Tails stabilize landing of gliding geckos crashing head-first into tree trunks

Geckos are renowned for their climbing abilities, utilizing specialized feet and tails for agile movements. Previous research has shown the role of the tail in various gecko behaviors, including aerial acrobatics and water running. This study focuses on the landing mechanism of the Asian flat-tailed gecko (*Hemidactylus platyurus*) during short-range glides in its natural rainforest habitat. Unlike specialized gliders, which typically reduce velocity before landing, these geckos crash head-first into tree trunks at considerable speeds. The research question is whether the gecko's tail plays a crucial role in stabilizing the landing and preventing falls. This is important because understanding how geckos manage high-speed impacts in the wild can offer insights into bio-inspired design principles for robotics and other fields. The study's innovative approach combines field observations of natural gecko behavior with dynamic mathematical modeling and robotic experiments to investigate the tail's function in stabilizing landings. By integrating these methods, the study aims to gain a comprehensive understanding of the biomechanics involved.

Existing literature highlights the diverse landing strategies employed by animals on vertical surfaces. Studies on geckos have shown the significance of their specialized feet in adhesion and climbing. The role of the tail in various gecko behaviors, including aerial acrobatics and rapid changes in posture during vertical running, has also been documented. Previous work suggests a potential stabilizing function of the tail during gliding, though quantitative data in natural settings were lacking. Research on other gliding animals, such as flying squirrels and lizards, shows that these animals typically reduce velocity and employ aerodynamic control before landing. The lack of extensive aerodynamic control in the Asian flat-tailed gecko, however, necessitates an alternative landing strategy, prompting the investigation of the tail's role in stabilizing high-speed impacts.

The study employed a multifaceted methodology incorporating field observations, mathematical modeling, and robotic experiments. Field observations were conducted in the Southeast Asian rainforest using high-speed video cameras to record the gliding and landing behaviors of wild *Hemidactylus platyurus* geckos. Two camera angles (long-range and close-range) were used to capture the trajectories and landing details. The data collected included glide approach trajectories, glide and landing velocities, foot trajectories, and landing impact forces. A dynamic mathematical model was developed to investigate the effect of tail length on the fall-arresting response (FAR) during landing. The model simplified the gecko's body as a rigid body and the hind legs as a pin joint, considering forces from the tail, gravity, and the substrate reaction. Three scenarios were analyzed: constant angular deceleration, constant tail force, and a proportional force, each aiming to capture different aspects of the gecko's tail action. To experimentally test the model's predictions and provide an independent line of evidence, a robotic physical model with an active tail reflex was built. The robot was launched towards a vertical force platform, mimicking the gecko's landing scenario. Landing success, pitch-back trajectory, and landing foot forces were measured for both passive and active tails of varying lengths. The force data were also analyzed to confirm the force predictions from the mathematical model. The effect of tail length on landing success was assessed using statistical tests (χ² test for independence and one-way ANOVA).

High-speed video analysis revealed that *H. platyurus* geckos typically accelerate throughout their glides and impact tree trunks head-first at speeds of approximately 6.0 ± 0.9 m/s. Upon impact, geckos exhibited a distinctive fall-arresting response (FAR), characterized by a substantial pitchback of their torso away from the trunk, anchored by their hind limbs and tail. The dynamic mathematical model predicted that the force required to maintain hindfoot contact during landing is inversely proportional to tail length, implying that longer tails reduce the forces on the feet. This prediction was validated by experiments with the robotic physical model. The robotic experiments showed that longer tails, particularly with an active tail reflex, resulted in significantly lower adhesive foot forces compared to shorter or passive tails. Specifically, a tail shortened to 25% required over twice the adhesive foot force to land successfully. Tailless geckos showed a drastically lower success rate in landing experiments, supporting the hypothesis that the tail is crucial for stabilizing landings. The timing of the tail reflex in the FAR was comparable to that observed in the geckos’ "kickstand response" during vertical walking, suggesting a conserved tail mechanism utilized in different contexts. The study also noted that the geckos maintained a near-constant approach angle during gliding, with limited ability to change their trajectory using aerodynamic control, further emphasizing the tail's importance in impact stabilization. The quantitative data from the robotic experiments confirmed the model's predictions, suggesting a strong inverse relationship between tail length and landing forces. Successfully landing geckos exhibited a pitchback angle of approximately 114° ± 16°, with a duration of approximately 138 ± 15 ms. While geckos possess both claws and adhesive pads, the study emphasizes that the mechanism involved could predominantly be attributed to adhesive pads during the FAR.

The findings strongly support the hypothesis that the tail plays a crucial role in stabilizing the landing of gliding geckos. The combined results of the field observations, mathematical modeling, and robotic experiments provide robust evidence for the tail's function in mitigating high-impact forces and preventing falls. The inverse relationship between tail length and landing forces suggests that the tail acts as a shock absorber, reducing the forces exerted on the feet and limbs during landing. This mechanism is critical for preventing foot slippage and maintaining contact with the substrate, ensuring successful landings. The conserved nature of the tail reflex observed in both the FAR and the kickstand response suggests that it might be a general mechanism for stabilizing the gecko's posture in challenging locomotor situations. The study highlights the importance of considering both active and passive mechanisms in animal locomotion, especially in high-impact scenarios. Furthermore, this study's findings contribute to a deeper understanding of bio-inspired design principles, providing valuable insights for the development of robots capable of robust and stable landings on challenging surfaces.

This study demonstrates the crucial role of the tail in stabilizing the high-speed landings of gliding geckos. The unique fall-arresting response, combining head-first impact with a controlled pitchback, showcases the remarkable adaptability of gecko locomotion. The quantitative data from field observations, mathematical modeling, and robotic experiments provide strong support for the tail's function as a dynamic shock absorber, reducing the forces on the feet and preventing falls. Future research could investigate the contribution of claws versus adhesive pads to attachment during landing, explore the neural control mechanisms behind the tail reflex, and further investigate the potential applications of these findings in bio-inspired robotics.

The study primarily focused on short-range glides, and the findings might not be fully generalizable to longer glides. The mathematical model employed several simplifications, such as assuming the gecko's body as a rigid body and neglecting certain frictional forces. The robotic model, although dynamically similar to the gecko, cannot perfectly replicate all aspects of gecko biology, including the complex musculature and sensory feedback systems. Further investigation could consider the effects of different environmental conditions, substrates, and body sizes on the landing mechanism.