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Morpho-functional adaptations to digging in Australian marsupials

Martin, Meg Louise (2020) Morpho-functional adaptations to digging in Australian marsupials. PhD thesis, Murdoch University.

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Digging behaviour has evolved across multiple lineages of Australian marsupials and monotremes, some of which are amongst the most specialised diggers in the world. These animals forage for subterranean food sources, while others dig extensive burrow systems for shelter. The scratch-diggings, in turn, assist in soil turn over, water infiltration, nutrient cycling and dispersal of fungi and seeds, thus playing important roles in ecosystem health.

Digging species are capable of generating high out-forces with their forelimbs to excavate soil. As form follows function, forelimb musculoskeletal morphology is expected to be driven by the forces that are imposed by their day-to-day activities, within the constraints imposed by phylogenetic background. In this thesis, I present four studies that quantitatively investigate intra- and inter-specific variation in forelimb adaptations to digging in Australian monotremes and marsupials to examine the link between skeletal morphology and muscle architecture. Representatives of all extant lineages of marsupials (Diprotodontia, Dasyuromorphia, Peramelemorphia, Notoryctemorphia) and monotremes (Monotremata) were used in a correlative study to examine the extent to which functional patterns of limb morphology are influenced by digging behaviour.

Study 1 examined ontogenetic development of muscle architecture (muscle mass (mm) and muscle physiological cross-sectional area (PCSA)) in the Quenda (Isoodon fusciventer). This data demonstrated differential development of the muscles acting as main movers of the power-stroke during digging in comparison to recovery-stroke muscles for force production (PCSA) but on the whole suggested mechanical similarity throughout ontogeny in the sample. Study 2 examined the intraspecific relationship between the ontogenetic development of muscle architecture and 2D and 3D measures of bone shape to reveal that the shape of the scapula, humerus and third metacarpal show significant covariation with muscle anatomy. However, the relationship was not well-represented by bone indices.

In study 3, the covariation between muscle PCSA and bone shape was quantified across a range of species. Bone shape was significantly different between species of different digging abilities; however, differences were not apparent after phylogenetic correction with the exception of the ulnar and third metacarpal shape. A significant link between muscles PCSA and shape was evident, especially for the scapula, humerus and third metacarpal. Study 4 extended the range of species examined for bone measures to reveal that ulnar shape and bone indices show significant differences between behaviour; this relationship was less evident in the scapula, humerus and third metacarpal.

Overall, this collective body of work has quantified the extent to which forelimb muscle architecture and bone shape covary. This thesis also highlights the importance of ontogeny in quantitative studies of muscle architecture, and provides novel models of analysis of post-cranial anatomy. This information furthers the understanding of the complex links between vertebrate form and function. The application of these results will assist in making inferences of the behaviour and ecology of extinct species and the roles they may have played within the Australian ecosystem through time and space.

Item Type: Thesis (PhD)
Murdoch Affiliation: Environmental and Conservation Sciences
Supervisor(s): Fleming, Trish, Warburton, Natalie and Travouillon, Kenny
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