Leaping lizards landing on leaves: escape-induced jumps in the rainforest canopy challenge the adhesive limits of geckos

Abstract

The remarkable adhesive capabilities of geckos have garnered attention from scientists and the public for centuries. Geckos are known to have an adhesive load-bearing capacity far in excess (by 100-fold or more) of that required to support their body mass or accommodate the loading imparted during maximal locomotor acceleration. Few studies, however, have investigated the ecological contexts in which geckos use their adhesive system and how this may influence its properties. Here we develop a modelling framework to assess whether their prodigious adhesive capacity ever comes under selective challenge. Our investigation is based upon observations of escape-induced aerial descents of canopy-dwelling arboreal geckos that are rapidly arrested by clinging to leaf surfaces in mid-fall. We integrate ecological observations, adhesive force measurements, and body size and shape measurements of museum specimens to conduct simulations. Using predicted bending mechanics of petioles and leaf midribs, we find that the drag coefficient of the gecko, the size of the gecko and the size of the leaf determine impact forces. Regardless of the landing surface, safety factors for geckos range from a maximum of just over 10 to a minimum of well under one, which would be the point at which the adhesive system fails. In contrast to previous research that intimates that gecko frictional adhesive capacity is excessive relative to body mass, we demonstrate that realistic conditions in nature may result in frictional capacity being pushed to its limit. The rapid arrest of the lizard from its falling velocity likely results in the maximal loading to which the adhesive system is exposed during normal activities. We suggest that such activities might be primary determinants in driving their high frictional adhesive capacity.

Multi-modal locomotor costs explain sexual size but not shape dimorphism in a leaf-mimicking insect

“…Not only were large males relatively poor flyers, they also were more likely to detach and fall when landing. The largest males had the lowest dynamic safety factors for frictional attachment forces, which are fundamental for accommodating impact forces when landing (Higham et al, 2017). Indeed, we estimated that the largest males were likely to slip when landing on small and stiff leaves (i.e., SFdynamic < 1).…”

“…Only frictional attachment forces (as opposed to adhesive forces) were used to compute these SFdynamic as they are the most important for accommodating deceleration and impact forces when landing (Higham et al, 2017).…”

“…Deflection was calculated as a function of the weight of the insect. Given the relatively light weight of male leaf insects, and in contrast with Higham et al, 2017, who were considering geckos (i.e., roughly 28 times heavier), we only accounted for small leaf deflections (deflection angle θ < 5°) and therefore used the following equation to estimate the deflection of the tip of the leaf δ (Goodno and Gere, 2018):…”

“…where L is the total length of the leaf (m) and EI is its flexural stiffness (N.m 2 ). We used the range of values estimated by Higham et al, 2017, for leaf length and corresponding EI (i.e., L1 = 23 cm, EI1 = 2.67 × 10 -3 N.m 2 and L2 = 86 cm, EI2 = 28.48 × 10 -3 N.m 2 ) to explore the possible range of SFdynamic.…”

Digital hyperextension has no influence on the active self‐drying of gecko adhesive subdigital pads

“…If a gecko's toe pads were to become soaked with water in its natural habitat, it is possible that the adhesive system could be damaged during an event where a gecko is under high shear forces. Recent work has shown that the adhesive system of geckos performing aerial escape maneuvers may be subjected to remarkably high shear force, potentially approaching the adhesive force capacity of the system under such conditions (Higham, Russell, & Niklas, ). As such, if water compromises the structural integrity of the gecko adhesive system and a gecko undergoes such an escape maneuver with recently soaked toe pads, damage to the adhesive system may occur.…”