Limb posture and locomotion in the Virginia opossum (<i>Didelphis marsupialis</i>) and in other non‐cursorial mammals

TLDR

Past studies of mammalian posture and locomotion have focused mainly on cursorial species, leading to a stereotyped view that limbs are vertically oriented and move parasagittally. The study examines walking movements in eight mammalian species, including the Virginia opossum, to assess limb posture and excursion. Using cineradiography, the authors measured limb posture and excursion relative to parasagittal and horizontal planes in the eight species. The results show that only the cat and hyrax exhibit the classic vertical, parasagittal limb posture, while the other six species—including the opossum—display more horizontal and oblique limb orientations with variable excursion patterns, indicating that the traditional view of mammalian posture is inaccurate and that non‑cursorial mammals provide more realistic models for ancestral locomotion.

Abstract

Past studies of mammalian posture and locomotion have been made principally on cursorial species and current concepts are stereotyped accordingly. Mammalian limbs are usually characterized in terms of vertical orientation and parasagittal excursion; the assumption prevails that this type of stance and limb movement is typical of the class. In the present study, walking movements in eight mammalian species (Tachyglossus aculeatus, Didelphis marsupialis, Tupaia glis. Mesocricetus auratus, Rattus norvegicus, Mustela putorius, Heterohyrax brucei , and Felis domestica ) were studied cineradiographically with particular attention given to limb posture and excursion relative to the parasagittal and horizontal planes. Only the cat and, to a lesser degree, the hyrax conform to the postural and locomotory pattern that has been regarded as characteristic of most terrestrial, quadrupedal mammals. In the other six species, the humeri and femora usually function in positions more horizontal than vertical and at angles oblique to the parasagittal plane. Furthermore, the excursion pattern of these species have interspecific differences; some patterns are more variable than others. The classical conception of “mammalian posture” and “mammalian locomotion” is inaccurate both as a description of the features possessed in common by living terrestrial mammals and as a hypothetical approximation of the condition in ancestral mammals. At present, non‐cursorial mammals such as the opossum and tree shrew are more realistic models on which to base deductions concerning posture and locomotion in eatly mammals.