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First published online December 22, 2003
Journal of Experimental Biology 207, 553-563 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00774

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The tale of the tail: limb function and locomotor mechanics in Alligator mississippiensis

Jeffrey S. Willey^1,*, Audrone R. Biknevicius^2,, Stephen M. Reilly¹ and Kathleen D. Earls²

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

View larger version (55K): [in a new window]   Fig. 1. Configurations of the trackway for capture of ground reaction forces for (A) individual limbs, (B) tail and (C) whole-body mechanics (to which tail data were subsequently added). The black segment represents the instrumented part of the trackway; the remaining (hatched) trackway is isolated from the force platform.

View larger version (16K): [in a new window]   Fig. 2. Kinematic summary of high walking in Alligator mississippiensis. (A) The gait graph (based on Hildebrand, 1976) confirms that high walks are walking trots. (B) The gait plot reveals asynchronous footfalls and liftoffs of diagonal couplets. The mean timing of footfalls and their 95% confidence intervals are shown.

View larger version (19K): [in a new window]   Fig. 3. (A) Representative forelimb (left) and hindlimb (right) ground reaction forces in A. mississippiensis (trials 2C26 and 2C29; 0.202 m s-1 and 0.198 m s-1, respectively). Negative and positive values in the craniocaudal forces are reflective of braking and propulsive efforts, respectively. Positive mediolateral forces reflect lateral pushes by the limbs. Data are unfiltered in order to preserve the impact spike on the hindlimb record. Abbreviations: Fx, mediolateral forces; Fy, craniocaudal forces; Fz, vertical forces. Plots of (B) peak vertical force and (C) net craniocaudal impulse against speed. Symbols: open square, forelimb; filled square, hindlimb.

View larger version (18K): [in a new window]   Fig. 4. Whole-body mechanics in A. mississippiensis during a single step of a walking trot (trial GC13; 0.174 m s-1). (A) Representative plots of whole-body ground reaction force, velocity of the center of mass and vertical displacement of the center of mass versus time. Footfalls () and liftoffs () are indicated for individual feet during the step (F, forelimb; H, hindlimb; L, left; R, right). (B) Representative profiles of kinetic (Ek), gravitational potential (Ep) and total mechanical energies (Em) of the center of mass versus time.

View larger version (12K): [in a new window]   Fig. 5. (A) Phase shift between the fluctuations in gravitational potential and total kinetic energies for high walks in Alligator mississippiensis. (B) Percent recovery of mechanical energy by pendular mechanics for high walks in A. mississippiensis. Symbols: open squares, energy recoveries computed without the mediolateral component; closed squares, energy recoveries computed with all three components.

View larger version (32K): [in a new window]   Fig. 6. Summary of the differential roles of limbs and tail in high walking in A. mississippiensis (see Discussion).

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