Tree Frogs Alter Their Behavioral Strategies While Landing On Vertical Perches

As an arboreal animal, tree frogs face diverse challenges when landing on perches, including variations in substrate shape, diameter, flexibility, and angular distribution, with potentially significant consequences for failed landings. Research on tree frog landing behavior on perches, especially concerning landing on vertical substrates, remains limited. This study investigated the landing strategies (forelimb, abdomen, and hindlimb) of tree frogs on vertical perches, considering perch diameter. Although all three strategies were observed across perches of different diameters, their frequencies differed. Forelimb landing was most common across all perch diameters, with its frequency increasing with perch diameter, while abdomen and hindlimb landing strategies were more prevalent on smaller diameter perches. During the process from take-off to landing, the body axis underwent some deviation owing to the asymmetric movement of the left and right limbs; however, these deviations did not significantly differ among landing strategies. Additionally, different landing strategies led to variations in the landing forces, with abdominal landings generating significantly higher impact forces than the other two strategies. These findings provide insights into the biomechanics and biological adaptations of tree frogs when landing on challenging substrates, such as leaves or branches.

The aero body righting of frog Rana rugulosus via hindleg swings

Frogs can keep an excellent aerial balance for landing and achieve consecutive jumps reliably. A safe landing requires an accurate body righting in the air. However, there is no systematic study on how the frogs adjust the aerial postures and body attitudes after jumping. The stretched long hindlegs swung quickly in the aerial phase, which revealed a clear relationship with the body attitudes. This study aimed to verify the function of frogs' hindlegs on aero body righting in the air. We captured the motions of both hindlegs and found the hindlegs adopted two movement modes, the bilateral parallel, and separated swings. The hindleg-induced torques by the two movements were negatively correlated with the body's angular accelerations on pitch and roll, respectively. Moreover, an analytical model was derived based on the conservation of angular momentum and verified by the dynamic simulations. Thus, we confirmed that the hindlegs are the dominant mechanism in aerial pitch and roll controls. We anticipate our achievements to inspire the design of air-righting tools.

The integration of sensory feedback in the modulation of anuran landing preparation

Controlled landing requires preparation. Mammals and bipedal birds vary how they prepare for landing by predicting the timing and magnitude of impact from the integration of visual and non-visual information. Here, we explore how the cane toad Rhinella marina - an animal that moves primarily through hopping - integrates sensory information to modulate landing preparation. Earlier work suggests that toads may modulate landing preparation using predictions of impact timing and/or magnitude based on non-visual sensory feedback during takeoff rather than visual cues about the landing itself. We disentangled takeoff and landing conditions by hopping toads off platforms of different heights while measuring electromyographic (EMG) activity of an elbow extensor (m. anconeus) and capturing high-speed images to quantify whole body and forelimb kinematics. This enabled us to test how toads integrate visual and non-visual information in landing preparation. We asked two questions: (1) when they conflict, do toads correlate landing preparation with takeoff or landing conditions? And (2) for hops with the same takeoff conditions, does visual information alter the timing of landing preparation? We found that takeoff conditions are a better predictor of the onset of landing preparation than landing conditions, but that visual information is not ignored. When hopping off higher platforms, toads start to prepare for landing later when takeoff conditions are invariant. This suggests that, unlike mammals, toads prioritize non-visual sensory feedback about takeoff conditions to coordinate landing, but that they do integrate visual information to fine-tune landing preparation.

The Influence of Visual, Vestibular, and Hindlimb Proprioceptive Ablations on Landing Preparation in Cane Toads

Coordinated landing from a jump requires preparation, which must include appropriate positioning and configuration of the landing limbs and body to be successful. While well studied in mammals, our lab has been using the cane toad (Rhinella marinus) as a model for understanding the biomechanics of controlled landing in anurans, animals that use jumping or bounding as their dominant mode of locomotion. In this article, we report new results from experiments designed to explore how different modes of sensory feedback contribute to previously identified features of coordinated landing in toads. More specifically, animals in which vision, hindlimb proprioception, or vestibular feedback were removed, underwent a series of hopping trials while high-speed video was used to record and characterize limb movements and electromyographic (EMG) activity was recorded from a major elbow extensor (anconeus). Results demonstrate that altering any sensory system impacts landing behavior, though loss of vision had the least effect. Blind animals showed significant differences in anconeus EMG timing relative to controls, but forelimb and hindlimb movements as well as the ability to successfully decelerate the body using the forelimbs were not affected. Compromising hindlimb proprioception led to distinctly different forelimb kinematics. Though EMG patterns were disrupted, animals in this condition were also able to decelerate after impact, though with less control, regularly allowing their trunks to make ground contact during landing. Animals with compromised vestibular systems showed the greatest deficits, both in takeoff and landing behavior, which were highly variable and rarely coordinated. Nevertheless, animals in this condition demonstrated EMG patterns and forelimb kinematics similar to those in control animals. The fact that no ablation entirely eliminates all aspects of landing preparation suggests that its underpinnings are complex and that there is no single sensory trigger for its initiation.

