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First published online March 31, 2005
Journal of Experimental Biology 208, 1489-1494 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01554

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Giant Galápagos tortoises walk without inverted pendulum mechanical-energy exchange

Peter A. Zani^*, Jinger S. Gottschall and Rodger Kram

Department of Integrative Physiology, University of Colorado, Boulder, Colorado 80309-0354, USA

View larger version (25K): [in a new window]   Fig. 1. Vertical, horizontal, and lateral force tracings from a typical walking trial (corresponding to Fig. 3A). Tortoise mass=171 kg. Data were filtered using a fourth order low-pass filter at 20 Hz. At 5.73 s, the animal placed its left front foot on the force platform, followed by the right front at 7.43 s. Full support on the force platform occurred with the lifting of the right hind foot at 11 s. The step analyzed in Fig. 3A occurred with right front contact at 13.33 s to left front contact at 15.28 s. Only the hind feet remained on the platform by 19 s and the last hind foot lifted from the platform at 24.5 s.

View larger version (13K): [in a new window]   Fig. 2. Limb–phase relationships for giant Galápagos tortoises during walking. Numbers within each limb indicate order of footfalls (i.e., diagonal couplet). Numbers outside each limb indicate limb phase, that is, proportion (mean for all trials ± 1 S.D.) of the total stride time elapsed between the limb contact times.

View larger version (26K): [in a new window]   Fig. 3. Fluctuations in kinetic energy (KE; broken line), gravitational potential energy (GPE; thick solid line), and total energy (TotE; thin solid line) of COM (hatched circles in drawings) during one walking step (right-front (RF) to left-front (LF) footfalls as in drawings) for three tortoises. Note absence of similar amplitude or out-of-phase relationship for fluctuations in KE and GPE. (A,B) Two trials from Male 1; (C,D) two trials from Male 2; (E,F) two trials from Female 2. Scale bars (10 J) reference both KE and GPE; y-axis zeros reference KE only. Thin horizontal lines above drawings indicate each foot's contact during step in A; data in B–F from a similar RF–LF step (foot contacts not shown). Vertical broken lines link tortoise cartoon footfalls (RF contact, LF lift, and LF contact) to that point in each step.

View larger version (25K): [in a new window]   Fig. 4. Relationship between velocity and (A) mechanical-energy recovery using the inverted-pendulum mechanism, and (B) external work per unit mass per distance. Individuals (as in Table 1) are designated by symbol; all steady-state walking trials are included (N=64). Trend lines are for least-squares regressions including all trials.