Foraging behaviour and sexual segregation in bighorn sheep

Abstract

Like many sexually dimorphic ungulates, bighorn sheep,Ovis canadensis, form sexually segregated groups. Nursery groups include females, lambs and subadult males, while adult males form bachelor groups. Previous hypotheses to account for sexual segregation in ungulates have suggested sexual differences in energy requirements, predation risk and social preferences. I tested the hypothesis that differing nutritional demands, due to sexual dimorphism in body size, would lead to different movement patterns and time budgets. If ruminating/foraging schedules differed according to body size, males and females could not synchronize their activities and therefore would segregate by sex. To test this hypothesis, I observed a population of marked bighorns. I recorded the time males and females spent lying, grazing or walking during 8–14h of focal-animal observations. Initial and final location of groups and steps taken per minute foraging were also noted. Females spent more time foraging and walking and had longer foraging and walking bouts than adult males, but did not differ in numbers of steps taken while foraging. Males spent more time lying than females. Subadult males switched between nursery and bachelor groups and changed their foraging behaviour depending on the type of group they were in. The distance moved was on average almost four times greater for female than for male groups. I suggest that sexual difference in time budgets and movement patterns make it difficult for males and females to stay in the same group and therefore lead to sexual segregation.