They do supplement their naps by dozing standing up, but remain alert during these periods, leaving one eye open, and constantly moving their eight-inch ears. Besides spending a lot of time watching for danger, giraffes spend a lot of time browsing. Like cattle, giraffes are ruminants, eating quickly and barely chewing their food the first time around. They typically browse for several hours after dawn and several more near dusk, reserving the hot middle part of the day for chewing their cud. Giraffes grasp branches with their long, black, prehensile tongues and strip off the leaves with their comb-like canine teeth, which are broad and divided into several lobes.
Acacias are a nearly perfect food for giraffes.
The leaves are up to 75 percent water, which may help explain why these animals can go so long without drinking, and contain virtually all the nutrients they need. The only drawback to acacias is that they bristle with stout, pin-sharp thorns up to two inches long. Giraffes are remarkably well-equipped to deal with these thorns. They can close their nostrils completely, their upper lips are protected by a thick barrier of hair, and they produce great quantities of viscous saliva to help the thorns go down more easily. Moreover, their tongues are nimble enough to reach between thorns to snag a leaf.