[Originally written as a response to a question on the TortoiseForum.Org. Reprinted with permission.

Coprophagy
(the consumption of feces) is practiced by young torts to establish
intestinal flora necessary for the digestion of food materials
particularly cellulose. Typically, adult torts do not consume their own
or other torts (of the same species) feces. They will consume other
animals feces as a protein or nutrient source and just to gross out
their owners!

In response to a comment made about many hind-gut fermenters that eat their own feces and need to do so for health…
The
‘double-whammy’ or ‘recycling’, otherwise known as caecotrophy, is the
production of partially digested pseudo-fecal (caecal) pellets which
are excreted and immediately consumed. It is limited to a few high
metabolism organisms like rabbits, hamsters and certain other small
mammals. The digestive product of caecal pellets are feces which are
not consumed. This mechanism evolved to eliminate prolonging the
digestive process necessary for fiber fermentation, reducing the load
in the digestive tract and still be able to utilize the nutrients
generated by the shorter fermentation. True caecotrophs will starve to
death if prevented from consuming the caecal pellets.

On ‘hind-gut fermentation as a method of digesting fiber better…
Cellulose
is a structural constituent of plant cell walls. In certain parts and
types of plants the cellulose serves as ‘armor’. Vertebrates are
incapable of producing cellulase (notice the “A”), the enzyme
responsible for breaking down (digesting) cellulose. Certain microbes
in the intestinal flora of many animals, even termites, can produce
cellulase and in most herbivores are harbored within the intestines for
this purpose.

Fiber
(cellulose) itself is not absorbed during digestion. It is actually a
barrier to digestion. The purpose of chewing, foregut fermentation by
ruminants, acid hydrolysis in the stomach and hindgut fermentation in
many higher animals; is an attempt to release and/or convert released
nutrients from within the plant cells and convert some of the cellulose
(via microbial action) into useable nutrients.

Tortoises,
in general, have a rather slow digestive process. The purpose of the
slow digestion is to allow time for the microbial digestion of the
plant cell walls so as to release the nutrients within and the
production of additional nutrients by the microbes themselves for
absorption in the colon like certain B vitamins, vitamin K and fatty
acids. The amino acids, proteins and certain other nutrients generated
by these microbes are mostly lost as there is no means for them to be
absorbed in the posterior (distal) colon.

Overall,
hindgut fermentation does not generate great quantities of nutrients
responsible for energy production. Although up to 30% may occur in the
anterior (proximal) and mid colon, the majority occurs in the small
intestines and ‘functional’ caecum (an eccentric dilation of the
anterior colon in tortoises). The relatively low energy available
precludes hindgut fermentation from providing long-term sustenance. In
almost all cases, prolonged activity during periods of extended fasting
is fueled by fat reserves.

The
research that demonstrates a direct correlation between fiber content
and digesta retention time simply illustrates that low fiber (greens,
fruits, flowers, shoots, etc.) diets, similar to our own, do not
require substantial cellulose fermentation whereas higher fiber
(grasses, sedges, twigs, etc.) diets do. Higher fiber diets reduce the
average digestibility (the amount utilized vs. the amount eaten) as
well – the expected scenario for most tortoises. Tortoises, among
others, are able to modify the rate at which digesta move through the
colon, including reverse peristalsis in some instances, thereby
maximizing the efficiency of the fermentation by retaining the
materials in the digestive tract longer.

Eating
daily does not equate to excreting daily necessarily. Once digested,
the large masses of material are reduced considerably in volume – most
food items are largely composed of water which is absorbed. The
utilized components of the diet comprise a surprisingly small
percentage of the total mass consumed. Depending upon the food
materials consumed defecation may occur from once every several days to
multiple times per day.