Ulnar curvature has long been recognized as an indicator of locomotor behavior in mammals, although its relevance has yet to be thoroughly tested in a phylogenetically restricted and functionally diverse group. Extant cercopithecids exhibit a wide range of quadrupedal locomotor behaviors and substrate use, making them an ideal benchmark to test form-function relationships between ulnar curvature and locomotion. While their ulnar curvature has been partly investigated through its anteroposterior curvature, the mediolateral curvature remains largely unexplored. We hypothesized that ulnar curvature covary with habitual substrate use (i.e., terrestrial versus arboreal) and locomotor behaviors (e.g., suspension versus climbing) in both sagittal and coronal planes. In this study, we provide a comprehensive assessment of ulnar curvature in extant cercopithecids, based on an extensive and taxonomically diverse sample of 23 species and 167 individuals, to assess inter- and intraspecific morphological variation. As expected, our analyses confirm previous findings regarding anteroposterior curvature, with terrestrial quadrupeds exhibiting an anteriorly convex ulna, and arboreal taxa showing an anteriorly concave ulna. Regarding mediolateral curvature, arboreal taxa exhibit a lateral convexity, while terrestrial quadrupeds show a more complex sigmoid curvature, possibly reflecting resistance to the various mediolateral constraints generated by hand postures. Although the two curvatures seem to distinguish arboreal and terrestrial locomotor behaviors, their moderate covariation (~55%) suggests that curvature responds to partially distinct biomechanical factors. Suspensory taxa, previously thought to possess relatively straight ulnae, are revealed to have noticeable anteriorly concave bones, consistent with notable brachialis contraction during suspension. Unexpectedly, climbers show intermediate morphologies between arboreal and terrestrial quadrupeds, supporting the idea that ulnar curvature does not allow their distinction, implying that this behavior is difficult to infer through ulnar curvatures.

Understanding terrestrial locomotion in walking fish species can unlock new insights into vertebrate evolution and inspire versatile robotic systems capable of traversing diverse environments. We introduce a novel, single-actuator continuum robot inspired by the terrestrial locomotion of the gray bichir (Polypterus senegalus), which employs a simple rotating helix to reproduce realistic undulatory movements. We hypothesized that a simplified robotic model with minimal actuation could accurately replicate the terrestrial locomotion patterns observed inP. senegalus. Using a 'robot-twin' methodology, we developed four helix configurations directly informed by the observed gait postures of real fish specimens and compared robotic performance and kinematics against biological data. We found that helix geometry significantly influenced both locomotion speed and lateral stability, with designs closely mimicking biological curvatures often exhibiting trade-offs between accuracy and performance. The fastest helix configuration produced the greatest lateral oscillation, whereas the most biologically accurate shape resulted in reduced locomotion efficiency. Additionally, integrating passive leg structures greatly enhanced stability, mirroring the biomechanical function of pectoral fins in the real fish. These findings underscore the value of minimalistic robotic designs in understanding fish-like locomotion and pave the way for future robotic platforms using reduced degrees of freedom.

Modeling and Oscillation Suppression for a Rigid-flexible Coupled Tail in a Crocodile-inspired Robot During Terrestrial Locomotion

Sea turtle hatchlings display maneuvering capabilities across diverse aquatic and coastal terrains. While turning behavior is crucial in aquatic environments, it is equally vital for terrestrial locomotion by hatchlings that must quickly navigate obstacle-rich terrain on their way to the sea. This study introduces a robotic prototype that emulates the turning strategies of juvenile sea turtles to optimize turning rate and energy consumption across diverse terrestrial surfaces. The research investigates the rotational displacement capabilities of a bioinspired robot across five distinct gait configurations: one involving all flippers in a unique pattern, and four employing reduced flipper combinations, including front, diagonal, back, and single flippers. We investigated the robot's turning capabilities on diverse granular and compliant media, including four specified rock sizes, a consistent foam platform, and dry sand. Comparative analyses were conducted using rigid and soft flipper designs. Key locomotion features, including roll, pitch, yaw, and lift height, were quantified for each configuration. The results reveal significant differences in rotational behavior across terrains and gait styles, highlighting the interplay between flipper design, gait strategy, and environmental adaptability. This research advances the understanding of bioinspired robotics for applications in complex and variable environments.

