SEXUAL SEGREGATION IN MOUNTAIN SHEEP - RESOURCES OR PREDATION

We studied mountain sheep (Ovis canadensis nelsoni) at Old Dad Mountai n, in the Kelso Mountains, and in the Marl Mountains in the eastern Mo jave Desert, San Bernardino County, California during 1981-90 to deter mine causes of sexual segregation. Forty-four mountain sheep were capt ured, fitted with radio collars, and located systematically from a fix ed-wing aircraft to determine differences in habitats used by males an d females. In addition, diet composition and forage quality and availa bility along with information on diets and distribution of predators w ere obtained to test 4 hypotheses potentially explaining sexual segreg ation in ungulates. Mature males and females were segregated from Dece mber to July and were aggregated from August to November. Mature males obtained higher quality diets than did females (based on values for f ecal crude protein) during 2 of the 3 years for which data were availa ble. Indices of predator abundance were substantially lower on ranges used by females and juveniles than on those used by mature males. Fema les occurred closer to permanent sources of water and in steeper, more rugged, and more open habitats than did mature males. Moreover, forag e was more abundant in habitats used primarily by mature male sheep. F emales with and without lambs did not differ in their distance from wa ter during aggregation or segregation, and females did not visit water more often during the period of peak lactation when compared with oth er times of the year. Female groups with lambs, however, occurred on s teeper slopes and in more rugged and open habitats during segregation, when lambs were very young. Based on our results, we refute the hypot heses (1) that females outcompete males for available resources, and a llometric differences between the sexes lead to sexual segregation; (2 ) that the constraints of lactation may be important in explaining sex ual segregation in this desert-adapted ungulate; and (3) that males se gregate to avoid competition with their mates, potential mates, and of fspring, at least in desert ecosystems. In contrast, our findings stro ngly support the hypothesis that, because of their smaller body size a nd potentially greater vulnerability to predation, and the need to min imize risk to their offspring, female ungulates and their young use ha bitats with fewer predators and greater opportunities to evade predati on than do mature males, but that males are able to, and do, exploit n utritionally superior areas. We conclude that sexual segregation likel y 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 con dition and horn growth while simultaneously incurring greater risks th an do females. In contrast, females appear to enhance their fitness by minimizing risks to their offspring, albeit at the expense of nutrien t quality. Further, we suggest that how food and risk of predation are arrayed in the environment may affect whether males or females inhabi t better quality ranges, as well as which sex moves to produce spatial separation.