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.

Social bonds between unrelated females increase reproductive success in feral horses

“…These results are consistent with findings on primates (15) between related females (16,17,38). Females benefit directly by reducing harassment received, and harassment reduces reproductive success in horses (6) and other mammals (2,39,40). Additional benefits could include reduced stress levels (41), because harassment increases stress levels and decreases body condition (6).…”

“…I n mammals, aggressive male behavior has costs for females (1)(2)(3), and female avoidance of male harassment has been suggested as an explanation for mating system structure (3)(4)(5). For example, in horses, male harassment better explains the unusual harem mating system than resource-based explanations (6).…”

Sexual conflict and the evolution of asexuality at low population densities

“…For example, female guppies move to habitats of higher predation risk to avoid male harassment [7]. Density often plays a role: in high-density feral sheep populations, female mortality peaks in summer-despite high resource availability-due to male harassment [8]. In damselflies, Cordero [9] has shown that at high densities more matings were forced compared with low-density populations, and that at extreme population density harassment by males can hinder oviposition.…”

Sexual Conflict, Life Span, and Aging

“…Because females vigorously resist mating with extra-pair males, forced copulation attempts can result in severe injuries, and in extreme cases even death, for the female (Adler 2010). Similarly, in feral sheep, females are exposed to high levels of male harassment and injury during the breeding season, resulting in a female-biased mortality rate (Réale et al 1996). Male-imposed injuries to females through coercive mating strategies are documented across taxa, sometimes resulting from bizarre adaptations, such as the "hypodermic genitalia" found in the true bug infraorder Cimicomorpha (Heteroptera), in which males inseminate females by stabbing them in the abdomen, often resulting in injury and infection (Stutt and Siva-Jothy 2001;Tatarnic et al 2014).…”