The passage of digesta markers through the gut of a folivorous marsupial, the koalaPhascolarctos cinereus

The passage of particulate and solute digesta through the alimentary tract of koalas fed the mature foliage ofEucalyptus punctata was investigated using radioisotopic markers. The solute marker was retained for longer than the particulate marker, the mean retention times of both markers (213 h and 99 h respectively) being longer than has been reported in most other mammals. Both the pattern of excretion in faeces of orally administered markers and data from a slaughter experiment indicated that the principal site of marker retention was the highly developed caecum/proximal colon which appeared to function as a single mixing compartment. An undettermined physiological mechanism caused solutes and fine particles of digesta to be selectively retained in the caecum/proximal colon. It is suggested that this caused the particulate marker to distribute in two pools situated in parallel producing a marker excretion curve different from any reported in other mammals. It is postulated that the rate of clearance of dietary fibre from the hindgut would be maximized by this mechanism and that this in turn would elevate the upper limit to intake of food, and hence digestible nutrient, by koalas. Such an adaptation would partially compensate for the physiological constraints on nutrient acquisition from fibrous diets imposed by small body size and thus may be important in enabling the koala to utilize a diet avoided by most other small herbivores.