Plant eaters: The teeth and digestion
Plant eaters have to have particularly good teeth. Not only do they use them for very long periods but the material they have to deal with is tough. Rats, like other members of the order Rodentia squirrels, mice, beavers, porcupines cope with that problem by maintaining open roots to their front gnawing teeth, the incisors, so that they continue to grow throughout the animal's life compensating for wear. They are kept sharp by a simple but effective self-stropping process. The main body of the rodent incisor is of dentine, but its front surface is covered by a thick and often brightly coloured layer of enamel which is even harder. The cutting edge of the tooth thus becomes shaped like a chisel. As the top incisors grind over the lower ones the dentine is worn away more quickly and this exposes the blades of enamel at the front keeping a sharp chisel edge.
Once gnawed, ground and pulped, the food has to be digested. This too presents major problems. Cellulose, the material from which the cell walls of plants are built, is one of the most stable of organic substances. Digestive enzymes produced by mammals are unable to break cellulose down, and this can be achieved by either mechanical means through extended chewing or by bacteria which are able to dissolve the cellulose through fermentation. Herbivore digestive systems maintain bacterial cultures to break down this cellulose. Even with bacterial help, digestion of an entirely vegetarian meal can take a long time.
In rabbits (order Lagomorpha) and rodents additional digestion is provided by re-eating soft faecalpellets (coprophagy), so that the material is twice processed and the last vestiges of nourishment are extracted. Only after this second processing are the faeces deposited outside the burrow as the familiar dry pellets.