Male harassment influences female movements and associations in Grevy's zebra (Equus grevyi)

In traditional models for social organization, female movements and association patterns track resource distribution, whereas males track females. More recently, this model has been expanded to include feedback effects of male behavior, especially sexual harassment, on female decisions. In Grevy's zebra (Equus grevyi), males defend territories containing resources attractive to females, who form unstable groups. Past research has explained female behavior based on resource distribution and needs alone. Lactating females have been found to have restricted movements and fewer male associates than nonlactating females, a pattern we find in our study. This pattern has previously been attributed solely to the higher water needs of lactation. However, in our population, both lactating and nonlactating females are typically close to water. We test the hypothesis that male harassment also influences female ranging and associations with males. This effect is predicted to be greater for lactating females because harassment has higher costs to them. We find that lactating females experience higher harassment rates than those of nonlactating females. Lactating females tend to move faster during harassment periods, whereas nonlactating individuals do not. Lactating females experience lower harassment rates if they spend more time with a particular male, whereas nonlactating females' harassment rates do not depend on their allocation of time to a primary male. We suggest that females concentrate their time with one male in order to reduce male harassment. Even in species such as Grevy's zebra without strong male--female bonds, social interactions may be a significant driver of female distribution. Copyright 2007, Oxford University Press.