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Sun - Mar 26


By Cilla Norris

The Ringtail Possum is a Foliovore, which means that it eats leaves, such as native Eucalypts. The energy source obtained is almost all from the cellulose in the plant fibre of the leaves. However cellulose is not only a poor food source but is also difficult to break down and digest. So how can a ringtail get enough nutrition from such a highly-lignified (woody) and fibrous diet as that of gum leaves? For a ringtail has relatively low gut volume compared to its body mass, and has high nutrient requirements for its high metabolic rates as a very active little animal. This problem is solved by specialised bacteria in its Caecum, and a clever 'separation-system', which will now be explained.

Below is a diagram of the Gastro-intestinal tract of the Ringtail Possum


The Ringtail Possum is a Hind-Gut-Fermenter, which means that it ferments the leaves it eats in a large fermentation chamber, in the form of a greatly-enlarged Caecum, which is towards the hind end of the Gastro-Intestinal-Tract (GIT). Within this large caecum are specialised bacteria that ferment the cellulose leaf matter to make it easier to digest. Without these bacteria, the possum would not be able to digest its food, and would become weakened and eventually die. Also in the caecum are microbes that synthesise protein and B-vitamins vital to the possum. The fermentation takes time, which may account for the large size of this 'fermentation-chamber' which is able to hold a large volume of matter. The caecum takes up a lot of space in the abdominal cavity. It is important to note that starchy foods, such as oats and bread, should not be fed to possums, as the digestion of these raises the body temperature of the animal, which in turn may cause disease.

A ringtail chews its leaves very thoroughly, grinding them down using its molars at the back of the mouth, while at the same time producing saliva which mixes in with the leaves. The saliva contains enzymes which start to breakdown the plant matter, in particular any starches, to make it slightly more digestable. The chewed-up leaf-matter then passes into a small glandular stomach, not unlike our own, where different digestive enzymes break it down further; then it passes into the small intestine for more processing. By then, although any carbohydrates, proteins and fats are being digested, the cellulose plant fibre is still fairly intact. To process this cellulose, specific bacteria are required, and these are found in the caecum. And so the plant-matter passes into the caecum as a sort of temporary detour before it is lost from the system.

Once the cellulose plant matter has been fermented and digested, it passes out of the caecum into the large intestine, then is excreted out of the body from the cloaca, in pellet-form. However, cellulose is a poor food source and there will still be more nutrients left in those excreted pellets that the fermentation and digestion processes were unable to extract. And that is apparently too good to waste. So the possum 'recycles' it. It eats the pellets it has just excreted, and so, that partially fermented and partially digested plant matter, goes right through the whole system again. So to salvage all the nutrients, ringtails re-ingest and re-digest their pellets. But it's not quite that simple....

Ringtail Possums produce 2 types of pellets, namely Faecal pellets and Caecal pellets. Faecal pellets are hard, dark and low in protein, and contain the unwanted, re-cycled matter. The possum excretes these at night while it's climbing through the trees. These faecal pellets are the one's seen on the ground in the morning. However, the caecal pellets are soft and greenish, high in protein, and still contain undigested nutrients. It is these that are re-eaten by the possum during the day, while the possum is asleep. A ringtail sleeps sitting on its haunches, curled in a ball, with its nose next to its cloaca, ready to consume the caecal pellets. By passively consuming the caecal pellets, the ringtail is also saving more energy by not running around the trees all night to find the correct leaves to eat.

The nutritious, protein-filled, caecal pellets are often fed to the babies, in a similar way to Koalas feeding their babies green 'pap', which is well -chewed, gum leaf pulp. Koalas are also hind-gut-fermenters, along with Greater Gliders. This is done to increase the gut flora, or gut bacteria, enabling the baby to ferment and digest the leaf matter - a bit like a 'starter-culture'. Eating its own pellets defines the ringtail as Copraphagic. It is also Caecotrophic, which means that it is 'drawn towards' the caecal pellets. But how does this clever mechanism of seperating faecal matter from caecal matter work? Up until 2002/3, this was not understood, but the research done by Dr. Fiona Herron of the University of Sydney, has given us an answer.

Dr. Herron used 'radio-active' markers as a fundamental aid to her research, and when she got the results, she happily agreed to give a talk to WIRES SPAC (Sydney Possum Advisory Committee) on her findings. As it turned out we were privileged to be amongst the first people to hear these as-yet unpublished results ! Dr. Herron generously wrote us a simplified version of her thesis, an excerpt from which, is shown below in the following 3 diagrams.

Where A = stomach, B = small intestine, C = caecum, D = proximal colon and E = anus.
When the ringtail possum is out feeding at night, the separation mechanism is 'switched ON'. This means that the small particles and easily digestible materials are returned back into the caecum (the storage tank) for further digestion and fermentation. During this time, hard faecal pellets are produced. These are discarded and can be seen on the ground around the trees where the possums have been feeding.


Where A = stomach, B = small intestine, C = caecum, D = proximal colon and E = caecotrophes and F = anus.
During the day the ringtail returns to it's nest and curls into a ball. It is at this time that the separation mechanism is 'switched OFF' and the contents of the caecum are released as soft faecal pellets (or caecotrophes) and are eaten directly by the possum. It is in this way that the possum is able to gain maximum nutrients from it's feed by this second digestion, without having to exert any extra energy.


Proposed digesta flow in the ringtail possum

Initially, it was proposed that separation mechanism was in the proximal colon. However, my studies suggest that due to lack of anatomical structure in the proximal colon, the separation mechanism is more likely to be in the caecum minor, as illustrated in the above diagram.

However, further studies must be undertaken before this can be confirmed.


Thanks to Scientists like Dr. Fiona Herron, and her supervisor, the eminent Professor Ian Hume, renowned for his work on the gut of the Koala, Greater Glider and Ringtail Possum, we can now see how vital it is that we feed our Ringtails with their proper native diet.

©Cilla Norris 2004

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