An elephant's molars, pictured here at the Reid Park Zoo in Tucson, Arizona, necessary for grinding up plant material, are replaced six times during its lifetime. These molars form at the back of the jaw, and move slowly forward and upward. Each set is gradually worn down and replaced by the next set. The replacement of these molars can be linked directly to age so, by identifying the molars in use, the age span of the elephant can be determined. The teeth do not emerge from the jaws vertically like with human teeth. Instead, they have to move horizontally, like a conveyor belt. New teeth grow in at the back of the mouth, pushing older teeth toward the front, where they wear down with use and the remains fall out. When an elephant becomes very old, the last set of teeth is worn to stumps, and it must rely on softer foods to chew. Very elderly elephants often spend their last years exclusively in marshy areas where they can feed on soft wet grasses. Eventually, when the last teeth fall out, the elephant will be unable to eat and will die of starvation. Were it not for tooth wearout, their metabolism would allow them to live much longer.