ANATOMY AND PHYSIOLOGY
1.1 Gastrointestinal microbiota
The koala’s hindgut is the primary site for microbial fermentation. Molecular examination of the contents of the caecum and colon and faecal pellets suggests that the diversity of microbial flora is fairly consistent across different anatomical sites, but the ratios of predominant bacterial phyla (namely Firmicutes and Bacteroides) vary considerably, especially between the caecum and distal colon (and faecal pellet) (Barker et al.
2013). When faecal pellets are used for transfaunation (v. pap [caecal content]), these differences may significantly affect the success of hindgut recolonisation. At a population level, geographical location has a strong influence on the microbiome of koalas, such that populations closer to each other geographically tend to have more similar microbiomes (Blyton 2020). Differences in microbiome composition appear to be linked to digestibility of browsed leaf species (Brice et al. 2019; B Moore pers. comm.) and are accompanied by changes in the functional potential of the microbiome. This may have implications for koala translocation. Genetic sequencing has identified 37 species of coprophilous fungi in koala faeces from the Family Muc- oraceae, Phylum Ascomycota and Phylum Basidiomycota (Peterson et al. 2009). All fungi produce high levels of cellulases, tannases, hemicellulases and ligninases essential for enzymatic degradation of eucalypt leaf (Peterson et al. 2009). Alteration or loss of bacterial and fungal biota in the koala hindgut is likely to be a key factor in the pathogenesis of caeco-colic dysbiosis/typhlocolitis syndrome (see section 4.1.1). In particular, the loss of the koala-specific protein-tannin-complex-degrading bacterium Lone- pinella koalarum from the gut microbiome following antibiotic treatment was significantly correlated with fatal outcomes in koalas undergoing rehabilitation (Dahl- hausen et al. 2018). Microbiome profiling of urogenital swabs from Chlamydia-free (PCR-negative) koalas, with and without ‘wet bottom’ (urinary incontinence), identified differential variations in microbial diversity between the two groups (Legione et al. 2018). The significance of these variations is currently unclear, but may suggest an alternative aetiology for ‘wet bottom’ or reflect alterations in microbial diversity within the urogenital tract as a result of persistent incontinence.1.2 Thermoregulation
Intraperitoneal temperature-sensitive radio-transmitters and iButton® (iButtonLink Technology, Whitewater, WI, USA) data-loggers were used for evaluating core body temperature in free-ranging koalas in south-east Qld, as a means to understand habitat and microclimate selection (Adam et al. 2016). Although koalas pant and sweat from plantar and palmar skin, and use some behaviours to assist in cooling, they have a relatively inefficient suite of mechanisms to deal with hyperthermia. Free-ranging koalas are more likely to utilise understory trees or those with dense canopies during hot weather (S FitzGibbon pers. comm.) and rest with their chest pressed against the cooler trunk of smooth barked trees (commonly Queensland blue-gum [Eucalyptus terreticornis]) (Briscoe et al. 2014) during periods of high ambient temperature. During extremely hot weather koalas may take refuge in caves and hollow logs (B Nottidge pers. comm.). Results from implanted thermal loggers suggest that koalas undergo a diurnal rhythm in body temperature, with highest core body temperatures (maximum 37.7°C) in the late afternoon and early evening and lowest (34.2°C) in the morning. Mean variation in maximum and minimum body temperature between individuals was 2.6°C and 0.6°C, respectively, and may be reflective of individual activity level (Adam et al. 2016). Rectal temperature readings correlated positively with thermal logger readings, but often underestimated core body temperature by ~0.25°C (Adam et al. 2018).
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