House geckos across southern Asia hang out at night, waiting for a meal to appear. Blink and you might miss one scurry up a wall and vanish. This brief moment hides a complex story, played out over at least 40 million years of evolution. Climbing lizards like the Asian house gecko must be speedy, but at the same time grippy and energetically economical. These qualities are engaged in a tug of war – one improves at the expense of another.
Johanna Schultz and colleagues from the University of the Sunshine Coast, Australia, and Bremen University of Applied Sciences, Germany, wondered how these performance qualities – speed, grip and efficiency – play off against each other as climbing lizards optimise their ascent. One way to find out would be to watch real lizards scampering. The trouble is, lizards aren't the best at taking instructions; ‘climb faster!’ or ‘be more bendy!’ will probably be met with an unblinking stare. This is where the robots come in.
Schultz and colleagues decided to test their ideas on four 300 g robots, designed to mimic the climbing abilities of lizards such as Asian house geckos. The robots were each kitted out with on-board sensors, motors to move their limbs and spine, and claws to grip onto carpeted walls. They were then programmed to climb vertically, taking 10 full steps over and over again, varying their speed, the angles of their feet against the wall, and their limb and spine range of motion. Sometimes the robots fell, sometimes they climbed high. Schultz and colleagues studied how the robots performed and compared their achievements with those of real-life house geckos.
The best climbing robots didn't move especially fast or especially slow. Those moving at a moderate speed climbed the furthest; at slower speeds, they tended to slip and, at their maximal speed, they also detached. This mirrored the real geckos, which prefer to ascend at intermediate speeds, hinting that moderate speed is important for their feet to get a grip. At the slowest speeds, climbing was energetically most expensive, tending to drain the robots’ batteries after three ascents. Of course, real geckos aren't battery powered, but excessive power use is still best avoided.
The robots walked a tightrope between grip and stability. Their claws best engaged with the carpet when aligned parallel to the body – imagine gripping your fingers onto a ladder. The trouble is that this foot angle wasn't good for bracing the robot against their side-to-side wiggle, and that meant the robots were prone to slipping and falling. Instead, the robots climbed best when their back feet were turned outwards or inwards, with their front feet only slightly so. This configuration of grippy front feet and splayed back feet was found in the real geckos, showing that they face similar trade-offs between grip and stability.
Lastly, Schultz and colleagues varied the robots’ limb and spine range of motion, from no movement at all to very wiggly, and watched what happened. They found that the robots made the fastest and most economical climbs when the motions of the limb were larger than the side-to-side motion of the spine. This was also how the geckos climbed in real life, revealing how their wiggle is tuned towards speed and efficiency.
The robots showed the team how peak performance can be achieved and real-life lizards amazingly seem to follow the same recipe for success. That said, climbing is draining work for the robots, but despite these challenges it is clear that quintessentially synthetic creations can help us to understand the mechanisms of the flesh and blood world.
