The evolution of sibling interactions in humans

Studying the fitness consequences of living in the presence of siblings at different life history stages, and thus how siblings differ in their fitness maximising strategies, is crucial for understanding the evolution of family dynamics. Using a large demographic dataset from historical Finland, I investigate the effects of sibling interactions on key life history traits, including dispersal phenotype (i.e. dispersing or remaining philopatric) and its direct and indirect fitness outcomes.
First, I highlight opposite effects of sibling interactions depending on life history stage, and show that overall lifetime reproductive success was reduced by the presence of same-sex elder siblings, but increased by the presence of opposite-sex elder siblings (Chapter 2). These results suggest that individuals may switch their fitness maximising strategy in different life history stages. I then show that dispersal patterns are strongly influenced by intra-sex competition (over land resources for males and mating opportunities for females) with later born, disadvantaged siblings being more likely to disperse (Chapter 3). However, I find no evidence that later born children with lower access to parental resources benefitted more from dispersing than did first borns (Chapter 4). Finally, I investigate potential indirect fitness benefits of helping behaviour provided by relatives who remain philopatric and single, but I do not find evidence for alloparenting effects (Chapter 5).
Overall, these results suggest that differential access to parental resources could have qualitative as well as quantitative consequences on the fitness maximising strategies of siblings with different access to resources, namely that first born offspring gained fitness both directly and indirectly, whereas later born, disadvantaged siblings only seemed to invest in their direct fitness. This thesis provides a substantial step towards understanding sibling interactions over the entire life-course rather than only focusing on early-life consequences, and stresses the need for more long-term studies of sibling interactions.