Siblicide in the spotted hyena: analysis with ultrasonic examination of wild and captive individuals

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.