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.