Digestive Anatomy
Hindgut FermentationKoalas are hindgut fermenters, unlike many marsupial species who are actually foregut fermenters which synthesize microbial cells in the stomach to then digest in small intestine. Hindgut fermentation loses the microbial cells synthesized in the stomach to feces (Hume 1999). Another disadvantage to hindgut fermentation is that they cannot detoxify toxins as well as foregut fermenters. However, this excludes the phenolics and terpenes which are organic compounds found in eucalyptus, allowing koalas to use it as a food source.
Selective Retention in the CecumAccording to Hume (1999), the small intestine is rather short at 29% of the total intestine length, while the cecum is 23% and 35% of the total volume. Over one meter long, the cecum of the koala has 8 to 14 folds and is responsible, along with the proximal colon, for microbial fermentation and retention of digesta. These folds not only increase the area four-fold, but also allow epithelial interactions with about 45 species of bacteria, including an enterobacteria which degrades tannin-protein complexes, removing nitrogen to be absorbed in the cecum. It absorbs water and nutrients through the intercellular spaces and has a relatively neutral pH at 6.5 and a temperature of 36 degrees Celsius, along with the colon.
The koala retains digesta for quite and amount of time in the hindgut. This allows for an increase in the number of nutrients to be digested. Hume (1999) found that mammals with longer particle retention times had higher proportions of small particles in the cecum and proximal colon than in the stomach or distal colon. This is the selective retention that is so advantageous for the koala. Not only is it able to increase digestion, but it can also ensure that less microbial protein is lost to feces, as well as reduces fibre in large particles. Ultimately, this process of selective retention allows koalas to successfully subsist on such a low nutrient diet (Hume 1999). As well, a product of unformed fecal material derived of the cecum, “pap" is what juvenile koalas, or “joeys,” ingest from their mother as a source of microflora. Reference: Hume, I. D. (1999). Marsupial nutrition. Cambridge: Cambridge University Press.
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Dentition and Particle SelectionAs discussed in the section Anatomy, the koala has an expansive cecum and colon. As hindgut fermenters, the koala performs fermentation in these gastrointestinal locations, selectively retaining certain beneficial solutes and expelling the rest. In the small intestine, the rate of fermentation is limited by particle size which is itself limited by the extent to which the food has undergone mastication. A study by Lanyon and Sanson (2011) found that the "efficiency of mastication” is influenced by tooth wear over the lifetime of the animal, such that “the length of the occluding enamel edges increases to a peak then declines with extreme wear.” As well, “animal with very worn teeth have much larger particles and a greater proportion of these present in the stomach that animals with unworn teeth. However, the increased proportion of large particles in the stomach of older animals is accompanied by only a slight increase in the percentage of the largest size class within the caecum” (Lanyon and Sanson, 2011). To this effect, older koalas are able to perform selective retention of finer particles in the hindgut despite the poorer dentition, potentially due to a greater intake and “gut passage rate.”
Reference: Lanyon JM, Sanson GD. 1986. Koala (Phascolarctos cinereus) dentition and nutrition. II. Implications of tooth wear in nutrition. J Zool Lond Ser A 209:169–181. MerycismIn 2006, M. Logan determined the first recorded account of merycism in the koala. The koalas used in the study were outfit with acoustic transmitters that found rhythmic bouts of “mastication-like” noises that were not a part of any ingestive feeding activity. As well, noises interpreted as guttural regurgitation and re-swallowing were detected. The paper suggests that koalas may perform merycism to re-masticate gastric digesta, "while in a resting position, and thus potentially save energy and increase the extent of food preparation…contribute[ing] to the koala's ability to consume a high fibre, poor quality diet."
While there are no known photos or videos of koalas performing merycism, Figure 1 to the left contains images from Matsuda et al. (2011) of proboscis monkeys performing merycism. References: 1. Logan, M. (2001). Evidence for the occurrence of rumination-like behaviour, or merycism, in the koala (Phascolarctos cinereus, Goldfuss). Journal of Zoology 255, 83–87. 2. Matsuda, I., T. Murai, M. Clauss, T. Yamada, A. Tuuga, H. Bernard, and S. Higashi. "Regurgitation and Remastication in the Foregut-fermenting Proboscis Monkey (Nasalis Larvatus)." Biology Letters 7.5 (2011): 786-89. Web. |