Sexual Segregation in Mountain Sheep: Resources or Predation?

Bleich, Vernon; Bowyer, R.; Wehausen, John

We studied mountain sheep (Ovis canadensis nelsoni) at Old Dad Mountain, in the Kelso Mountains, and in the Marl Mountains in the eastern Mojave Desert, San Bernardino County, California during 1981-90 to determine causes of sexual segregation. Forty-four mountain sheep were captured, fitted with radio collars, and located systematically from a fixed-wing aircraft to determine differences in habitats used by males and females. In addition, diet composition and forage quality and availability along with information on diets and distribution of predators were obtained to test 4 hypotheses potentially explaining sexual segregation in ungulates. Mature males and females were segregated from December to July and were aggregated from August to November. Mature males obtained higher quality diets than did females (based on values for fecal crude protein) during 2 of the 3 years for which data were available. Indices of predator abundance were substantially lower on ranges used by females and juveniles than on those used by mature males. Females occurred closer to permanent sources of water and in steeper, more rugged, and more open habitats than did mature males. Moreover, forage was more abundant in habitats used primarily by mature male sheep. Females with and without lambs did not differ in their distance from water during aggregation or segregation, and females did not visit water more often during the period of peak lactation when compared with other times of the year. Female groups with lambs, however, occurred on steeper slopes and in more rugged and open habitats during segregation, when lambs were very young. Based on our results, we refute the hypotheses (1) that females outcompete males for available resources, and allometric differences between the sexes lead to sexual segregation; (2) that the constraints of lactation may be important in explaining sexual segregation in this desert-adapted ungulate; and (3) that males segregate to avoid competition with their mates, potential mates, and offspring, at least in desert ecosystems. In contrast, our findings strongly support the hypothesis that, because of their smaller body size and potentially greater vulnerability to predation, and the need to minimize risk to their offspring, female ungulates and their young use habitats with fewer predators and greater opportunities to evade predation than do mature males, but that males are able to, and do, exploit nutritionally superior areas. We conclude that sexual segregation likely results from differing reproductive strategies of males and females among sexually dimorphic ungulates. Males may enhance their fitness by exploiting habitats with superior forage and thereby enhance body condition and horn growth while simultaneously incurring greater risks than do females. In contrast, females appear to enhance their fitness by minimizing risks to their offspring, albeit at the expense of nutrient quality. Further, we suggest that how food and risk of predation are arrayed in the environment may affect whether males or females inhabit better quality ranges, as well as which sex moves to produce spatial separation.

Sexual Segregation in Mountain Sheep: Resources or Predation?

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