title: Are elder siblings helpers or competitors? Antagonistic fitness effects of sibling interactions in humans.
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.
authors.affiliation: Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK. a.nitsch@sheffield.ac.uk
authors.forename: Aïda
authors.initials: A
authors.lastname: Nitsch
authors.forename: Charlotte
authors.initials: C
authors.lastname: Faurie
authors.forename: Virpi
authors.initials: V
authors.lastname: Lummaa
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D002648:Child; D003163:Competitive Behavior; D005260:Female; D005387:Finland; D056084:Genetic Fitness; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D012098:Reproduction; D012189:Retrospective Studies; D012737:Sex Factors; D012801:Sibling Relations; D012919:Social Behavior; D016019:Survival Analysis