Active tails enhance arboreal acrobatics in geckos

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   Fig. 1.

   Gecko tail response activated during rapid vertical climbing (SI Movies 1 and 2). (a) Dorsal view of a gecko running up a high traction vertical track with a slippery patch lacking traction. (b) Side view demonstrates that the tail remains clear of the surface before slipping but contacts the surface shortly after the forefoot slips. (c) Plot of tail tip position as a function of time shows tail response activation after the forefoot slips and subsequent depression of the tail tip. (d and e) Dorsal and side view of gecko climbing a moderate traction surface with an embedded force platform. (f) Plot shows tail tip normal force as a function of time. Substantial normal forces were measured when geckos pushed their tail into the wall after a forefoot slip (red arrow).

   
   
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   Fig. 2.

   Tail use in a running gecko and a legged climbing robot in response to a large pitch-back. (a) Repeated, large foot slips (t = 0 ms) resulted in pitch-back. To prevent overturning, an extreme posture similar to that of a bicycle kickstand was used by geckos (t = 126 ms; SI Movie 3), which enabled them to avoid falls and regain contact with the wall (t = 230 ms) to traverse gaps. (b) RiSE (Robot in Scansorial Environment), a quadrupedal, bio-inspired robot, will use an active tail as an emergency fifth limb to assist in climbing.

   
   
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   Fig. 3.

   Tail-induced air-righting maneuver in geckos (SI Movie 4). (a) At takeoff the gecko released from an upside down (supine) posture. (b and c) Counterclockwise tail rotation (red arrow; θ_T) induced a clockwise rotation of the body (blue arrow; θ_B). (d) As the gecko's body attained right-side up (prone) posture, the tail stopped rotating. The animal maintained a skydiving posture during the subsequent free fall. (e) Schematic of a supine gecko falling to show angle convention. (f and g) Rotation of body and tail segments as a function of time in tailed (f) and tailless animals (g).

   
   
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   Fig. 4.

   Tail-based turning maneuver of gecko during an equilibrium glide in a vertical wind tunnel that moved air upward. Time sequence from left to right of postural stages during a right turn while gliding. When viewed head-on, the tail rotated in a clockwise manner starting from the 12 o'clock position at t = 0 s (a) and sweeping to the right (3 o'clock position; b), down (near 6 o'clock; c), and swinging back past the 9 o'clock position (d and e), and finally stopping near the 12 o'clock position at t = 0.9 s (f). Geckos that rotated their partially dorsi-flexed tail in this clockwise direction initiated a clockwise turn to the right in yaw when viewed from above. (SI Movie 5 shows that a counterclockwise tail rotation correspondingly produced a left turn.)