Geckos glide, crash-land, but don’t fall thanks to tail
Soft perching robot validates the benefit of having a fifth leg
IMAGE: A GECKO ON A LEAF view more
CREDIT: MPI FOR INTELLIGENT SYSTEMS
Stuttgart – Geckos’ impressive climbing abilities give them agility rarely surpassed in nature. With their highly specialized adhesive lamellae on their feet, geckos can climb up smooth vertical surfaces with ease and even move on a ceiling hanging upside down. Their ability to run on water adds is another superpower. Now another can be added.
A scientific study published on September 2nd 2021 in Nature Communications Biology by researchers who work at the intersection between robotics and biology shows the geckos are capable of even more. In the publication titled Tails stabilize landing of gliding geckos crashing head-first into tree trunks, the authors Rob Siddall, Greg Byrnes, Robert Full and Ardian Jusufi present footage showing that geckos with no major specializations for flight are in fact capable gliders. Experiments with a gecko-inspired robot confirm the reptile’s locomotion abilities are not entirely down to its feet. The tail plays just as much a pivotal role, the team from the Max Planck Institute for Intelligent Systems in Stuttgart, Siena College in New York, and the University of California at Berkeley discovered.
In their work, the scientists begin by showing that the multi-talented lizard known as Hemidactylus platyurus is capable of gliding. In its natural habitat, it lives in trees and can jump many meters from one tree trunk to the next to avoid predators. When trees are close and the jump is short, the gecko is still accelerating so that everything between jump and landing happens at the blink of an eye. The gecko experiences an unbraked collision. Surprisingly, the gecko can cope with smashing full-on into a tree trunk.
Ardian Jusufi, who initiated the study, set up several experiments in a wildlife reserve in the rainforests of Singapore. At the Max Planck Institute for Intelligent Systems, he leads the Cyber Valley research group “Locomotion in Biorobotic and Somatic Systems”. He has spent many years investigating geckos and their many locomotion abilities. In the tree canopy, Jusufi explored a situation where reptiles both with and without a tail face the challenge of a short accelerating glide.
Placed on a platform seven meters above the ground, a tail-equipped gecko leaps down into the deep and glides to a nearby tree. High speed cameras capture the fall and show that the jumping gecko reaches 6 m/s, just over 21 km/h. Unlike a car that would be heavily dented after driving into a tree at this speed, the footage shows the gecko lands on the trunk without falling off. It moves away as if nothing happened. With tailless animals, it was quite the opposite. Geckos who had naturally lost their tails couldn’t maintain their grip after the crash and, consequently, fell off the tree trunk after landing.
As can be seen in the corresponding video (https://youtu.be/LXRAWypJBPI), the mechanism the animal applies to cushion the impact is bending its torso backward as far as 100 degrees. During the bend, the front feet lose grip. Only the rear legs remain attached. This pitch-back of the torso dissipates energy as it pushes the tail hard into the trunk. Animals that have lost tails could not dissipate sufficient energy and fell. The tail acts as a fifth leg, helping the gecko stabilize after the impact, they believed. But without a control experiment can one conclusively show that the tail has this stabilizing effect? Hence, they set off to the lab.
The scientists created a physical model of a gecko to better understand the forces the animal experiences. Their gecko-inspired robot features a soft torso, where the tail can be taken off and put back on. When the front foot hits a surface, the robot is programmed to bend its tail just like the reflex that Jusufi discovered previously in climbing geckos. The information is processed via a microcontroller on the shoulder. This signal activates the motor to pull on a tendon and hence pushes the tail into the wall to slow the head over heels pitchback.
Back in the Locomotion in Biorobotic and Somatic Systems lab, Robert Siddall and Ardian Jusufi began by catapulting a soft robotic lizard onto a wall with an embedded force-sensitive scale (the simulated tree trunk) which is lined with felt, to which the robot’s Velcro-lined feet can stick. The robot hit the force plate as abruptly as the geckos hitting the tree, tilting back its torso at a right angle to the surface. The roboticists then measured the force the front and back feet of the robot endured upon impact. The longer the tail, they discovered, the lower the force pulling the back feet away from the surface. The lower that force, the easier it is for the robot (and likely the animal) to hold on. Without a tail, however, the forces on the back feet become too high – the robot loses grip, bounces off, and falls. This experiment validated the scientists’ hypothesis that the tail is essential for the gecko to be able to stabilize itself on a vertical surface after colliding with it at high speed – findings that could make a significant contribution to robot landings and beyond.