A trade-off between aquatic and terrestrial locomotion is self-evident at broad phylogenetic scales. While the effects of more subtle trade-offs in the evolution of closely related species are less clear, they are hypothesized to drive ecological speciation and adaptive radiation. Amphibious animals strike a balance between aquatic and terrestrial activity, and the need to maintain performance in one medium is hypothesized to constrain evolution of high performance in the other (the running-swimming dilemma). Closely related species of Desmognathus salamanders partition local habitats along a gradient from mid-stream to stream edge to completely terrestrial. The trade-off hypothesis predicts that these species will differ in relative running vs. swimming performance depending on the relative importance of each mode of locomotion in their niches. Here, I show that primarily aquatic Desmognathus ecomorphs are superior swimmers and inferior runners relative to semi-aquatic ecomorphs using paired escape performance trials in aquatic and terrestrial arenas. I measured performance as the velocity of the fast-start response to simulated predator attack. I tested two species of each ecomorph, representing two divergent clades with parallel evolution of aquatic and semi-aquatic species. Notably, the different southern clade ecomorphs have been genetically isolated for millions of years, whereas the northern clade ecomorphs share a recent common ancestor and interbreed regularly. My results showed a negative correlation between aquatic and terrestrial performance, with aquatic ecomorphs being faster swimmers and semi-aquatic ecomorphs being faster runners. While there was a possible trend consistent with faster swimming speeds of northern forms relative to their southern counterparts, the functional differences between ecomorphs were similar in both clades. These results are consistent with the hypothesis that trade-offs between aquatic and terrestrial locomotion have contributed to a consistent pattern of habitat partitioning during parallel speciation.

The uncinate process (UP), a dorsocaudal projection from the vertebral ribs, represents a pivotal adaptation in the avian thoracic skeleton, serving as a mechanical brace that enhances respiration and stabilizes the trunk during flight. This literature review synthesizes anatomical, functional, developmental, and evolutionary perspectives on UP morphology across diverse bird taxa. A systematic search spanning 2015–2025 identified 20 relevant studies detailing the structural variability of UPs—categorized as short, intermediate, or long—correlating with flight styles, such as soaring, diving, or terrestrial locomotion. Long UPs, observed in species like penguins and cormorants, are associated with enhanced ventilatory efficiency and thoracic rigidity for high-energy propulsion, while short UPs in flightless birds reflect reduced respiratory demand. Developmental studies reveal ontogenetic shifts from cartilaginous to ossified UPs, aligned with increasing locomotor activity. Evolutionary analysis underscores the UP as a conserved synapomorphy of Aves, with convergent elongation in unrelated taxa emphasizing its functional significance. Structural integration with intercostal musculature and adjacent ribs enables efficient force transmission and thoracic stabilization, vital for maintaining trunk posture during flight. Beyond its evolutionary relevance, UP morphology has practical implications in avian health, surgical ventilation strategies, and bioinspired biomechanical applications. This review highlights the UP as an underappreciated yet essential component of avian musculoskeletal architecture, offering insights into respiratory evolution, ecological adaptation, and comparative vertebrate anatomy. Future investigations should expand morphometric databases and apply high-resolution imaging and biomechanical modeling to further elucidate the functional roles of the UP in avian physiology and evolution.

A mechano-resistance mechanism in skin adapts to terrestrial locomotion.

