[Photo credit: Larry D. Moore. License: CC-BY-SA 3.0]
The teeth of herbivores are adapted for cropping grass and grinding plant matter to break down the cellulose cell walls. The incisors of a rabbit are long and chisel-shaped. They bite against each other to chop off pieces of grass.
The incisors wear away fast but the teeth can grow continuously because of a good supply of blood into the pulp cavity. Hence, these teeth that grow continuously are known as open teeth.
There are no canines in herbivores. Instead, between the incisors and premolars, there is a gap, the diastema. When the rabbit eats, the cheeks project into the mouth in this region and in doing so, help to push the food from the front of the mouth to the grinding teeth behind.
The premolars and molars have broad, rigid surfaces. These allow the food to be ground into fine particles when the teeth of the upper and lower jaw grind against each other. Premolars and molars also grow continuously. The rabbit's jaws are loosely linked. They can be moved side to side, as well as, up and down.
Plants matter contains a lot of cellulose which has to be broken down to glucose before it can made use of. Rabbits cannot make cellulase, the enzyme needed to digest cellulose. Instead, the rabbit has cellulase-producing bacteria in its gut. This enables the rabbit to digest the cellulose in its food to glucose.
The bacteria benefit as they have a warm and safe place to live in, as well as a constant supply of food. This is an example of a mutual(symbiotic) relationship. The bacteria are found in the caecum, a large thin-walled sac, that lie between the small and large intestines.
One problem arises during the digestion of food in the rabbit. By the time cellulose digestion is complete, the contents would have moved past the small intestine where absorption can take place. The food is passed out as soft, mucus-covered pellets. The rabbit quickly eats the pellets; and the glucose they contain is absorbed in the small intestine.
The intestine of rabbit is quite long because food needs longer time to digest.
OTHER HERBIVORES
Herbivores generally have three special adaptations that enable them to digest cellulose:
- They have long intestines. Digestion is slower than in other animals.
- They have a caecum and appendix for harboring their mutual partners - the cellulase-producing bacteria.
The first part is the rumen or paunch, where the unchewed food is temporarily stored and contains protozoans and bacteria which digest cellulose.
The rumen gradually passes it to the second chamber, the reticulum or honey comb bog where food is moistened and formed into ball-like masses called boluses or cuds. When the animal is at rest, these pass back (regurgitate) up to the oesophagus into the mouth for thorough chewing and mixed with saliva.
When swallowed, the food passes into the third chamber, the omasum or psalterium.
It then passes into the fourth chamber, abomasum or true stomach which bears the gastric glands. Here, the stomach's part of digestion is completed.
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