By Meg Martin. Digging marsupials play an especially important ecological role in Australian ecosystems by helping with soil turnover, nutrient mixing, seed dispersal and increasing breakdown of organic materials. Many of these species are highly specialised diggers – with strong forlimbs and long claws.
Historically, the interactions between bones and muscle during behaviour has been based on descriptive (qualitative) relationships, but now quantitative approaches are enabling deeper understanding of the evolutionary interactions between muscles and bones. Using CT scans and computer modelling, we can now quantify bone shape in three dimensions. And identify even subtle differences in key landmarks.
The internal architecture of muscles can be measured and then used to estimate how much force the muscle could have produced (physiological cross-sectional area or PCSA). This tells us how the size, shape, architecture of a muscle correspond to the shapes of the bones that it is interacting with.
My first study (Martin et al. 2019 Digging muscles in quenda) was the first to report muscle PCSA as animals grow (ontogeny), providing important information on the intraspecific variation of these characteristics (and one that is often over-looked in other studies). This study showed that in the Quenda (local bandicoot species), muscles used in digging grow at a much greater rate than non-digging muscles.
My second study (Martin et al. 2019 covariation in quenda muscle and bones) demonstrated strong covariation between muscle anatomy and bone shape in the Quenda. This shows that the shape of forelimb bones are driven by muscle development – as the muscles grow, the forces they apply to bones warp and twist bone growth.
A review (Martin et al. 2020 PCSA review) of the methods used to calculate muscle force generation (PCSA) has received international recognition as an important contribution to the field, with correspondence reaching us within days from research groups in North America, the UK, Italy and South Africa. To calculate PCSA, fascicle length, muscle mass and pennation angle are measured. This review provides a ‘one-stop-shop’ for anyone looking to begin collecting this sort of data, and describes the advantages and limitations of each different technique.
The last chapters of my thesis will investigate the links between muscle and bone shape across a range of marsupial species (including bandicoots, wallabies, and carnivorous marsupials) in order to explain varied aspects of forelimb morphology with respect to digging behaviours.
The MU functional morphology lab within the Veterinary Anatomy Department is equipped with computer workstations that support 3-D visualisation and analysis software, photography, photogrammetry, 3-D microscribe, and a wealth of specimens just waiting to be studied. Oh, and did I mention, great supervisors?