When marsupials can’t run: hindlimb morphology and escape behaviour in critical weight range species

Natasha Tay

Australian fauna within the ‘critical weight range’ (CWR; 35 g–5.5 kg) have been disproportionately affected by introduced eutherian predators such as the red fox (Vulpes vulpes) and feral cat (Felis catus). The impact of these predators is more than double that of native Australian predators, likely due to the lack of co-evolution between predator and prey. While anatomy and biomechanics are commonly listed as key factors determining escape success, few studies examine anatomical constraints on escape behaviour. Understanding the anatomy of our native fauna may help determine if there are intrinsic traits that limit CWR marsupials’ ability to escape from introduced predators with hunting strategies that they did not evolve alongside. I first quantified and characterised the escape behaviour of eight CWR terrestrial taxa representing quadrupedal bandicoots and bipedal macropodoids to determine if differences in how they escape indicate their ability to respond appropriately and effectively to introduced predators (Chapter 2). Wild-caught animals were filmed escaping through a runway, and measures relating to their gait, speed and path characteristics were recorded. I found a strong link between the phylogenetic relatedness of species and their use of linear speed or agility when fleeing. This suggests specific escape tactics may be constrained by morphology, although animals increased the intensity of their response after repeated exposure, suggesting training could enhance effective antipredator responses. To further explore the effects of repeated predator exposure on escape behaviour, I present a case study of the burrowing bettong (Bettongia lesueur) and compare two fenced populations: one that had been purposely exposed to feral cats, while the other had been maintained without introduced predators (Chapter 3). My results suggest the cat-naive animals were less motivated to flee than their cat-exposed counterparts. Introducing low levels of predation pressure can successfully increase the intensity of escape behaviour but did not alter the overall escape tactic (i.e. linear speed vs. agility from Chapter 2). To investigate the musculoskeletal system driving locomotion, I have described the hindlimb anatomy in macropodoids and investigated modifications of the hindlimb musculature that relate to locomotor specialisations within this diverse group (Chapter 4). I present data from dissections of seven species and synthesise the available literature to examine patterns in hindlimb muscles across 11 genera. Species that inhabit complicated substrates (arboreal or rocky habitats) had more differentiated muscles throughout the limb. In contrast, species that employ a more specialised endurance hopping gait had less differentiation throughout the limb and greater relative masses in their gluteal and adductor muscles. These patterns provide a platform for understanding the adaptations within the hindlimb for different locomotor strategies within marsupials. To further characterise how the hindlimb anatomy may differ between bandicoots and macropodoids, I assessed how muscle architecture scaled with increasing body mass in the two groups (Chapter 5). I found strong positive allometry in muscles that contributed to hip and knee extension in the macropodoids but not in bandicoots. These differences reflect specialisations not only for the bipedal gait but also for strong forward propulsion in macropodoids, which contributes to their specialisation for efficient locomotion at high speeds. Overall, my research presents an integrated view of morphology and locomotor behaviour to understand better the mechanistic basis of escape performance in CWR terrestrial marsupials.

When marsupials can’t run: hindlimb morphology and escape behaviour in critical weight range species

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