Abstract
Mortality generally is higher around birth and then progressively declines through the juvenile stage. In species where offspring depend upon their parents during maturation, a factor behind this mortality decline could be sibling replacement: offspring sacrifice their survival to benefit future or present siblings as early as possible in order to minimize losses in parental investment. Here, we propose a kin-selection model of sibling replacement. Theoretical analysis of the model and its application to demographic data of mammals suggest that sibling replacement consistently generates a selective incentive for increasing juvenile mortality at early ages when this mortality increment is the result of positive selection for juvenile altruism within the nuclear family. The model highlights how sibling replacement goes beyond optimal allocation of parental resources into dependents and can provoke greater mortality closer to birth also in response to a more favorable ratio of actors to recipients of altruism among siblings.