SKEWED OFFSPRING SEX-RATIOS AND SEX COMPOSITION OF TWIN LITTERS IN SERENGETI SPOTTED HYAENAS (CROCUTA-CROCUTA) ARE A CONSEQUENCE OF SIBLICIDE

Long-term data on the sex ratio among offspring and sex composition of litters in a population of several hundred individually known spotted hyaenas in the Serengeti National Park, Tanzania are presented. Femal es produced singleton or twin litters. The overall sex ratio (proporti on of males) was unbiased at 0.52 (n = 318) but masked significant dev iations in opposite directions in litters of different size. Sex ratio in singleton litters was female biased at 0.43 (n = 91 individuals, P = 0.07). The sex ratio in twin litters was significantly (P < 0.05) m ale-biased at 0.56 (n = 228 individuals); there were significantly too many mixed-sex and too few all-female twin litters. Three hypotheses are considered that may explain these sex ratio biases: (i) the primar y sex ratio may be skewed, (ii) cub mortality may be female-biased, le ading to the loss of more females than males, or (iii) the biases are due to siblicide. The first two hypotheses predicted the same sex rati o biases for both singleton and twin litters, while the third hypothes is predicted opposite trends for singleton and twin litters. The resul ts are inconsistent with the two first hypotheses but support the thir d. The study suggests that the lack of all-female twin litters and the female bias amongst singletons is due to a higher incidence of siblic ide in all-female twin litters than among mixed-sex or all-male twin l itters. Siblicide is the major source of post-natal sex ratio biases t o the age of independence in this population. (C) 1997 Elsevier Scienc e B.V.