Sexual segregation in desert-dwelling mountain sheep

Abstract

Mountain sheep (Ovis canadensis nelsoni) were studied in the eastern Mojave Desert, San Bernardino Co., California to test 4 hypotheses potentially explaining sexual segregation in ungulates. Mature males and females were segregated from December to July, and aggregated from August to November. Mature males obtained higher quality diets than did females (based on fecal crude protein) and forage was more abundant on ranges used by these males. Indices of predator abundance were substantially lower on ranges used by females and young 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. Female groups with and without lambs did not differ in their distance from water during aggregation or segregation. Female groups with lambs, however, occurred on steeper slopes and in more rugged and open habitats during segregation, when lambs were very young. I refute the hypotheses that: (1) Males enhance their fitness by segregating from females and their own offspring; (2) Females outcompete males for available resources, and allometric differences between the sexes lead to sexual segregation; and (3) The constraints of lactation may be important in explaining sexual segregation in this desert-adapted ungulate. In contrast, my observations strongly support the hypothesis that, because of their smaller body size and potentially greater vulnerability to predation, female ungulates use habitats with fewer predators and more opportunities to evade predation than do males, but males are able to exploit nutritionally superior areas. Sexual segregation likely results from differing reproductive strategies of males and females among sexually dimorphic mammals. Males may enhance their fitness by exploiting habitats with superior forage, and thereby enhance body condition, 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 intake.