Christian E. Brown

Humans have been fascinated by amphibians that leap and glide from trees for centuries, possibly millennia. Here, I discuss aerial behavior and functional morphology along a gradient of arboreality to better understand the biomechanics of and adaptations for leaping and gliding in amphibians. Even nonarboreal frogs and toads experience short-term airborne flight after takeoff and reliably engage in aerial righting, forelimb-forward and controlled landing, and quick recovery that assist in terrestrial locomotion. However, arboreal amphibians are exposed to additional locomotor challenges, including long-term airborne flights, and subsequently exhibit more sophisticated aerial control. Gliding treefrogs have morphological features that maximize surface area and pressure drag and behavioral postures that enhance aerodynamic torque and maneuverability during flight. Likewise, some arboreal salamanders readily jump, albeit using entirely different biomechanics than anurans. The most arboreal species have flattened bodies, semi-webbed feet, and long limbs that provide high frontal area and pressure drag, enhancing maneuverability during flight. Although arboreal anurans and salamanders independently evolved to jump using completely different takeoff strategies, they seem to have converged upon similar functional morphologies and adaptive behaviors for gliding flight. The biomechanics of leaping and gliding in amphibians help inform our understanding of arboreal locomotion in tetrapods more generally.

For over a century researchers have marveled at the square‐shaped toe tips of several species of climbing salamanders (genus Aneides ), speculating about the function of large blood sinuses therein. Wandering salamanders ( Aneides vagrans ) have been reported to exhibit exquisite locomotor control while climbing, jumping, and gliding high (88 m) within the redwood canopy; however, a detailed investigation of their digital vascular system has yet to be conducted. Here, we describe the vascular and osteological structure of, and blood circulation through, the distal regions of the toes of A. vagrans using histology in tandem with live‐animal videos. Specifically, we sectioned a toe of A. vagrans at 0.90 μm, embedded it in Spurrs resin, and stained the tissue with toluidine blue. An additional three toes were sectioned at 10 μm, embedded in paraffin, and after sectioning and mounting, treated with Verhoeff and Quad stains. For living salamanders, we recorded real‐time videos of blood flowing within individual toes upon a translucent surface oriented both horizontally (0°) and vertically (90°) to simulate both prostrate and vertical clinging scenarios, then analyzed the image sequences using ImageJ. We found that the vascularized toe tips have one large sinus cavity that is divided more proximally into two chambers via a septum, and there are mucous and granular glands in the dorsal and dorsolateral integument of the digit tips. Live‐animal trials revealed variable sinus‐filling both within and between toes, seemingly associated with variable pressure applied to the substrate when standing, stepping, clinging, and climbing. We conclude that A. vagrans , and likely other climbing salamanders, can functionally fill, trap, and drain the blood in their vascularized toe tips to optimize attachment, detachment, and complex arboreal locomotion (e.g., landing after gliding flight). Such an adaptation could provide insights for bioinspired designs.

Recent observations of wingless animals, including jumping nematodes, springtails, insects, and wingless vertebrates like geckos, snakes, and salamanders, have shown that their adaptations and body morphing are essential for rapid self-righting and controlled landing. These skills can reduce the risk of physical damage during collision, minimize recoil during landing, and allow for a quick escape response to minimize predation risk. The size, mass distribution, and speed of an animal determine its self-righting method, with larger animals depending on the conservation of angular momentum and smaller animals primarily using aerodynamic forces. Many animals falling through the air, from nematodes to salamanders, adopt a skydiving posture while descending. Similarly, plant seeds such as dandelions and samaras are able to turn upright in mid-air using aerodynamic forces and produce high decelerations. These aerial capabilities allow for a wide dispersal range, low-impact collisions, and effective landing and settling. Recently, small robots that can right themselves for controlled landings have been designed based on principles of aerial maneuvering in animals. Further research into the effects of unsteady flows on self-righting and landing in small arthropods, particularly those exhibiting explosive catapulting, could reveal how morphological features, flow dynamics, and physical mechanisms contribute to effective mid-air control. More broadly, studying apterygote (wingless insects) landing could also provide insight into the origin of insect flight. These research efforts have the potential to lead to the bio-inspired design of aerial micro-vehicles, sports projectiles, parachutes, and impulsive robots that can land upright in unsteady flow conditions.

