Age- and sex-dependent associations between the number of older siblings and early-life survival in pre-industrial humans

Siblings are an important part of an individual's early-life environment and may therefore play an important role in shaping an individual's survival. The quantification of sibling effects on survival is challenging, however, especially in long-lived species with extended parental care and overlapping generations, such as humans. Here, we use historical parish data from Switzerland to quantify how the number of older siblings and their survival status, age and sex are associated with childhood survival. Across 2941 focal individuals born between 1750 and 1870, the total number of older siblings did not predict an individual's childhood survival probability. However, distinguishing between siblings by their survival status, age and sex revealed several associations, which in some cases also interacted with the sex of the focal individual: while older brothers close in age reduced the survival of girls (but not boys), having more older sisters close in age improved their younger sibling's survival. Our results therefore suggest that older siblings play an important role in shaping early-life survival and highlight that the strength and direction of sibling-related associations are context-dependent and can arise through both biological and cultural factors.