FEMALE-BIASED MORTALITY INDUCED BY MALE SEXUAL HARASSMENT IN A FERAL SHEEP POPULATION

In contrast to most populations of sexually dimorphic ungulates, a 3-y ear study revealed a tertiary sex ratio (the number of males per femal e at sexual maturity) strongly biased towards males in the Ile Longue feral sheep (Ovis aries) population (Kerguelen subantarctic archipelag o). This population presents a main winter lambing season and a second ary summer lambing period. The sex ratios of lambs and lamb carcasses were even. The skewed sex ratio of adults was therefore attributed to shorter longevity of females. We showed that (i) male monthly mortalit y followed a bimodal distribution with peaks corresponding to the two rutting periods. Male mortality was, however, highest during the winte r even though less rutting occurred in this season than in the summer, suggesting that harsher winter conditions enhance the cost of mating competition; (ii) female mortality was related to the rutting activity of males but not to lambing. Female mortality peaked in the summer de spite plentiful food resources. This period corresponds to the main ru tting peak, suggesting a high survival cost of mating for females. Fem ale mortality due to male harassment was confirmed by direct observati ons and examination of females injured during rutting periods. This si tuation may be due to a strongly male-biased operational sex ratio in the population, associated with an inability of males to establish a s table dominance hierarchy within a large flock.