Pet
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Review

Your rabbit's digestive system

Your rabbit has a rather strange digestive system, when compared with ours, but evolution has ensured that it is highly efficient, so that a rabbit can extract the maximum amount of nutrients from the often sparse forage available in the wild.

The digestive system of the rabbit is evolved to eat large amounts of grass with a high fibre content. Fibre is fermented by bacteria in the large bowel to produce caecotrophs which are expelled and then eaten to provide vitamins and other essential nutrients.

This is how this remarkable system works:

In an adult (4-4.5 kg) or semi-adult (2.5-3 kg) rabbit the total length of the alimentary canal is 4.5 to 5 m. After a short oesophagus there is a simple stomach which stores about 60-80 g of a rather pasty mixture of feedstuffs.

The adjoining small intestine is about 3 m long and nearly I cm in diameter. The contents are liquid, especially in the upper part. Normally there are small tracts, about 12 cm long, which are empty. The small intestine ends at the base of the caecum. This second storage area is about 40-45 cm long with an average diameter of 3-4 cm. It contains 100-120 g of a uniform pasty mix with a dry matter content of about 20 percent.

Very near the end of the small intestine, at the entrance to the caecum, begins the exit to the colon. The caecum thus appears to be a blind pouch branching off from the small intestine-colon axis (Figure 2). Physiological studies show that this blind pouch-reservoir forms part of the digestive tract: the contents circulate from the base to the tip passing through the centre of the caecum, then return towards the base, along the wall. The caecum is followed by a 1.5 m colon: this is creased and dented for about 50 cm (proximal colon) and smooth in the terminal section (distal colon).

The alimentary canal, which develops rapidly in the young rabbit, is nearly full size in an animal of 2.5 kg, when it has reached only 60-70 percent of adult weight.

Two major glands secrete into the small intestine: the liver and the pancreas. Bile from the liver contains bile salts and many organic substances but no enzymes. Bile aids digestion catalytically. The reverse is true of pancreatic juice which contains a sizable quantity of digestive enzymes allowing the breakdown of proteins (trypsin, chymotrypsin), starch (amylase) and fats (lipase).

DIGESTIVE TRACT AND CAECOTROPHY

Feed eaten by the rabbit quickly reaches the stomach. There it finds an acid environment. It remains in the stomach for a few hours (3-6), undergoing little chemical change. The contents of the stomach are gradually "injected" into the small intestine in short bursts, by strong stomach contractions. As the contents enter the small intestine they are diluted by the flow of bile, the first intestinal secretions and finally the pancreatic juice.

After enzymatic action from these last two secretions the elements that can easily be broken down are freed and pass through the intestinal wall to be carried by the blood to the cells. The particles that are not broken down after a total stay of about 11/2 hours in the small intestine enter the caecum. There they have to stay for a certain time, from 2 to12 hours, while they are attacked by bacterial enzymes. Elements which can be broken down by this new attack (mainly volatile fatty acids) are freed and in turn pass through the wall of the digestive tract and into the bloodstream.

The content of the caecum is then evacuated into the colon. Approximately half consists of both large and small food particles not already broken down, while the other half consists of bacteria that have developed in the caecum, fed on matter from the small intestine.

So far, the functioning of the rabbit's digestive tract is virtually the same as that of other monogastric animals. Its uniqueness lies in the dual function of the proximal colon. If the caecum content enters the colon in the early part of the morning it undergoes few biochemical changes. The colon wall secretes a mucus which gradually envelops the pellets formed by the wall contractions. These pellets gather in elongated clusters and are called soft or night pellets (more scientifically, caecotrophes). If the caecal content enters the colon at another time of day the reaction of the proximal colon is entirely different.

Successive waves of contractions in alternating directions begin to act; the first to evacuate the content normally and the second to push it back into the caecum. Under the varying pressure and rhythm of these contractions the content is squeezed like a sponge. Most of the liquid part, containing soluble products and small particles of less than 0.1 mm, is forced back into the caecum. The solid part, containing mainly large particles over 0.3 mm long, forms hard pellets which are then expelled. In fact, due to this dual action, the colon produces two types of excrement: hard and soft.

The hard pellets are expelled, but the soft pellets are recovered by the rabbit directly upon being expelled from the anus. To do this the rabbit twists itself round, sucks in the soft faeces as they emerge from the anus, then swallows without chewing them. The rabbit can retrieve the soft pellets easily, even from a mesh floor. By the end of the morning there are large numbers of these pellets inside the stomach, where they may comprise three quarters of the total content.

From then on the soft pellets follow the same digestive process as normal feed. Considering the fact that some parts of the intake may be recycled once, twice and even three or four times, and depending on the type of feed, the rabbit's digestive process lasts from 18 to 30 hours in all, averaging 20 hours.

Half the soft pellets consist of imperfectly broken-down food residues and what is left of the gastric secretions, and half of bacteria. The latter contain an appreciable amount of high-value proteins and watersoluble vitamins. The practice of caecotrophy therefore has a certain nutritional value. (source www.fao.org)

If the rabbit’s diet consists of a high percentage of cereal-based pellets containing easily digested carbohydrate rather than grass or hay which is high in fibre, an excessive number of caecotrophs will be produced. These stick to the bottom and may look like diarrhoea. This is known as “sticky bottom”.

See also Rabbit Feeding