Soft robot motion performance has long been a core focus in scientific research. This study investigates the motion capabilities of soft robots constructed using poly(N-isopropylacrylamide) (PNIPAM) hydrogels, with key innovations in material design and functional enhancement. By optimizing the hydrogel formulation and incorporating molybdenum disulfide (MoS2) to endow it with photothermal response properties, the material achieves muscle-like controllable contraction and expansion deformation—a critical breakthrough in mimicking biological motion mechanics. Building on this material advancement, the research team developed a series of soft robotic prototypes to systematically explore the hydrogel’s motion characteristics. A flytrap-inspired soft robot demonstrates rapid opening–closing movements, replicating the swift responsiveness of natural carnivorous plants. For terrestrial locomotion, a hexapod crawling robot utilizes the photo-induced stretch-recovery mechanism of both horizontally configured and pre-bent feet to achieve stable directional propulsion. Most notably, a magnetically driven rolling robot integrates magnetic units to realize versatile multimodal movement: it achieves a stable rolling speed of 1.8 cm/s across flat surfaces and can surmount obstacles up to 1.5 times its own body size. This work not only validates the strong potential of PNIPAM hydrogel-based soft robots in executing complex motion tasks but also provides valuable new insights for the development of multimodal soft robotic systems, paving the way for future innovations in adaptive and bio-inspired robotics.

Musculoskeletal comparison of the pectoral fin in mudskippers (Gobiidae: Oxudercinae).

In response to the challenges faced by unmanned aerial vehicles (UAV) in cluttered environments such as forests, ruins, and pipelines, this study introduces a ground–air amphibious UAV specifically designed for personnel search and rescue in complex environments. By innovatively designing and applying a separation cage structure, the UAV’s capabilities for ground movement and aerial flight have been enhanced, effectively overcoming the limitations of traditional single-mode robots operating in narrow or obstacle-dense areas. This design addresses the occlusion issue of sensing components in traditional caged UAVs while maintaining protection for both the UAV itself and the surrounding environment. Additionally, through the innovative design of an H-shaped quadcopter frame skeleton structure, the UAV has gained the ability to perform steady-state aerial flight while also better adapting to the separation cage structure, achieving a reduced energy consumption and significantly improving its operational capabilities in complex environments. The experimental results demonstrate that the UAV prototype, weighing 1.2 kg with a 1 kg payload capacity, achieves a 40 min maximum endurance under full payload conditions at the endurance speed of 10 m/s while performing real-time object detection. The system reliably executes multimodal operations, including stable takeoff, landing, aerial hovering, directional maneuvering, and terrestrial locomotion with coordinated steering control.

A new trackway produced by crawling fishes, which includes imprints of the trunk, snout, tail, body drag traces, and pectoral fins, was discovered in the Lower Devonian (middle-upper Emsian) marginal marine deposits in the Holy Cross Mountains, Poland. The snout imprints are represented by a low-angle variant of the already described Osculichnus tarnowskae, which has generally been interpreted as a hunting trace of fishes. However, in this case, it is considered an imprint of a fish's snout, used for anchoring in the sediment during the locomotion of at least partially emerged fish. This compound trackway provides the first evidence of the previously unknown life behaviour and locomotion abilities of dipnoan fishes in the early stage of their evolution and documents a testing land mobility skills of vertebrates, predating by about 10million years fully terrestrial tetrapods locomotion traces. Similar trackways are produced by extant lungfish during terrestrial locomotion. The trackway co-occurs with a new resting trace produced by a dipnoan fish supporting itself with one or two pairs of fins on the bottom.

Screw propulsors have attracted increasing attention for their potential applications in amphibious vehicles and benthic robots, owing to their ability to perform both terrestrial and underwater locomotion. To investigate their hydrodynamic characteristics, a two-stage numerical analysis was carried out. In the first stage, steady-state simulations under various advance coefficients were conducted to evaluate the influence of key geometric parameters on propulsion performance. Based on these results, a representative configuration was then selected for transient analysis to capture unsteady flow features. In the second stage, a Detached Eddy Simulation approach was employed to capture unsteady flow features under three rotational speeds. The flow field information was analyzed, and the mechanisms of vortex generation, instability, and dissipation were comprehensively studied. The results reveal that the propulsion process is dominated by the formation and evolution of tip vortices, root vortices, and cylindrical wake vortices. As rotation speed increases, vortex structures exhibit a transition from ordered spiral wakes to chaotic turbulence, primarily driven by centrifugal instability and nonlinear vortex interactions. Vortex breakdown and energy dissipation are intensified downstream, especially under high-speed conditions, where vortex integrity is rapidly lost due to strong shear and radial expansion. This hydrodynamic behavior highlights the fundamental difference from conventional propellers, and these findings provide theoretical insight into the flow mechanisms of screw propulsion.

