Morphological correlates of pentapedal locomotion in kangaroos and wallabies (Family: Macropodidae)

Across the animal kingdom, many animals engage the tail as part of the locomotor repertoire. However, nothing compares to the use of the tail as a fifth leg while moving at slow speeds, known as pentapedal locomotion. During pentapedal locomotion, the tail is used to support the body weight while the hindlimbs are drawn forward, and to provide power to propel the body weight forward in kangaroos and wallabies. The central aim of this project is to identify behavioural and morphological correlates of pentapedal locomotion within the Macropodidae.

The adoption of pentapedal locomotion is often attributed to all members of the subfamily Macropodinae. Study 1 of this project tested this assumption using a landmark-based approach to classify locomotion of 16 macropodine species as either pentapedal locomotion or quadrupedal bounding (no use of tail support) at slow speeds. Macropus agilis, M. fuliginosus, M. irma, M. robustus, M. rufogriseus, Onychogalea unguifera and Wallabia bicolor species used pentapedal locomotion, while Petrogale xanthopus, M. eugenii, P. lateralis, M. parma, Thylogale thetis, T. stigmatica, T. billardierii, and Setonix brachyurus engaged in quadrupedal bounding at slow speed. The utilization of pentapedal locomotion was linked with the lengthening of the tibia (relative to the femur) and the preference for open habitat.

Study 2 investigated the caudal musculature of the pentapedal western grey kangaroo, M. fuliginosus. This was carried out to establish a detailed account of the muscular anatomy prior to subsequent studies of bony morphology. The remarkable features of this species include the large ventral flexors along the entire tail, the heavy development of the lateral flexors that connect the proximal tail to the pelvis, and the fleshy sacrocaudal musculature that span from the end of the thoracic region to the eighth caudal vertebra. These features reflect the repositioning of the tail under the pelvis prior to the hindlimb swing phase of pentapedal locomotion, and the extension of the tail which occurs during the bipedal hopping cycle.

In Study 3, a comparative approach was undertaken to elucidate how the first fifteen caudal vertebrae (Ca) are specialised for the use of the tail during pentapedal locomotion. Seven functional measurements, based on caudal vertebral dimensions, were used to construct caudal vertebral profiles for 14 species of macropodines categorized into one of four groups with functionally different uses of the tail - pentapedal macropodines, quadrupedal bounding macropodines, rock-wallabies and tree-kangaroos. Pentapedal macropodines demonstrated adaptations of the centrum to resist sagittal plane bending and to bear load in Ca5-13. Additionally, it was evident that large terrestrial kangaroos have adaptations of caudal vertebrae that give stability during bipedal hopping, while smaller wallabies, rock-wallabies and tree-kangaroos have caudal vertebrae that are adapted for lateral mobility as part of their locomotor repertoire.

Study 4 used a geometric morphometric approach on three key caudal vertebrae (Ca1, Ca5 and Ca10) in sthenurines and macropodines. This methodology was used to test differences between the four groups of extant Macropodidae identified in Study 3, and to test the hypothesis that sthenurines (the extinct sister clade to Macropodines) did not engage in pentapedal locomotion. Pentapedal macropodines differ in morphology from all other macropodines in Ca1 and Ca5, but to a lesser extent in Ca10. Sthenurine caudal vertebral morphology was found to be significantly different to pentapedal macropodines in Ca1 and Ca5, and there were also significant differences between sthenurines and nonpentapedal macropodines. The outcomes from this study were two-fold. First, it corroborated the functional analyses of Ca1-15 in extant macropodines in Study 3. Second, sthenurines were found to lack adaptations associated for the use of the tail in pentapedal locomotion and bipedal hopping. These findings support the hypothesis that sthenurines were unlikely to have adopted a locomotor repertoire similar to extant macropodines. Moreover, sthenurine caudal vertebral morphology is indicative of the adoption of an upright bipedal posture and a striding gait.

These four studies found distinctive morphologies of the caudal vertebral and muscular anatomy that are associated with pentapedal locomotion. This thesis further highlights that the adoption of pentapedal locomotion is associated with extreme specialisation of the body plan for bipedal hopping within the Macropodinae. Importantly, the diversity in tail use as part of the locomotor repertoire reflects the adaptive radiation that has characterised the evolution of the Macropodidae.