“This field discovery on the perching behavior of geckos has important implications for our understanding of tails as multi-functional appendages that animals can rely on. Ranging from inertial to contact tails, they facilitate the most extreme transitions, such as from gliding flight to collision with a wall,” says Ardian Jusufi, the senior and corresponding author.
“One of the most dramatic transitions we can think of in multi-modal locomotion is to alight on a vertical surface from high-speed gliding flight to a standstill,” continues Ardian Jusufi.
Larger gliding specialists appear to avoid engaging in short glides, as there is not sufficient vertical drop height to reach terminal velocity, stop accelerating, and begin a dedicated landing maneuver with a stall prior to impact. Smaller animals may be able to use mechanically mediated solutions to negotiate such situations.
However, no one had ever quantified this amazing animal’s gliding behavior before. Such video material from the rainforest is hard to come by. “Our attempts to film the small, camouflaged lizard in the rainforest revealed a fall arresting response nobody thought these geckos could do and showed us their tails were entirely underestimated. Previously contact tails were thought to be used to maintain grip during rapid wall-running, while the findings presented here suggest that geckos exhibit exaptation of the behavior to improve the success of landing in the wake of their directed aerial descent,” says Jusufi.
“With the robot, we were able to measure something we could not with geckos in the field. The wall reaction forces at the impact upon landing confirmed that the tail is an essential part facilitating the landing in subcritical glides. Our soft robotic lander not only helps to make an impact in another field, but it can also help improve robot locomotion by increasing robustness and simplifying control,” explains Ardian Jusufi.
"Nature has many unexpected, elegant solutions to engineering problems - and this is wonderfully illustrated by the way geckos can use their tails to turn a head-first collision into a successful perching maneuver. Landing from flight is difficult, and we hope our findings will lead to new techniques for robot mobility – sometimes crashes are helpful," Robert Siddall describes.
Gliding flight has evolved repeatedly in the Indomalayan lowland tropical rainforest of Southeast Asia.
CAPTION
A gecko bending back its torso after crash-landing
CREDIT
MPI for Intelligent Systems
CAPTION
Ardian Jusufi with a soft gecko-inspired robot
CREDIT
MPI for Intelligent Systems / A. Jusufi
Additional image and video material is available here:
https://bio.is.mpg.de/landingtail
Authors:
Robert Siddall and Ardian Jusufi*, Locomotion in Biorobotic and Somatic Systems, Max-Planck-Institute for Intelligent Systems, Germany. E-mail: rob@is.mpg.de;
*Corresponding author: ardian@is.mpg.de , Phone: ++491743230725.
Greg Byrnes. Biology Department. Siena College, NY, USA.
E-mail: gbyrnes@siena.edu. Phone: ++1 518-783-4249
Robert Full. Department of Integrative Biology, University of California at Berkeley, CA, USA. E-mail: rjfull@berkeley.edu. Phone ++1 510 642 9896
Contact Information for Comments on Article from Investigators Not Involved in this Research:
Professor Jake Socha (comparative biomechanics, gap-crossing)
Department of Biomedical Engineering and Mechanics
Virginia Tech University, USA
E-mail: jjsocha@vt.edu
Phone: 540-231-6188
Professor Prof. David Lentink (biomechanis of flight, robots)
Uroeningen University, Holland.
E-Mail: d.lentink@rug.nl
Professor Sharon Schwartz
Department of Ecology and Evolutionary Biology, and Professor of Engineering
Brown University, USA.
Sharon_Swartz@brown.edu
Professor Martin Whiting (herpetology, ecology, predator prey interactions)
Department of Biological Science
Macquarie Univ. Sydney
Email: martin.whiting@mq.edu.au
Tel +61(0)402752229
JOURNAL
Communications Biology
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Tails stabilize landing of gliding geckos crashing head-first into tree trunks
ARTICLE PUBLICATION DATE
1-Sep-2021
These geckos crash-land on rainforest trees but don't fall, thanks to their tails
Scientists find another use for lizards' versatile tails:
stabilization after headfirst crashes
Peer-Reviewed PublicationIMAGE: AN ASIAN FLAT-TAILED HOUSE GECKO, HEMIDACTYLUS PLATYURUS. VIDEOS OF THESE GECKOS, COMMON IN THE FORESTS OF SINGAPORE, SHOWED THAT THEIR TAILS ALLOW THEM TO RECOVER EFFECTIVELY FROM CRASH LANDINGS ON TREE TRUNKS. view more
CREDIT: ARDIAN JUSUFI
A gecko's tail is a wondrous and versatile thing.