The climbing perch, Anabas testudineus, is one of the most widely distributed freshwater amphibious fishes in South and Southeast Asia, exhibiting terrestrial movements. Our experimental study aimed to investigate endocrinological and biochemical changes in the blood of climbing perch associated with their terrestrial movements. To achieve this, the fish were divided into two groups: one group was exposed to aquatic conditions for twenty minutes, while the other group was subjected to terrestrial conditions for the same duration through rapid water level decrease. In terrestrial conditions, the fish predominantly exhibit movements on land, whereas in aquatic environments, they primarily remain immobile or swim. Elevated levels of stress-induced cortisol and glucose after short-term exposure indicate a high-stress response involving both neuroendocrine and metabolic mechanisms. Changes in the activity of aspartate aminotransferase and increased concentrations of triglycerides in the blood serum suggest energy mobilization through aerobic metabolic pathways. Extreme environmental changes did not affect thyroid axis function, including deiodination, thereby maintaining essential physiological activities under new conditions. Additionally, the anaerobic metabolic pathway appears to be minimally utilized at the onset of terrestrial movement, as no significant changes in lactate dehydrogenase concentrations were observed. Overall, the terrestrial movements of the climbing perch are likely predominantly forced and associated with high stress.

The ability to achieve controllable multimodal locomotion on complex terrains is crucial for the practical applications of small-scale legged robots. In this study, a novel magnetically actuated soft quadrupedal terrestrial millirobot was designed. Inspired by biological terrestrial locomotion modes, three distinct locomotion modes-quadrupedal bounding, quadrupedal pacing, and bipedal walking-were realized through a combination of various postures under a uniform external magnetic field and asymmetrical friction effects induced by magnetic torque. The characteristics of these modes were examined and compared, including the effects of magnetic field strength, swing angle, and surface roughness on stride length. Furthermore, the line-of-sight control method was implemented in path-tracking experiments, enabling closed-loop control on complex paths and improving tracking accuracy. This research holds significant potential for applying magnetically controlled small-scale robots in the bioengineering and industrial micromanipulation fields.

Abstract Amphibious fishes, with varying degrees of terrestriality, occur in a broad variety of marginal lacustrine and marginal marine settings. Most of these rely on axially driven locomotion (tail and torso bending) in terrestrial movement. Oxudercid gobiids (mudskippers) are distinctive in that their normal terrestrial locomotion is driven primarily by a pulling motion of their pectoral fins resulting in movement termed ‘crutching’. This contribution is focused on the traces emplaced in sand and mud by mudskippers, formed during the locomotory activity of the two most terrestrialized mudskipper genera: Periophthalmus and Periophthalmodon. The traces produced were compared with Devonian ichnofossils in order to frame the discussion of the nature and timing of the earliest vertebrate invasion of terrestrial realms. Mudskipper traces emplaced by large (> 20 cm body length) Periophthalmodon in subaerial, wet, but unsaturated mud produced exceptionally distinctive traces. These traces are characterized by distinct central tail and body grooves, bordered by small distinct pelvic fin traces and larger, arcuate pectoral fin traces. Fin traces in mud substrates commonly retain discrete fin ray impressions. The unique crutching motion results in an approximately symmetrical trace, with paired fin traces on each side of the central grooves. Traces made by smaller Periophthalmus may look similar or may be less distinctive due to the lighter weight of the animals. Traces made in sand may lack pelvic fin impressions and, in some cases may be limited to the central body groove. Mudskipper locomotion traces are similar to the arthropod ichnogenera Siskemia and Stiaria. We note the similarity of some mudskipper tracks to some trackways previously inferred to be made by tetrapods. Assessing the traces that modern amphibious fishes make can inform the search for similar behaviors recorded by Devonian traces and by traces made by previously unrecognized amphibious fishes throughout the post-Devonian interval.

