It's a feature of many sun drenched holidays. You're sitting relaxing in the sun, you glimpse a swift movement out of the corner of your eye, but as soon as you look it's gone; lizards are notoriously nimble, and geckos must be the ultimate reptile gymnasts. Able to trot up vertical surfaces, the tiny reptiles seem to defy gravity. Fascinated by almost all aspects of the gecko's agile lifestyle, Bob Full from Berkeley, California, was intrigued by their remarkable vertical mobility. Even though the reptiles seem able to run against gravity, their climbing gait is almost indistinguishable from the style they adopt when scampering along the flat. How could that be? Puzzled by the conundrum, Full and Keller Autumn from Lewis Clark College, Oregon,decided to measure the forces generated by running geckos, whether they were scrambling up a wall (p. 260) or trotting on the flat(p. 249), to find out how geckos use very similar running styles, no mater which direction they're running in.

Working with Autumn and a team of dedicated undergraduates and gradstudents, Full put the tiny gymnasts through their paces(p. 260). Filming the lizards at 1000 frames s-1 as they charged up a wall fitted with a vertical force plate, the team were able to correlate the reptiles' lightning fast movements with the forces generated by their legs. Amazingly, the geckos trotted up the plate at break-neck speed, taking an astounding 30 steps every second. Full explains that the animals are moving so fast that he was sure they would decelerate substantially as their feet attached to the wall. But he was surprised when he saw that the geckos continued climbing at a near constant speed, even though the force they generated dropped to zero at the instant they stepped from one set of toes onto the other; instead of stalling,the lizards were propelled on by their own momentum at a colossal 0.8 m s-1. The team was also able to see the geckos quickly detach one pair of sticky feet by peeling them off the surface at the same instant that they attached the other pair, a process taking 15 ms.

Analysing the forces generated by the gecko's fleeting feet, Full and Autumn realised that the instant the lizards' feet grabbed onto the wall, they pulled down while simultaneously pulling their legs inwards to get a good grip on the surface. While doing this, the team noticed that the tiny animals'heads pitched backwards, away from the wall as if they were about to fall off,but they recovered by pushing away from the wall with their rear legs while pulling their heads back in toward the wall with their front legs. Full saysI've never seen anything like this before'.

But how did the geckos' horizontal trotting compare with their wall climbing antics? This time the team set the lizards scuttling along a track over a horizontal force plate, filming their every move with high speed video(p. 249). Sure enough,the geckos careered across the plate with the same trotting gait that they used to ascend the wall, but the forces they generated were completely different from the forces during their vertical ascent.

Firstly, the reptiles' fore legs were decelerating the animals' bodies while their hind legs mostly accelerated, despite trotting forward at a constant average speed. But the biggest surprise came when Full realised that instead of pulling their legs inwards, as they did when climbing, the horizontally trotting lizards were pushing their legs outwards. The pattern of forces was more similar to the pattern generated by running insects that bounce from side to side as they scamper along. And what is more, this bouncing movement probably allows the geckos to recover automatically when they stumbled. Full explains that this passive dynamic stability' is probably an inherent aspect of the lizard's musculoskeletal system, helping it to recover effortlessly whenever it loses its footing.

So, despite their remarkable visual similarities, climbing geckos are doing something that is very different from geckos running across open ground; in the gecko's case, looks have been deceptive.

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Autumn, K., Hsieh, S. T., Dudek, D. M., Chen, J., Chitaphan, C. and Full, R. J. (
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