Frog tendon structure and its relationship with locomotor modes

Tendon collagen fibrils are the basic force-transmitting units of the tendon. Yet, surprisingly little is known about the diversity in tendon anatomy and ultrastructure, and the possible relationships between this diversity and locomotor modes utilized. Our main objectives were to investigate: (a) the ultra-structural anatomy of the tendons in the digits of frogs; (b) the diversity of collagen fibril diameters across frogs with different locomotor modes; (c) the relationship between morphology, as expressed by the morphology of collagen fibrils and tendons, and locomotor modes. To assess the relationship between morphology and the locomotor modes of the sampled taxa we performed a principal component analysis considering body length, fibrillar cross sectional area (CSA) and tendon CSA. A MANOVA showed that differences between species with different locomotor modes were significant with collagen fibril diameter being the discriminating factor. Overall, our data related the greatest collagen fibril diameter to the most demanding locomotor modes, conversely, the smallest collagen fibril CSA and the highest tendon CSA were observed in animals showing a hopping locomotion requiring likely little absorption of landing forces given the short jump distances.

Neuromuscular control of locomotion is altered by tail autotomy in geckos

Animal locomotion is driven by underlying axial and appendicular musculature. In order for locomotion to be effective, these muscles must be able to rapidly respond to changes in environmental and physiological demands. Although virtually unstudied, muscles must also respond to morphological changes, such as those that occur with tail autotomy in lizards. Tail autotomy in leopard geckos (Eublepharis macularius) results in a 25% loss of caudal mass and significant kinematic alterations to maintain stability. To elucidate how motor control of the locomotor muscles is modulated with these shifts, we used electromyography (EMG) to quantify patterns of in vivo muscle activity in fore- and hind limb muscles before and after autotomy. Forelimb muscles (biceps brachii and triceps brachii) exhibited no changes in motor recruitment, consistent with unaltered kinematics after autotomy. Amplitude of activity of propulsive muscles of the hind limbs (caudofemoralis and gastrocnemius) was significantly reduced and coincided with decreases in the propulsive phases of femur retraction and ankle extension, respectively. The puboischiotibialis did not exhibit these changes, despite significant reductions in femur depression and knee angle, suggesting that reduction in mass and vertical ground-reaction force by autotomy allows for the maintenance of a more sprawled and stable posture without increasing motor recruitment of the support muscles. These results highlight the significant neuromuscular shifts that occur to accommodate dramatic changes in body size and mass distribution, and illuminate the utility of tail autotomy as a system for studying the neuromuscular control of locomotion.

Strategy of landing behavior of the tree frog Hyla japonica

Toads on the ground control landing behavior by a simple strategy in which they extend their elbows at roughly the same time after take-off and continue extending them at the same velocity until they land. However, this simple strategy does not work well in much more complicated arboreal habitat. We analyzed properties of jumping and timing of forelimb extension of the tree frog, Hyla japonica, to demonstrate how the arboreal species land safely. The results showed that distance, duration, and velocity of jump were not affected by explicit optic flow information, whereas the all three parameters decreased significantly in the blindfolded animals. This suggests that the frogs do not use optic flow information during flight but need visual information to plan a jump to reach the destination appropriately. Many animals generate prelanding behavior when time-to-contact reaches a threshold value. However, the results showed that a significant positive correlation was found between jump duration and time-to-contact both in normal and blindfolded animals. The slopes and the intercepts of the linear regression lines were about 0.8 and about 50 msec, respectively, in both cases. These suggest that, without any visual inputs, the animal can make the decision for prelanding forelimb extension at 20% of jump duration and start extending the forelimb after a motor delay of about 50 msec as in the normal condition. This strategy enables the tree frog to always maintain 80% of the jump duration to prepare for landing in their complex habitat.

Evidence toads may modulate landing preparation without predicting impact time

Within anurans (frogs and toads), cane toads (Bufo marinus) perform particularly controlled landings in which the forelimbs are exclusively used to decelerate and stabilize the body after impact. Here we explore how toads achieve dynamic stability across a wide range of landing conditions. Specifically, we suggest that torques during landing could be reduced by aligning forelimbs with the body's instantaneous velocity vector at impact (impact angle). To test whether toad forelimb orientation varies with landing conditions, we used high-speed video to collect forelimb and body kinematic data from six animals hopping off platforms of different heights (0, 5 and 9 cm). We found that toads do align forelimbs with the impact angle. Further, toads align forelimbs with the instantaneous velocity vector well before landing and then track its changes until touchdown. This suggests that toads may be prepared to land well before they hit the ground rather than preparing for impact at a specific moment, and that they may use a motor control strategy that allows them to perform controlled landings without the need to predict impact time.

Forelimb kinematics during hopping and landing in toads (Bufo marinus)

Coordinated landing in a variety of animals involves the re-positioning of limbs prior to impact to safely decelerate the body. However, limb kinematics strategies for landing vary considerably among species. For example, human legs are increasingly flexed before impact as drop height increases while in turkeys, legs are increasingly extended before impact with increasing drop height. In anurans, landing typically involves the use of forelimbs to decelerate the body after impact. Few detailed, quantitative descriptions of anuran forelimb kinematics during jumping exist and it isn't known if they prepare for larger landing forces by changing forelimb kinematics. In this study, we used high-speed video of 51 hops from five cane toads (Bufo marinus) to test the hypothesis that forelimb kinematics change predictably with distance. We measured excursions of the elbow (flexion/extension) and humerus (protraction/retraction and elevation/depression) throughout every hop. Results indicate that elbow and humeral excursions leading up to impact increase significantly with hop length, but do so without any change in the rate of movement. Instead, because the animal is in the air longer during longer hops, near-constant velocity movements lead to the larger excursions. These larger excursions in elbow extension result in animals hitting the ground with more extended forelimbs in longer hops, which in turn allows animals to decelerate over a greater distance.