Wandering salamanders (Aneides vagrans), known to occupy the crowns of old growth coast redwood trees, have recently been found to decelerate and engage in controlled, nonvertical descent while falling. Closely related, nonarboreal species with seemingly minor morphological differences exhibit far less behavioral control while falling; however, the influence of salamander morphology on aerodynamics remains to be tested. Here, we examine differences in morphology and aerodynamics of two salamander species, A. vagrans and the nonarboreal ensatina salamander (Ensatina eschscholtzii), using a combination of traditional and contemporary techniques. Specifically, we compare morphometrics statistically, then use computational fluid dynamics (CFD) to characterize predicted airflow and pressure over digitally reconstructed models of the salamanders. While similar in body and tail lengths, A. vagrans are more dorsoventrally flattened with longer limbs and greater surface area of the foot relative to body size than the nonarboreal E. eschscholtzii. CFD results show dorsoventral pressure gradients differ between the two digitally reconstructed salamanders resulting in lift coefficients of approximately 0.02 and 0.00, and lift:drag ratios of approximately 0.40 and 0.00 for A. vagrans and E. eschscholtzii, respectively. We conclude that the morphology of A. vagrans is better suited for controlled descent than that of the closely related E. eschscholtzii and highlight the importance of subtle morphological features, such as dorsoventral flatness, foot size, and limb length, for aerial control. That our simulation reports align with real‐world performance data underscores the benefits of CFD for studying the link between morphology and aerodynamics in other taxa.

Wandering salamanders (Aneides vagrans) inhabit the crowns of the world's tallest trees, taking refuge in epiphytic fern mats within these complex arboreal environments. These salamanders readily jump from the canopy when disturbed and maintain stable postures while falling via fine adjustments of the limbs and tail in lieu of dedicated aerodynamic control surfaces, thus reliably carrying out non-vertical descent. Here, we examined the aerial behavior and performance of A. vagrans and three other species of plethodontid salamander across a habitat gradient of arboreality by recording salamanders falling from short heights and moving within the jet of a vertical wind tunnel. Kinematic performance of aerial behavior in plethodontid salamanders was correlated with a gradient of arboreal habitats; moreover, salamanders from arboreal niches were more effective in slowing and redirecting descent compared with other salamanders. Aneides vagrans and the closely related Aneides lugubris consistently engaged in parachuting and gliding when falling; their trajectories were very steep, but were sufficiently angled to enable contact with either the home trunk or nearby branches during falls or jumps from great heights. Aerial maneuvering in arboreal salamanders is similar to that seen in other vertebrates capable of non-vertical and controlled descent, suggesting that the long limbs and active tail of these arboreal plethodontids (often cited as adaptations for climbing) may also contribute to parachuting and gliding when falling from trees. These aerial behaviors within the redwood canopy warrant further investigations into other canopy residents that lack conspicuous surfaces for aerodynamic control.

Salamanders are often used as a model of early tetrapod locomotion, due to their generalized tetrapod body plan. We imaged five individuals of wandering salamander, Aneides vagrans, during horizontal and vertical locomotion and compared kinematic and gait adjustments made to compensate for each condition. We used ImageJ to calculate duty factor, stride length, stride frequency, contralateral foot spread, heading of climbing, and forward velocity. Like other terrestrial salamanders, A. vagrans use a diagonal couplet, lateral sequence walk on horizontal surfaces. When walking vertically, however, they use a single‐foot gait, taking smaller steps and extending the duty factor compared with level walking. Salamanders adjusted their limbs to have a wider contralateral foot spread between fore‐ and hindfeet when walking downward as compared to walking upward or horizontally. These gait adjustments may minimize the risk of slipping or falling by increasing the number of contact points during the stride cycle, and by bringing their center of mass closer to the surface. We found that salamanders moved at a significantly faster velocity when traveling horizontally compared to vertically. Furthermore, we found no significant difference between upward and downward velocity, despite greater stride lengths in upward locomotion; this was explained by a higher stride frequency in downward locomotion. We estimate travel time up or down an old‐growth redwood tree to be on the scale of hours, a significant time investment that may encourage alternatives when available. These results suggest that, in an ecological context, while salamanders are capable of traveling straight down a redwood tree, they may simply tend to jump.