Integrated field and laboratory studies of long-lived, large-bodied mammals are rare but offer unique opportunities to elucidate the behavioral ecology of these animals. Here, we used this approach to examine whether siblicide in spotted hyenas (Crocuta crocuta) is obligate or facultative. First, we tested predictions of obligate and facultative hypotheses by using ultrasonography to compare litter size before and after parturition and identify potential causes of litter reduction. Second, we compared litter size and composition between wild and captive hyenas to assess variation in offspring sex ratios. Third, we used demographic data to compare survivorship among litters of various sizes and compositions. Fourth, we compared sex ratios within twin litters born in the wild under conditions of high population density and intense feeding competition with those born when population density and intensity of feeding competition were reduced. Our data were inconsistent with the obligate siblicide hypothesis. Litter reduction occurred during roughly one-third of pregnancies in both wild and captive hyenas, and all such reductions among captives were due to fetal resorptions or stillbirths. Litter sizes and compositions differed little between wild and captive hyenas. However, sex ratios in twin litters varied in the wild with intensity of feeding competition. In conjunction with captive data, long-term study of a wild hyena population under varying environmental conditions suggests facultative siblicide is most likely to occur when feeding competition is most intense, thus offering an ecological explanation for earlier conflicting reports on siblicide in this species.

Sibling rivalry, occurring in the form of intense aggression directed at siblings, has been described in many birds, but few mammals. In the spotted hyaena, early aggression directed at littermates has been hypothesized to function as a mechanism for litter reduction by siblicide, particularly within same-sex litters. Alternatively, intralitter aggression might serve to establish and maintain a rank relationship between twin littermates. Our goal here was to use long-term observations of 46 litters to test these two hypotheses in wild Crocuta. Rates of sibling aggression were high during the first 2 months of life, but then dropped to low levels. During the first year of life, littermates fought mainly over milk and meat, both of which are critical food resources, and dominant cubs were heavier than subordinate siblings of the same age. Rates at which mixed-sex littermates fought did not differ significantly from fighting rates within same-sex litters. Rates of intralitter aggression were higher when local prey were scarce than when prey were abundant. Although we found no significant effect of maternal social rank on cub aggression rates, our results were consistent with the notion that aggression rates within litters vary with maternal rank. Our results did not support the hypothesis that intralitter aggression functions to mediate siblicide, particularly within same-sex litters. Instead, our results were consistent with the hypothesis that early sibling aggression in Crocuta functions to establish and maintain a rank relationship within the litter. (c) 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

We examined patterns of affiliation, association, and aggression to inquire whether spotted hyenas (Crocuta crocuta) can distinguish among various groups of maternal and paternal siblings. If so, and if these animals conform to predictions of kin selection theory, then behavioral interactions among hyenas should vary with relatedness. We also considered familiarity-based recognition and phenotype matching as mechanisms hyenas might use to recognize kin. Patterns of affiliative behavior indicated that hyenas favored full-sibling littermates over half-sibling littermates or any other group of half-siblings. Rates of dyadic aggression generally did not vary with kinship. Hyenas associated more closely with half-sibling littermates than with non-littermate half-siblings, and hyenas affiliated more with maternal half-siblings than with paternal half-siblings, suggesting that familiarity-based cues might mediate discrimination among these sibling classes. In addition, operation of a phenotype-matching mechanism was suggested by the preference hyenas demonstrated during affiliative interactions for full- over half-sibling littermates, and by their lack of preference in these interactions for half-sibling littermates over non-littermate half-siblings. Phenotype matching was also suggested by our observation that paternal half-siblings cooperated more, and fought less, than did non-kin. Our data indicate that hyenas can discriminate among various types of siblings, that their social behavior conforms to predictions of kin selection theory, and that they recognize kin using mechanisms of both familiarity and phenotype matching.