| Popis: |
Most studies of animal locomotor biomechanics examine movement on a level, flat trackway. However, small animals must negotiate heterogenerous terrain that includes changes in orientation and diameter. Furthermore, animals which are specialized for arboreal locomotion may solve the biomechanical problems that are inherent in substrates that are sloped and/or narrow differently from animals which are considered terrestrial. Thus I studied the effects of substrate orientation and diameter on locomotor kinetics and kinematics in the gray short-tailed opossum ( Monodelphis domestica ). The genus Monodelphis is considered the most terrestrially adapted member of the family Didelphidae, but nevertheless these opossums are reasonably skilled at climbing. The first study (Chapter 2) examines the biomechanics of moving up a 30° incline and down a 30° decline. Substrate reaction forces (SRFs), limb kinematics, and required coefficient of friction were measured. On sloped substrates, M. domestica moved more slowly with a higher duty factor, used more statically stable gaits (decline), and required a greater coefficient of friction to avoid slipping. These data suggest that a 30° slope is enough to perturb the opossums’ normal mode of locomotion, and they must therefore adjust their locomotor patterns to remain stable. On inclines, both limb pairs supported body weight equally, and the craniocaudal limb excursion increased; however, the forelimbs exerted greater propulsive impulse than hindlimbs. On the decline, the forelimbs were brought to bear far more body weight and braking effort than the hindlimbs; perhaps the greater forelimb protraction (at touchdown) was a way of accommodating a more substantial load during downhill locomotion. The second and third studies (Chapters 3 & 4) tested the effects of substrate diameter on locomotor kinetics and kinematics. On the arboreal substrate, many kinetic patterns were similar to those observed on the terrestrial. Forelimbs exhibited higher vertical impulse and peak vertical force than hindlimbs, and both limb pairs exerted a braking force followed by a propulsive force during each stride. However, the forelimbs exerted more than twice the braking and propulsive impulses than hindlimbs. The manus was placed higher around the circumference of the branch than the pes. The shifts in forces and limb placement resulted in a lower required coefficient of friction in the forelimb. Thus, the forelimbs are probably more stable than the hindlimbs, and this may explain why forelimbs have such a dominant role on the branch. Although vertical impulses were lower on the terrestrial substrate than on the arboreal support, this was most likely due to speed effects because the opossums refused to move as quickly on the arboreal trackway. Vertical impulse decreased significantly faster with speed on the arboreal substrate because most of these trials were relatively slow, and stance duration decreased with speed more rapidly at these lower speeds. A decrease in speed is a common behavioral adaptation to maintain stability. Stride length, frequency, and duration were well-correlated with speed, but spatial variables were not. Thus it is possible that timing variables were affected by speed, while substrate affected mostly spatial variables (joint angles and limb placement). The distal elements of the forelimb were significantly more adducted on the arboreal substrate, but otherwise there were few substrate effects on the forelimb. It is possible that the relatively stable placement of the manus permits the forelimb to make few kinematic adjustments for arboreal locomotion. In contrast, substrate had many significant effects on hindlimb kinematics. Like the forelimb, the distal elements of the hindlimb were significantly adducted on the arboreal trackway. On the arboreal trackway, the hindlimb was more protracted at touchdown and time of peak vertical force, and hip height was greater. The pelvic girdle of the opossums underwent lateral undulation regardless of substrate. The lack of crouching behavior on the branch may suggest that crouching behavior is not a universal adaptation to treacherous substrates. Finally, the posterior shift in weight support observed in Chapter 3 may be the result of relatively protracted hindlimbs on the arboreal trackway. |
