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

Integrated field and laboratory studies of long-lived, large-bodiedmammals are rare but offer unique opportunities to elucidatethe behavioral ecology of these animals. Here, we used thisapproach to examine whether siblicide in spotted hyenas (Crocutacrocuta) is obligate or facultative. First, we tested predictionsof obligate and facultative hypotheses by using ultrasonographyto compare litter size before and after parturition and identifypotential causes of litter reduction. Second, we compared littersize and composition between wild and captive hyenas to assessvariation in offspring sex ratios. Third, we used demographicdata to compare survivorship among litters of various sizesand compositions. Fourth, we compared sex ratios within twinlitters born in the wild under conditions of high populationdensity and intense feeding competition with those born whenpopulation density and intensity of feeding competition werereduced. Our data were inconsistent with the obligate siblicidehypothesis. Litter reduction occurred during roughly one-thirdof pregnancies in both wild and captive hyenas, and all suchreductions among captives were due to fetal resorptions or stillbirths.Litter sizes and compositions differed little between wild andcaptive hyenas. However, sex ratios in twin litters varied inthe wild with intensity of feeding competition. In conjunctionwith captive data, long-term study of a wild hyena populationunder varying environmental conditions suggests facultativesiblicide is most likely to occur when feeding competition ismost intense, thus offering an ecological explanation for earlierconflicting reports on siblicide in this species.