life-history traits. The presence of elder siblings improved the chances of younger siblings surviving to sexual maturity, suggesting that despite a competition for parental resources, they may help rearing their younger siblings. After reaching sexual maturity however, same-sex elder siblings' presence was associated with reduced reproductive success in the focal individual, indicating the existence of competition among same-sex siblings. Overall, lifetime fitness was reduced by same-sex elder siblings' presence and increased by opposite-sex elder siblings' presence. Our study shows opposite effects of sibling interactions depending on the life-history stage, and highlights the need for using long-term fitness measures to understand the selection pressures acting on sibling interactions.

Brothers delay menarche and the onset of sexual activity in their sisters

The higher costs of sons compared with daughters extends to a negative effect of brothers on the lifetime reproductive success of their siblings in subsistence and preindustrial societies. In societies with fewer resource constraints, one might expect that these effects would be limited or non-existent. This study investigates the costs of brothers and sisters in a contemporary western society of adult Australians. Girls with elder brothers had a delayed age at menarche. Younger brothers were associated with delayed onset of sexual activity in sisters, but not in brothers. Neither younger nor elder brothers influenced fitness parameters (number of pregnancies, number of children, age at first pregnancy or age at first birth) in siblings of either sex. This study provides evidence that brothers negatively affect their sisters' onset of reproductive maturity and sexual activity; however, this delay is not associated with a fitness cost in contemporary Australia. We suggest this is due to the long period of independence prior to child bearing.