Brief communication: Forelimb compliance in arboreal and terrestrial opossums

Primates display high forelimb compliance (increased elbow joint yield) compared to most other mammals. Forelimb compliance, which is especially marked among arboreal primates, moderates vertical oscillations of the body and peak vertical forces and may represent a basal adaptation of primates for locomotion on thin, flexible branches. However, Larney and Larson (Am J Phys Anthropol 125 [2004] 42-50) reported that marsupials have forelimb compliance comparable to or greater than that of most primates, but did not distinguish between arboreal and terrestrial marsupials. If forelimb compliance is functionally linked to locomotion on thin branches, then elbow yield should be highest in marsupials relying on arboreal substrates more often. To test this hypothesis, we compared forelimb compliance between two didelphid marsupials, Caluromys philander (an arboreal opossum relying heavily on thin branches) and Monodelphis domestica (an opossum that spends most of its time on the ground). Animals were videorecorded while walking on a runway or a horizontal 7-mm pole. Caluromys showed higher elbow yield (greater changes in degrees of elbow flexion) on both substrates, similar to that reported for arboreal primates. Monodelphis was characterized by lower elbow yield that was intermediate between the values reported by Larney and Larson (Am J Phys Anthropol 125 [2004] 42-50) for more terrestrial primates and rodents. This finding adds evidence to a model suggesting a functional link between arboreality--particularly locomotion on thin, flexible branches--and forelimb compliance. These data add another convergent trait between arboreal primates, Caluromys, and other arboreal marsupials and support the argument that all primates evolved from a common ancestor that was a fine-branch arborealist.