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Small hcrbivorous animals living on coarse food during cold or dry periods require a mechanism to be able to eat large quantities of food without losing the micro-organisms they need for fermentation. They use colonic separation mechanisms (CSM) that allow rapid transport of the less digestible food particles through the digestive tract whilst at the same time they retain micro-organisms and easily digestible food particles in their caecum for sufficiently long time periods in order for fermentation and microbial reproduction to take place. As a result of CSM, micro-organisms accumulate in the caecum. All species provided with a CSM utilise microbial proteins and vitamins via caecotrophy. A comparison is made between species belonging to the order Lagomorpha (hare, rabbit, pika) and species belonging to two suborders of Rodentia [myomorph rodents (rat, lemming, vole) and caviomorph rodents (guinea pig, chinchilla, nutria)].The effects of the CSM are similar amongst the species studied, however, the efficiency of the CSM differs between individual species independent of the orderisuborders to which they belong. The anatomy of the caecum and proximal colon is similar within the lagomorph order, but different compared to the rodents. It is more or less similar within each of the rodent suborders but differs between suborders. The mode of trapping and returning micro-organism is similar in myomorph and caviomorph rodents but different compared to the lagomorph species. Outside of the lagomorph and rodent orders CSM producing similar effects are found only in a few marsupial species. The anatomical and physiological background of the CSM in these species is not known.

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Morphologically the large intestine of the nutria resembles that of other caviomorphs, notably the guinea pig. The cecum is voluminous: it contributes 8.6% of the total intestinal length and 12.7% of the total intestinal surface area (considering the surface enlargement factor). It contains 27-32% of the wet ingesta and 20-23% of the dry matter in the gastrointestinal tract. In the colon the corresponding figures are: 21.8% of length, 12.6% of surface area, 16-21% of wet ingesta, and 16-40% of dry matter. The colon can be subdivided both structurally and functionally into two sections, the proximal and the distal colon, the border between the two being the apical flexure of a long parallel loop. The proximal colon (42% of colonic length) displays on the mucosal surface of its mesenterial side a narrow furrow bordered by ridges, which is absent in the distal colon. The ridges contain subepithelial accumulations of coiled tubuloalveolar mucoid glands, entwined by bundles of muscle fibers. Determinations of nitrogen in the contents near the furrow suggest a concentration of bacteria in this part of the lumen. It is hypothesized that the structural differentiations of the proximal colon provide mechanisms for the transport of bacteria from the proximal colon back into the cecum to maintain the fermentation function. The slopes of the longitudinal profiles for dry matter and for concentrations of sodium, potassium and calcium in the luminal contents change at the tip of the parallel loop. The electrical potential difference "intestinal lumen - blood" is particularly large in the proximal colon, indicating active electrogenic ion transport in this region.