Body composition and the evolution of the Macropodidae (Potorous, Dendrolagus, and Macropus).
The segmental distribution of body weight and the proportions of skin, muscle, and bone are compared for three genera of the Macropodidae (Potorous, Dendrolagus, and Macropus) and one genus of the Petauridae (Pseudocheirus). Potorous and Macropus possess high proportions of muscle mass to total body weight, high concentrations of musculature in the lumbar extensors, thigh, and tail, and disproportionate ratios of forelimb: hindlimb bone and forelimb: hindlimb muscle which correspond to disproportions of intermembral length. These species converge with high-speed terrestrial runners in some traits and remain distinctive in others. Macropus, larger, more muscular, and faster than Potorous, appears to store and return energy to the hopping cycle more efficiently. Dendrolagus has less than three-fourths the musculature of the other macropod genera, low proportions of the back extensor muscles compared to the other macropods, and relatively more equal ratios of forelimb:hindlimb bone and forelimb:hindlimb muscle. This species converges with slow-moving arboreal climbers such as Pseudocheirus. These data on body mass and tissue proportions translate directly into center of gravity, strength-to-weight ratio, and muscular (kinetic) chains, key elements of macropod evolution. The geometric similarity of muscle between smaller potoroids and larger macropodids, and assumption critical to allometric comparison, is not confirmed.