In more than 15 years of research on geckos, scientists at the University of California, Berkeley, and, more recently, the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, have shown that geckos use their tails to maneuver in midair when gliding between trees, to right themselves when falling, to keep from falling off a tree when they lose their grip and even to propel themselves across the surface of a pond, as if walking on water.
Many of these techniques have been implemented in agile, gecko-like robots.
But Robert Full, UC Berkeley professor of integrative biology, and Ardian Jusufi, faculty member at the Max Planck Research School for Intelligent Systems and former UC Berkeley doctoral student, were blown away by a recent discovery: Geckos also use their tails to help recover when they take a header into a tree.
Those head-first crashes are probably not the geckos' preferred landing, but Jusufi documented many such hard landings in 37 glides over several field seasons in a Singapore rainforest, using high-speed video cameras to record their trajectories and wince-inducing landings. He clocked their speed upon impact at about 6 meters per second, or 21 kilometers per hour — more than 200 feet per second, or about 120 gecko body lengths per second.
"Observing the geckos from elevation in the rainforest canopy was eye-opening. Before take-off, they would move their head up-and-down, and side-to-side to view the landing target prior to jumping off, as if to estimate the travel distance," Jusufi said.
The videos show that when this gecko — the common Asian flat-tailed house gecko, Hemidactylus platyurus — collides head-on with a tree, it grabs the trunk with its clawed and padded toes so that, as its head and shoulders rebound, it has leverage to press its tail against the trunk to prevent itself from tumbling backward onto the ground and potentially ending up as someone’s dinner.
“Far from stalling, some of these lizards are still accelerating upon impact,” Jusufi said. “They crash headfirst, pitch back head over heels at an extreme angle from the vertical — they look like a bookstand sticking away from the tree — anchored only by their rear legs and tail as they dissipate the impact energy. With the fall-arresting reflex happening so fast, only slow motion video could reveal the underlying mechanism.”
This surprising behavior, and a demonstration that robots with tails that act similarly also can successfully recover from crash landings, will be reported this week in the Nature journal Communications Biology. Though this type of headfirst crash landing has not been documented previously among geckos or other gliding animals, the scientists suspect that other small, lightweight leapers — in particular, other lizards — use this as a backup when a perfect jump is impossible.
“They may have longer glides that are more equilibrium glides, and they land differently, but, for example, if they are trying to escape, they choose to do this kind of behavior, in part because size matters,” Full said, noting that the lizards measure only a couple of inches from snout to tail tip. “When you're that small, you have options that aren't solutions for big things. So, this is sort of a body-mediated solution that you don't have if you're bigger.”
Jusufi and Full note that structures similar to gecko tails could be used to help stabilize flying robots, such as drones, when they land on vertical surfaces.
According to the researchers, this unusual behavior, which they're the first to document, mathematically model and reproduce in a soft robot, is an example of how an evolutionary innovation like a tail can be used in unforeseen ways. Vertebrate tails evolved in aquatic animals, likely as a means of propulsion in the water — something Jusufi also studies and models with soft robots that undulate. But the tail turned out to be such a versatile thing that the lizard evolved various exaptations, a term for structures that were shaped by natural selection for a particular function or adaptation, but that have been used for other behaviors.
“Exaptations are structures that have been co-opted for many behaviors, no matter what that structure evolved for originally, and here's one that you wouldn't expect,” Full said. “You can see how that incredible capability of being robust can allow these exaptations.”
“Until recently tails had not received as much attention as legs or wings, but people are now realizing that we should think of these animals as five-legged, in a way — pentapedal,” Jusufi said.
Full said that as robotic engineers attempt to add more and more functions to robots, they are finding that they can’t introduce a new part for every capability. A tail is one structure that, as lizards found out, can have multiple purposes.
