Musculoskeletal design in macropodoid marsupials: Acceleration mechanics, body size and habitat use

The goal of this dissertation was to examine hindlimb musculoskeletal design and its relationship to hopping mechanics, body size and habitat use in macropodoid marsupials to better understand the potential trade-offs that may exist with musculoskeletal specialization. Kangaroos and wallabies (superfamily Macropodoidea) share a similar general body plan specialized for hopping locomotion, yet they span a wide range of body size and inhabit many diverse environments which likely place a range of mechanical demands on their musculoskeletal system.;In this dissertation I employ a broad range of experimental techniques to test the hypothesis that a division of labor exists within the hindlimb which enables specialized distal muscle-tendon units to store and return elastic energy, while proximal muscles are employed to modulate mechanical work during steady, and more importantly, non-steady locomotion. Further, I test the hypothesis that a musculoskeletal system which favors elastic energy storage may limit acceleration capacity. I explore the ramifications of this in terms of the habitat specialization of two species of wallabies that inhabit different environments. Finally, I examine the implications of body size and habit utilization on musculoskeletal design in a broad sample of macropodoid species. The results of this work show that although all macropodoids retain a similar body plan, some musculoskeletal variation exists, specifically at the ankle extensor muscle-tendon units. Variation in distal limb design enables different species to meet the demands of their environment. This variation does appear to represent a trade-off in locomotor performance, such that rock adapted species that must generate high forces for jumping have thicker ankle extensor tendons which provide higher tendon safety factors but store less elastic strain energy compared with species that inhabit relatively level environments. In at least one wallaby species (M. eugenii) known to benefit from elastic energy storage in distal tendons, the ability to accelerate is not limited by tendon thin tendons and a division of labor clearly exists such that proximal muscles provide the work required during incline hopping and accelerations. Finally, elastic energy storage capacity is strongly correlated with body size and the trade-off that exists between energy storage capacity and tendon safety factor likely limits body size in the Macropodoidea.