Diets of the Macropodidae inferred through Dental Microwear Texture Analysis

Author: Sam Arman

  • Thesis download: available for open access on 7 Jan 2021.

Arman, Sam, 2019 Diets of the Macropodidae inferred through Dental Microwear Texture Analysis, Flinders University, College of Science and Engineering

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Kangaroos are the principal endemic vertebrate herbivores of Australia. Evolving in response to increasing aridity in the late Neogene, Pleistocene forms are dominated by two main groups; long-faced macropodines, and short-faced sthenurines. Differences in functional morphology suggest that diet differentiates these groups. Macropodine kangaroos possess high-crowned molars, inferred as an adaptative response to the wear produced by consuming abrasive grasses. In contrast the complex morphology of sthenurine molars is interpreted as an adaptation to browsing on tougher leaves of shrubs and other dicotyledonous plants. Skull length, which most obviously distinguish these groups is also considered a dietary adaptation. The longer diastema between the molars and incisors of macropodines assist in orally manipulating long grasses, while the shorter sthenurine skulls increase the forces of mastication required to break down tough browse.

Inferring diet through functional morphology is, however, limited. Morphology often may only reflect the most restrictive element of an animal’s diet. Moreover, it tells us little about finer-scale differences in diet within or between species. Such fine-scale differences are key to understanding more about these species. Greater understanding of diet can inform about niche partitioning, where different food resources are utilised by different species, facilitating greater regional diversity than competition would usually allow. Understanding dietary change over time can inform on how adaptable species are to changes in vegetation, and potentially feed into conservation of living kangaroos. A narrow diet may leave a species vulnerable to extinction, either directly through loss of that resource, or indirectly through flow-on effects of being bound to the physiological or geographical constraints of that resource. This has particular implications for sthenurine kangaroos, all of which went extinct in the late Pleistocene. Their extinction, alongside numerous other “megafaunal” groups, is the subject of ongoing debate in relation to the influence of climate change and human interaction. Understanding diets of sthenurines, as well as those of all kangaroos, may provide key information on these extinctions, and by understanding how plant resources have been used by different species in the past may help manage kangaroo populations today.

Here we investigate the diets of kangaroos through Dental Microwear Texture Analysis. This method operates by considering the impacts that food make on tooth enamel as animals chew their food. Physical characteristics of different foods, alongside grit adhering to, and phytoliths within foods, alter markings produced on tooth enamel during occlusion. To analyse these markings, high-resolution 3D scans are taken of the molar wear facets. Algorithms are applied to scans to quantify differences in surficial characteristics, and have been shown in numerous mammalian groups to distinguish between species with different diets.

To embark on a study of macropodid microwear first requires that a baseline is established, to allow the diets of extinct kangaroos to be inferred by comparison with living kangaroos of known diets. Such baselines are necessary for each group being studied to ensure that broad cranio-dental differences between mammalian groups do not bias results. To construct this baseline, and get a better understanding of modern kangaroo diets, we collated all published literature of dietary intake for kangaroos. A coarse classification was then established to sort living species into dietary groups: fungivores, browsers, mixed-feeders and grazers. Most species were classified as mixed feeders, which contrasts to similar analyses of herbivorous groups elsewhere. This is a possible adaptation to the often unpredictable environmental conditions in Australia. Very few species are specialist browsers, which supports the notion that this niche may have been largely filled by the now-extinct sthenurine species.

As microwear data collection began, a hurdle was encountered regarding comparability of data collected on different instruments. To minimise the effects of this, a series of filters were established to allow comparability between instruments. This led to further consideration of variation within species. Such questions have been considered elsewhere, with the result being that modern microwear sampling is limited to avoid effects of intraspecific variability. Such sampling practices, however, are limiting for palaeontological purposes, where sample sizes are inherently low. In addition, some differences within species, such as geographic origin of modern specimens, may reflect dietary differences that could assist in fine-tuning the dietary signal. With these matters in mind, we turned to general linear mixed-modelling to incorporate intraspecific factors into models of differences between species. This method includes factors in models only when they can improve the ability of the model to describe the data. Models generated indicated a small number of factors, including facet scanned and ecoregion of specimen origin to have the greatest effect on microwear data. Most important, however, appeared to be the inclusion of each specimen modelled as a random effect, likely to encapsulate natural inter-individual variation. Utilising modelling thus enables broader sampling practices at the same time as incorporating intraspecific variability where present, to enhance our ability to differentiate between species with different diets.

Turning finally to dietary analysis of macropodid microwear, analyses of modern species suggested that only 10 of 28 dental microwear texture analysis algorithms utilised showed any ability to differentiate between species with known differences in diet. Palaeontological analyses added the diverse assemblage from Victoria Fossil Cave, Naracoorte, South Australia. Findings revealed that most sthenurine species were indeed browsers, but that differences were evident between species. Some sthenurine species were strict browsers, while others had more mixed diets or may have been frugivorous. Considerable dietary overlap was also present between most sthenurine and macropodine kangaroos from the same deposit, supporting dietary flexibility as a core feature of all kangaroo diets. These results suggest that sthenurine extinctions were unlikely the result of any climate driven vegetation change.

Keywords: Dental Microwear, Macropodidae, palaeodiet, diet, herbivory

Subject: Biology thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Professor Gavin Prideaux