Female-biased mortality induced by male sexual harassment in a feral sheep population

Abstract

In contrast to most populations of sexually dimorphic ungulates, a 3-year study revealed a tertiary sex ratio (the number of males per female at sexual maturity) strongly biased towards males in the Île Longue feral sheep (Ovis aries) population (Kerguelen subantarctic archipelago). This population presents a main winter lambing season and a secondary 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 mortality followed a bimodal distribution with peaks corresponding to the two rutting periods. Male mortality was, however, highest during the winter 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 despite plentiful food resources. This period corresponds to the main rutting peak, suggesting a high survival cost of mating for females. Female mortality due to male harassment was confirmed by direct observations and examination of females injured during rutting periods. This situation may be due to a strongly male-biased operational sex ratio in the population, associated with an inability of males to establish a stable dominance hierarchy within a large flock.