Dental microwear texture analysis of extant koalas: clarifying causal agents of microwear

Microscopic wear patterns on teeth, that is, dental microwear, are capable of recording observed dietary behaviour in a diversity of extant and extinct animals. However, recent work has questioned the utility of dental microwear at clarifying dietary behaviour, instead suggesting that dental microwear textures are reflective of grit consumed and not the dietary properties of ingested food. Some suggest that dental microwear cannot reflect the textural properties of vegetation consumed because phytoliths are too soft to form microwear. Koalas (Phascolarctos cinereus) are model organisms for examining dental microwear formation because they consume a specialized diet consisting almost exclusively of eucalyptus leaves, which notably lack phytoliths. Here, we assess if koala dental microwear records a diet consistent with the consumption of tough leaves - despite the absence of phytoliths in their primary food source. Dental microwear texture data of koalas are consistent with tough folivorous diets, with high anisotropy values indistinguishable from folivorous primates and grazing bovids. However, koalas have significantly higher complexity (indicative of hard object feeding) than the folivorous primate Alouatta palliata, and are indistinguishable in this variable from mixed feeding and browsing bovids. While higher complexity values in koalas may result from increased dust and/or grit on the landscape in comparison to folivores that occur in wetter environments, it may also result from the mastication of woody browse. We also determined that complexity and textural fill volume are not significantly greater in drier environments - as one would predict if grit was the major contributor to dental microwear formation. Koalas may instead be selecting softer younger leaves in drier regions where water is more limited. Collectively, microwear associated with tough object feeding can be formed in the absence of phytoliths and grit may interact with food to form microwear reflective of diet, as opposed to overprinting dietary signals.