Limb development in pouch young of the brushtail possum (Trichosurus vulpecula) and tammar wallaby (Macropus eugenii)

Abstract

AbstractWe examined the allometric scaling of limb bones, forelimb and hindlimb musculature, and bone mineral content of the brushtail possum Trichosurus vulpecula and tammar wallaby Macropus eugenii eugenii to determine whether the pattern of development of these parameters in early pouch development fits in with the mechanical requirements necessary for true climbing following birth. In the brushtail possum the rates of increase of the forelimb and hindlimb bone widths with body mass were generally significantly lower than those predicted from the hypothesis of ‘static strain similarity’, that is 0.375, and were not significantly different from isometry. Conversely, the rates of increase of limb bone lengths with body mass were significantly greater than predicted, that is >0.25 in this species. In the wallaby the rates of increase in the widths of forelimb bones with body mass were lower than predicted by the hypothesis of static strain similarity whereas those of the hindlimb bones were higher. Again the rates of increase in forelimb and hindlimb bone lengths were all greater than predicted by the static strain hypothesis. These findings show that the neonates of both species had short wide forelimb bones better able to withstand the strain of body mass. Mineral content was lower in the forelimb bones of the pouch young of smaller body size in both species whereas the mineral content of wallaby hindlimb bones was relatively decreased in larger animals. The length of long bones scaled with positive allometry in relation to the associated mass of muscles in the limbs of both species, indicating that greater muscle mass per unit length in the neonate may compensate for the relatively low leverage of the neonatal limb bones. However, the more positive allometric scaling of the length of the hindlimb bones in relation to muscle mass indicates that the neonatal hindlimb has relatively greater muscle mass per unit length of bone. Given the relatively gracile form of the hindlimb bones, this suggests that neonatal forelimb morphology in these two species may result from the timing of birth in relation to limb development rather than the precocious specialization of the forelimb for early locomotion.