Many herbivorous animals perform coprophagy, that is they eat their own faeces, in order to gain more value from the relatively low value food that they eat.
Digestion in the stomach and small intestine of rabbits is similar to many other animals. The food that reaches the hindgut is primarily fibre. Rabbits, however, are unlike other animals in that they are able to separate small fibre particles from large ones in the proximal (first section) colon (large intestine). The large fibre particles pass distally (towards the anus) where they are excreted as hard faecal pellets, the small fibre particles pass back up to the caecum to be fermented. The fermentation process produces a paste that contains volatile fatty acids which are absorbed for energy, as well as vitamins, enzymes, amino acids (proteins), and micro-organisms.
For the majority of the day a rabbits colon performs this separation process. Periodically, usually in the morning and evening, the motility of this first part of the colon changes completely. At this point the paste from the caecum is directed rapidly along the colon to be passed as a soft faecal pellet known as a caecotroph. Caecotrophs are different from normal faeces in that they are higher in moisture and protein. These are re-ingested as an extra nutritional source for the rabbit.
The separation process in the proximal colon of a rabbit is achieved in two ways. Firstly, by haustra (sacculations in the wall of the colon) which retain the small particles, leaving the large particles in the lumen of the colon. Movement in the haustra directs the small particles back to the caecem, whereas movement in the lumen directs the large particles to the anus. In addition, rabbits demonstrate a ‘wash-back’ mechanism, fluid secreted into the proximal colon washes bacterial matter back to the caecum. The rabbits’ colonic separation mechanism is very efficient, allowing them to have a lighter digestive tract and therefore maintain the ability to be fast and agile, in comparison to other herbivores.
Lagomorphs (rabbits, hares and pikas), are unique in that they are the only herbivore to separate faeces in this way, i.e excreting the large fibre particles as the hard faecal pellets following only one passage through the gastrointestinal tract (GIT), whilst moving small fibre particles back to the caecum for fermentation before being passed as caecotrophs.
Guinea pigs also perform coprophagy, however, they do not produce caecotrophs. Guinea pigs have a relatively large caecum for their body size, containing 65% of the total contents of the GIT at any one time. Like rabbits, digestion in the stomach and small intestine is similar to other animals. Food exits the stomach after approximately 2 hours, but then will spend approximately 20 hours in the caecum undergoing the fermentation process. Total GIT transit time can be between 8-30hrs. Guinea pigs do exhibit a colonic separation mechanism but this is unlike rabbits in that it traps and returns only bacteria from the caecum, it does not separate fibre particles as in the rabbit. The separation mechanism of the guinea pig is known as the mucus-trap method. Guinea pigs have a groove or furrow in their proximal colon that contains mucous which traps the bacteria from the colon and returns it to the caecum.
The colonic separation mechanism of guinea pigs is less efficient than that of rabbits and therefore their colon is relatively larger and heavier, making them less agile than rabbits. The faeces that guinea pigs produce following one passage through the GIT may be slightly softer than that following a second passage through the GIT but they are not called caecotrophs because they haven’t come directly from the caecum following the fibre particle separation exhibited in rabbits.
Coprophagy is important to guinea pigs and contributes to their nutritional needs. When coprophagy is prevented guinea pigs lose weight, digest less fibre and excrete more minerals. However, guinea pigs require a dietary source of 7 of the 10 B vitamins, whereas rabbits only require a dietary source of 3, as caecotrophs are very rich in B vitamins. (Ferrets, Rabbits and Rodents Clinical Medicine and Surgery, 4th Edition).
Historically, I believe that it was thought that guinea pigs (and other herbivorous rodents) produced caecotrophs. As veterinary medicine has progressed and more detail of the digestive processes have been understood it I has been acknowledged that only lagomorphs produce caecotrophs due to their unique ability to separate fibre particles in their food. Guinea pigs whilst practicing coprophagy do not produce caecotrophs since they do not produce these two distinct excreta resulting from the process of separation in the colon.