Anatomy and development of the koala, Phascolarctos cinereus: an evolutionary perspective on the superfamily Vombatoidea

Fifteen koalas (Phascolarctos cinereus) - pouched young from 4 to 6.5 month s and 10 adults from 5 to 16.5 years - were analyzed for functional paramet ers (body composition, limb segment and muscle mass, post-cranial skeletal characters) and developmental expressions (growth of body, brain, musculatu re). These data were compared with a convergent eutherian, the three-toed s loth, Bradypus infuscatus, and with the koala's distant (Macropodid; wallab ies) and proximate (Vombatid; wombats) marsupial relatives. Musculoskeletal structures correlated with sitting and climbing, the growth of the young a nd the physiological demands of adulthood correlated with the low-quality d iet of Eucalyptus foliage. The gestalt of the ancestral Vombatoids (pronogr ade quadrupeds, generalist browsers and social conservatives with low basal metabolism and attenuated development) provided the baseline essential for their locomotor and nutritional divergence into arboreal browsers, the koa las, and fossorial grazers, the wombats.