Geckos are well known for a gripping feat: the remarkable adhesiveness of their feet, which allows them to climb slippery surfaces like windows where other animals would struggle to get a foothold. But some geckos use other means for travel. They glide with the help of skin flaps that help to keep them airborne. When a team of researchers led by Robert Siddall and Ardian Jusufi from the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, with colleagues from Siena College, USA, and the University of California Berkley, USA, filmed geckos crashing head-first into tree trunks in the rainforest in Singapore they were surprised to see such awkward landings and wanted to know how the geckos managed to remain in contact with the tree.
Having set up a platform perched 6.6 m above the ground and 3.8 m away from a tree trunk, the researchers used two cameras to film the geckos gliding to the tree from the platform; one that captured the entire glide from start to finish and a second that provided a zoomed view of the final portion as the geckos collided with the trunk. From these videos, the researchers could distinguish each stage of the landing manoeuvre and calculate how fast the geckos were gliding.
The videos showed that directly following impact with the tree, once all four feet were in contact with the trunk, the geckos curled their tail slightly inward to form a C-shape just as the forelimbs recoiled and they began to lose grip. Then, the lizard slapped its tail down onto the tree to act as a kickstand as its head and body pivoted backward. Eventually, the head and body recoiled so far back, bending the spine more than 90 deg to contort the lizard like the bullet-dodging characters from The Matrix action movie. In addition, the team recorded the small lizard's landing speed at a whopping 6 m s−1, which is 140 body lengths per second – the equivalent of 238 m s−1 for a 1.70 m tall human – and the entire episode was over in 60 ms. The videos also revealed that the geckos didn't arrest their fall much with their gliding flaps: these only reduced their speed by 6% before impact, which is much lower than in other specialized gliding animals that can reduce speed by up to 60%. This illustrates that the gliding geckos’ snappy tails make up for their lack of aerial agility.
To learn more about the importance of the tail for stabilizing landings, the team ran computer simulations of the foot and tail forces generated during an impact and built a robot gecko to investigate how tail length influences the gecko's ability to ‘stick’ the landing. The simulations showed that tail length helps to reduce the forces that the gecko's feet must generate to produce a foothold and the robot demonstrated that tails that are 25% shorter than their normal length doubled the amount of force required for the feet to get a grip. The model also incorporated a reflex that triggered the tail's stabilising ‘call to action’, which suggests that the gecko may also take advantage of a tail reflex to rapidly assume its kickstand pose.
Siddall and colleagues have shown how a whip of the gliding gecko's tail is important during head-first crash landings. The results may inform robot design by showing how a quick tail can provide controlled landings for robots on varied terrain and surfaces. However, crashlanding geckos are still likely to need to manage a sore head, unlike a robot. Fittingly, some species are better prepared for collisions, possessing a specialized skull that reduces the risk of injury; a clever adaptation for a head-banging gecko.
