Abstract
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.
Highlights
Determining the importance of family settings for individual success interests scientists from several disciplines, such as developmental psychology, genetics, social sciences and evolutionary biology
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
In order to investigate the net effect of elder siblings on lifetime fitness, we combined the effect of elder siblings on survival to sexual maturity and on reproductive success to estimate the lifetime fecundity expected at birth
Summary
Determining the importance of family settings for individual success interests scientists from several disciplines, such as developmental psychology, genetics, social sciences and evolutionary biology. Each model included as fixed effects mother’s age, father’s SES (included as a three-level categorical variable), mother’s survival (mother alive or dead at the end of childhood), birth order (firstborn or laterborn to account for lower survival of firstborns in the study population) [29] and the total number of siblings (to control for overall competition for resources in the family). In order to investigate the net effect of elder siblings on lifetime fitness, we combined the effect of elder siblings on survival to sexual maturity and on reproductive success to estimate the lifetime fecundity expected at birth This latter variable was estimated by multiplying the predicted probability of surviving to age 15 years by the probability of reproducing and by the expected offspring count, for all possible numbers of elder brothers and elder sisters, obtained from the previous models. Covariates were arbitrarily fixed to a specific level, as no interactions were significant in the models: high for family SES, laterborn for firstborn status and alive for mother’s survival during childhood
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