KOALA AND WOMBATS
Koalas (Phascolarctos cinereus) and wombats are hindgut fermenting herbivores (Fig. 14.1b). The wombat colon is like the horse with haustra between longitudinal bands of muscle (Bryant and Reiss 2008).
Koala digestive anatomy and physiology and Eucalyptus spp. harvest, storage and presentation is covered in Blanchard and Bodley (2008). Wombats are generalist terrestrial grazing herbivores while koalas are arboreal specialist folivores, highly adapted to a diet of Eucalyptus leaves with high levels of plant secondary metabolites (PSMs). Their low BMRs and energy requirements allow them to survive on low energy diets (Table 14.2) (Casey et al. 2021, 2023).A comparison of gut microbiomes has identified commonalities and differences that allow koalas and wombats to process their natural diets (Shiffman et al. 2017). Microbial involvement in lignocellulose degradation and urea cycling are apparent in both. Koalas and wombats have the highest relative proportion of cellulases (9 and 12% respectively) compared to foregut fermenting herbivores like the wallaby (4%) or cow (2%). Koalas use prolonged chewing, mercycism and both caecal and colon fermentation to extract as much nutrition from their diet as possible (Logan 2001). Koala gastrointestinal microbes appear to influence diet selection, be maternally inherited and contain species in the Family Synergistaceae, which may be specialised to metabolise PSMs (Blyton et al. 2022). Faecal transplant resulting in acceptance of less preferred Eucalyptus species may have powerful implications to translocations (Blyton et al. 2019; 2023).
Koalas prefer young eucalyptus leaves and tip. Preference is not evidence that tip provides better nutrient value than mature leaves but rather may be driven by the higher nitrogen and lower fibre content as well as increased moisture (Cochrane et al. 2023).
Climate change may be playing a role in reducing the water content of leaves to less than 55-65% resulting in free-ranging koalas seeking fresh water (Seebacher and Franklin 2012; Mella et al. 2019). Harvesting and storage procedures for koala browse that minimises moisture loss are essential. Maintaining as much moisture as possible by reducing time from harvest to feeding, storing in water and keeping cut browse in shade is important to maintain hydration. Keeping browse clean and free from soil and dust will also slow dental wear as animals age (Hedberg and DeSantis 2017).The wombat has a relatively small caecum and fermentation primarily occurs in the colon (specifically the proximal region), which is ~70% of the total GIT capacity (Eisenhofer et al. 2022). The wombat microbiome provides more xylanases (7.6% v. 1.9%) to process a diet higher in hemicellulose while the koala’s diet is higher in lignin and phenolics that are metabolised with increased levels of lignases, peroxidases, and tannases (Casey et al. 2021). Wombat diets too high in simple carbohydrates and low in abrasive grasses, forbs and sedges, create the opposite issue to koalas of insufficient dental wear (Fagan and Ullrey 2008; Stannard et al. 2021). Due to their open- rooted, continuously growing teeth, this can lead to malocclusion (see Chapter 37). As wombats exhibit an opportunistic feeding strategy, overconsumption of invasive plant species, such as those high in oxalates, increases their risk of associated dental disease both ex situ and in situ (Casey et al. 2023).
7.