Rabbit Alimentary System

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Introduction

The rabbit is a monogastric hindgut fermenters and is a herbivore. Microbes in the hindgut produce VFAs which are available to the animal for energy. Microbes also produce vitamins and protein, which are available only in minimal quantities as they are produced in the hindgut. Most microbial fermentation occurs in the caecum (as opposed to the horse where most occurs in the colon). Rabbits usually eat at dusk.

Stomach

Simple and large

Small Intestine

Functions similar to other animals but there are anatomical differences.
The caudal flexure of the duodenum is long and coiled, and may be referred to as the transverse part of the duodenum.
The sacculus rotundus exists at the ileocaecal junction. It is an enlargement of the large intestine and contains lymphoid tissue.

Large Intestine

Caecum

Large, about twice the length of the abdominal cavity and 10 times the capacity of the stomach, 40% of the entire alimentary tract.
Folded into four parts that flex upon each other.
Thin-walled.
Visible on the external surface is a spiral constriction that runs 20-30 times around the caecum. This is related to the folding of the mucosa internally.
There is an appendix at it's distal end containing lymphoid tissue.

Colon

Has ascending, transverse and descending parts.
The ascending colon is very long and occupies most of the ventral abdomen.
First part has taenia and haustra, the second part does not and is arranged into coils that lie in the dorsal part of the abdominal cavity.
The transverse and descending colons have the same anatomical arrangement as in other species.

Coprophagia

Rabbits eat some of their faecal pellets, called caecotropes, directly from the anus.
Caecotropes differ to normal pellets as they are softer, covered in mucus, smaller and contain large amounts of bacteria and microbes.
The purpose of this may be to gain access to the vitamins produced by bacteria in the caecum.
Some microbial protein may also be available, but is thought to contribute little the animal's requirement, as most is digested in the colon.
- Therefore some amino acids (lysine and methionine) are added to commercial rabbit food.
  - This behaviour is also seen in rodents.

Calcium Metabolism

Rabbits have a unique calcium metabolism.
Calcium is absorbed across the intestinal wall in the absence of vitamin D, which other mammals require for calcium absorption.
Less regulated calcium absorption results in calcium being excreted in the urine.
Parathyroid hormone and calcitonin regulate blood levels.
- Kidney stones and calcification of soft tissue may occur if a high calcium diet is fed to rabbits for a prolonged period of time.