“As we evolve our robots and physical systems, engineers all want to do more things. And guess what? At some point, you can't optimize a robot for everything,” he said. “You have to use things for other behaviors in order to get those behaviors.”
CAPTION
The Asian flat-tailed gecko would prefer a four-point landing after leaping to a tree trunk, but if it can't slow down sufficiently, it may have to crash head first into the trunk, rebounding but stabilizing itself with its tail. Researchers at the Max Planck Institute for Intelligent Systems in Germany built a soft robot with an active tail to replicate this behavior.
CREDIT
Photos by Ardian Jusufi, illustration by Andre Wee
A robot catapult
In Singapore, Jusufi and his colleagues used high-speed cameras to record geckos leaping to trees that were too close to allow gliding. Even though the flat-tailed gecko is not particularly adapted to gliding — some geckos have skin flaps that are like parachutes – it has some ability to glide and thus maneuver in midair. But gliding requires reaching terminal velocity so that the lizard can maneuver in midair, and the leaps weren't long enough for that.
Unable to glide or slow themselves by stalling before landing, the geckos crashed hard, usually headfirst. When they analyzed the trajectories and mechanics of the falling geckos, the researchers found that some were still accelerating on impact. Most could not maintain a grasp on the tree with their front feet.
“Our field observations of these small, agile lizards in the rainforest revealed highly dynamic, fall-arresting responses nobody thought these geckos could execute with their tails,” Jusufi said. "Our field observations suggest they exapted tail behavior thought to be for climbing to perching after gliding flight."
The researchers modeled the behavior mathematically to confirm that what they were seeing made sense physically, but to really determine what the geckos were experiencing, they decided to build a soft robot at the Max Planck Institute that resembles a gecko and launch it with a catapult into the wall. This way, they could measure the forces actually sustained by the geckos when they crash-land, and the forces produced by the feet.
They built the tailed robot from parts made by a state-of-the art 3D printer, Carbon M2, that is specifically designed to print soft structures. The feet were outfitted with Velcro to stick upon contact, and to the tail they added a mechanism that would make it press downward when the front legs hit a surface and slip, like the gecko’s tail reflex.
Surprisingly, the tailed robot had similar success when making hard landings. In the wild, 87% of geckos with tails successfully landed on a vertical surface without falling, while tailless geckos fell more frequently. (Geckos often shed their tails to escape from predators or their rivals, and regrow them later.) Tailless robots were only able to land successfully on a vertical surface in 15% of the trials, compared to 55% of trials involving the tailed robot.
The researchers also found that, beyond a certain length, longer tails aren’t necessarily that much better than shorter tails: Robots with tails only half the length of the head and body combined were nearly as successful as those with tails equal to the snout-vent length. Short-tailed robots, however, required twice the foot force to stay attached to the tree.
Full and Jusufi continue to study the behavior of geckos in search of principles that can be applied to the design of robots — in particular, soft robots that can perch in trees and land on vertical surfaces — but also to explore the evolutionary origins of animal locomotion. One key takeaway, Full said, is that, while engineers may seek to design the optimal robot, nature never does.
“Evolution is not about optimality and perfection, but instead, it’s about sufficiency. The just-good-enough solution really plays into giving you a breadth of capabilities so that you're far more robust in challenging environments,” Full said. “Evolution looks like more like a tinkerer who never really knows what they'll produce and uses everything that's at their disposal to make something that's workable.”
“Small arboreal animals without obvious morphological adaptations for flight are increasingly being found to exhibit surprising ability for mid-air maneuvering. Soft robotic physical models can help decipher the control of such mechanically mediated solutions to landing,” Jusufi said.
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Co-authors are Rob Siddall, a postdoctoral fellow in the Locomotion in Biorobotic and Somatic Systems Group at the Max Planck Institute, and former UC Berkeley doctoral student Greg Byrnes, now an associate professor of biology at Siena College in Loudenville, New York. The work was supported by grants from the Max Planck Institute, the Cyber Valley Research Fund, the Swiss National Science Foundation and the U.S. Army.
JOURNAL
Communications Biology
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Tails stabilize landing of gliding geckos crashing head-first into tree trunks
ARTICLE PUBLICATION DATE
2-Sep-2021
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