Kangaroo: The teeth
The second difficulty that grass eaters must overcome is the wear and tear on their teeth. Grass is tough, due to the silicates that occur in them, and breaking it down into a pulp in the mouth is very abrasive on the teeth. Grazers elsewhere have molars with open roots so that wear can be compensated by continuous growth throughout the animal's life. In kangaroos the roots of the teeth are closed, and they have evolved a different system of tooth replacement. There are four pairs of cheek teeth on either side of the jaws. Only the front ones engage. As they are worn down to the roots, they fall out and those from the rear migrate forward to take their place. By the time the animal is fifteen or twenty years old, its last molars are in use.