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Dall’s sheep (Ovis dalli dalli Nelson, 1884) sexual segregation : interactions between two hypotheses Corti, Paulo

Abstract

Four hypotheses have been proposed to explain sexual segregation in sexually dimorphic ungulates. I tested two of these on a Dall's sheep (CV/'s dalli dalli) population in Kluane National Park in Yukon, Canada. In the first, the reproductive-strategy hypothesis, males are predicted to feed in the best foraging areas to enhance their condition for intrasexual competition. Females are predicted to use areas with lower predation risk to raise offspring. In the second, the sexual dimorphism-body size hypothesis, females should use the best forage areas to satisfy the nutritional demands of gestation and lactation, and males due to their greater absolute metabolic requirements and larger body size have to forage on more available forages, but lower quality. I found Dall's sheep had a high social segregation but limited habitat segregation between males and females. Males were further from security cover in more gentle terrain than were maternal groups that used cliff or talus slopes. Maternal groups were located at higher altitudes than were males because most security cover was close to mountain peaks. Lamb presence was a factor increasing predation risk and affecting maternal groups' behaviour and distribution towards security cover. Group distance from security cover was negatively correlated with the proportion of individuals lying down for maternal groups without lambs. When this group type left security cover they were constantly active, either feeding or moving. The forage density index (FDI) values varied significantly, with the areas used by males having higher FDIs than areas used by maternal groups. Nutritional components were similar, but fibre content was significantly higher in areas used by males. Signs of wolves (Canis lupus) and predation events on Dall's sheep were recorded only at male sites. These data support the idea that male areas have a greater risk of predation. My results primarily support the reproductive-strategy hypotheses, indicating that predation plays a key role in the development of sexual segregation in Dall's sheep. However, I also found evidence to support one prediction of the sexual dimorphism-body size hypothesis, where males use areas higher in forage availability but lower in quality than female areas.

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