Grevy's zebras graze primarily on tough grasses, but they also browse on leaves, which may constitute up to 30 percent of their diet. However, Grevy's Zebras are water dependant and will migrate to grazing lands within daily reach of water.  Although zebras prefer to drink daily, adults can tolerate two to five days without water, but lactating females can only go one to two days without drinking water. Mixed herds graze away from water in the early morning, rest in the shade during mid-day, and return to water in the mid-afternoon.  They graze away from water overnight.  When grazing, Grevy's often make an open arc formation which enables them to watch for predators and be less competitive in feeding.