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Most vegetation is coarse, difficult to break down, and has low energy content. Therefore herbivores spend a great deal of time feeding and processing their food. Many have striking adaptations for feeding, such as the trunk (a flexible, gripping nose) of the elephant or the huge bill of the fruit-eating toucan, which can be half as long as its body. Many types of grinding, rasping, cutting, and shredding mouthparts have evolved in invertebrates for ingesting plant material, and the teeth of herbivorous vertebrates have been shaped by selection to tear, crush, and grind coarse plant matter.

The elephant provides an example of the importance of teeth. Elephants have six chewing teeth per quadrant, which appear one at a time. The first is present at birth, and subsequent ones appear at the back of the jaw and move forward as the elephant ages. The first falls out at 2–3 years of age, the second at 4–6 years of age, and so forth until the last, and biggest, one (20 cm long and 4 kg) is worn out sometime past 60 years of age. With the loss of that last tooth, the elephant starves and dies.

The digestive processes of herbivores can be quite specialized. An example is the koala, which almost exclusively eats leaves of eucalyptus trees. These leaves are very fibrous, low in usable energy and protein, and high in toxic chemicals. The koala has strong jaws for grinding the leaves, a very long gut for fermenting them, enzymes in its liver for detoxifying chemicals in the leaves, and a low metabolic rate (i.e., it expends little energy) to compensate for low energy intake.