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hal nit 19 spe d p fa ess herherbivores 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. Ó 1999 The Association for the Study of Animal Behaviour In this study we consider competition among littermates in free-ranging spotted hyaenas, Crocuta crocuta. Sibling rivalry appears to contradict the biological premise of maximizing inclusive fitness (Hamilton 1964), particu-larly when it has fatal consequences. Aggressive brood reduction has been defined as the use of fighting amongst siblings to establish a dominance hierarchy with the lowest-ranked member being most likely to die (Mock & Parker 1997). Mortality may result from physical damage and/or socially enforced starvation (Mock 1984); in fact any act of an individual that substantially reduces the survival of one of its siblings is included in this concept of siblicide. Siblicide (both facultative and obligate) has been defined and documented in a variety of avia