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First published online November 5, 2004
Journal of Experimental Biology 207, 4325-4336 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01231

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The biodynamics of arboreal locomotion: the effects of substrate diameter on locomotor kinetics in the gray short-tailed opossum (Monodelphis domestica)

Andrew R. Lammers^1,* and Audrone R. Biknevicius²

¹ Department of Biological Sciences, Ohio University, Athens, OH 45701, USA
² Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, OH 45701, USA

View larger version (39K): [in a new window]   Fig. 1. Arboreal locomotion in Monodelphis domestica. (A) Resolution of substrate reaction forces (SRFs) into normal and shear components (Fnormal,ML and Fshear,ML, respectively) as illustrated for a fore limb and its mediolateral SRF (FML). XFL, YFL and ZFL are coordinates of the estimated center of fore limb pressure. (B) Resolution of vertical SRFs into shear and normal components (Fnormal,V and Fshear,V, respectively). (C) Cropped representative image of M. domestica on the arboreal trackway illustrating the limb landmarks: (1) distal tip of the third manual digit; (2) lateral aspect of the wrist joint; (3) distal tip of the fifth pedal digit; (4) lateral aspect of the metatarsophalangeal joint. Note that the heel (see arrow pointing to the ankle marker) was typically not in contact with the substrate during arboreal and terrestrial trials. Scale bar (4 cm) denotes the length and location of the arboreal force transducer.

View larger version (13K): [in a new window]   Fig. 2. Phylogeny of some American marsupials, based on Palma and Spotorno (1999) and Nowak (1999). Although scansorial and arboreal locomotor adaptations evolved more than once in the family Didelphidae, it is likely that the common ancestor was a terrestrial form. Furthermore, Nowak (1999) and Cartmill (1972) suggest that the terrestrial Monodelphis genus retains the primitive condition to the greatest degree.

View larger version (14K): [in a new window]   Fig. 3. (A) Symmetrical gait plot for M. domestica during terrestrial and arboreal locomotion following Hildebrand (1976). Terrestrial and arboreal trials lie mostly within trots, although arboreal trials extend into smaller limb phases (lateral-sequence diagonal-couplet gait). (B) Relationship between stance duration and speed.

View larger version (26K): [in a new window]   Fig. 4. Representative substrate reaction force (SRF) profiles from the terrestrial and arboreal trackways (speed indicated on each plot). (A,B) Fore limb and hind limb arboreal trials. (C,D) Typical terrestrial trials for the fore limb and hind limb. (E,F) Slow terrestrial fore limb and hind limb trials with double-peaked vertical force traces. Negative craniocaudal forces indicate a braking effort and positive indicates propulsion. Negative mediolateral force designates a medially directed SRF (laterally directed limb force) and positive designates a medially directed limb force. For clarity, craniocaudal force is shown in gray.

View larger version (38K): [in a new window]   Fig. 5. Relationship of kinetic variables versus speed. (A) Peak vertical force. (B) Vertical impulse. (C) Braking impulse. (D) Propulsive impulse. The sample ellipses emphasize substrate and limb groups. The dimensions of the ellipses were determined from the standard deviations of the y and x variables; sample covariance between y and x determines the orientation of the ellipse.

View larger version (18K): [in a new window]   Fig. 6. Box-and-whisker plots of net mediolateral impulse for each substrate and extremity group. The line in the middle of each box plot represents the median; each box and each whisker corresponds to a fourth of the data; asterisks designate outliers; circle denotes extreme outliers. Positive values indicate a medially directed limb force [laterally directed substrate reaction force (SRF)], and negative values indicate a laterally directed limb force. Substrates were significantly different (N=75, P<0.00001), but there were no differences between limbs within substrate groups.

View larger version (15K): [in a new window]   Fig. 7. (A) Manus and pes placement about the arboreal trackway. Location of the center of pressure of each foot is drawn to scale relative to branch cross-sectional shape. (B) Representative required coefficient of friction data from M. domestica on the arboreal trackway (1.10 m s–1). High values occur at the foot touchdown and again at the end of the step. The broken line indicates the median value for this record (0.528). (C) Median required coefficient of friction for fore limbs and hind limbs on horizontal terrestrial and arboreal substrates. Ellipses are used to make each group more visible, and are calculated as in Fig. 5.