Models that address facultative siblicide in avian species predict that the costs and benefits of sibling aggression will change in relation to the level of food provisioning by parents. In spotted hyaenas, Crocuta crocuta, one of the few mammalian species in which facultative siblicide occurs, aggression rates between siblings were highest when cubs competed for access to maternal milk. In the Serengeti National Park, Tanzania, nursing of cubs by spotted hyaena mothers was influenced by the migratory movements of herbivores and the social status of the mother. Consistent with predictions from avian models, rates of sibling aggression among spotted hyaena littermates were inversely related to levels of maternal input in terms of lactation. Long-distance foraging trips by mothers (commuting trips) resulted in low cub growth. Rates of aggression were higher in years when cub growth rates were low than in years when cub growth rates were medium or high. Rates of aggression between littermates belonging to high-ranking mothers, who nursed their cubs more frequently than low-ranking mothers, were lower than those between siblings of low-ranking mothers. Aggression rates in all-female and all-male litters were higher than in mixed-sex litters. Identical payoff expectations of same-sex litters may provide an explanation for this result. In accordance with predictions from life history theory, aggression rates declined with age and increasing reproductive value of siblings. Copyright 1999 The Association for the Study of Animal Behaviour.