Long-bone microanatomy in elephants: microstructural insights into gigantic beasts

Microscopic robots exhibit efficient locomotion in liquids by leveraging fluid dynamics and chemical reactions to generate force asymmetry, thereby enabling critical applications in photonics and biomedicine. However, achieving controllable locomotion of such robots on terrestrial surfaces remains challenging because fluctuating adhesion on nonideal surfaces disrupts the necessary asymmetry for propulsion. Here, we present a microscopic robot composed of three-dimensional nanomembranes, which navigate diverse terrestrial surfaces with omnidirectional motion. We propose a general mechanism employing nonreciprocal shape morphing to generate stable asymmetric forces on surfaces. This nonreciprocal shape morphing is realized through a laser-actuated vanadium dioxide nanomembrane, leveraging the material's inherent hysteresis properties. We demonstrate that these robots can be fabricated in various shapes, ranging from simple square structures to bioinspired "bipedal" helical designs, enabling them to directionally navigate challenging surfaces such as paper, leaves, sand, and vertical walls. Furthermore, their omnidirectional motion facilitates applications in microassembly and microelectronic circuit integration. Additionally, we developed an artificial intelligence control algorithm based on reinforcement learning, enabling these robots to autonomously follow complex trajectories, such as tracing the phrase "hello world". Our study lays a theoretical and technological foundation for microscopic robots with terrestrial locomotion and paves a way for microscopic robots capable of operating on surfaces for advanced nanophotonic, microelectronic, and biomedical applications.

Cutting‐edge applications require robots to be capable of navigating in different environments to improve the time and energy efficiency of travel and exploration. Amphibious robots address this need by operating in both aquatic and terrestrial settings, leveraging biomimetic designs and integrated mechanical propulsion systems. While these robots has traditionally drawn inspiration from reptiles, crustaceans, and amphibians, there has been comparatively less exploration of swimming‐capable mammals and birds as biological models. In this work, DuckyDog, a quadruped amphibious robot using erect posture has been proposed and developed to take full advantage of the high mobility of mammals on land. By integrating a duck‐like body and passive fins, it also has excellent swimming ability on the water surface. Whereas many robots rely on soft materials to construct compliant legs, DuckyDog is distinguished by its fused deposition modeling‐printed legs made from polylactic acid (PLA) filament, featuring structurally induced variable stiffness and actuated through a tendon‐driven mechanism. A series of experiments are conducted to evaluate DuckyDog's terrestrial and aquatic locomotion performance in both laboratory settings and complex natural environments, where it achieves maximum speeds of 0.40 body lengths per second on land and 0.30 in water.

This paper introduces a novel robot designed to exhibit two distinct modes of mobility: rotational aerial flight and terrestrial locomotion. This versatile robot comprises a sturdy external frame, two motors, and a single wing embodying its fuselage. The robust frame not only facilitates mobility but also makes the robot collision-tolerant in both modes, enhancing its resilience in challenging environments. The robot is capable of vertical takeoff and landing in mono-wing flight mode, with the unique ability to fly in both clockwise and counterclockwise directions, setting it apart from traditional mono-wings. While relying on just two actuators, the robot seamlessly transitions to ground locomotion mode, utilizing its frame to facilitate rolling motion on the ground. The unique design ensures that the robot avoids any unrecoverable states, making it exceptionally robust and reliable in diverse operational scenarios. The robot demonstrates the capability to perform controlled ascents and descents on inclined surfaces with gradients of up to 15°. The integration of ground rolling mode significantly enhances operational efficiency. The paper provides a comprehensive overview of the robot’s design, dynamic model and functionality, along with experimental results for waypoint tracking in both aerial flight and terrestrial locomotion, its turning ability on various surfaces, the bi-directional transition, power consumption, and an example application outdoors. The combination of minimal actuators enabling different modes of mobility and robustness against unrecoverable states highlights the distinct capabilities and reliability of this platform.

Underwater sensor deployment in military applications requires high precision, yet existing robotic solutions often lack the maneuverability and adaptability required for complex aquatic environments. To address this gap, this study proposes a bio-inspired underwater robot modeled after the marine iguana, which exhibits effective crawling and swimming capabilities. The research aims to develop a compact, multi-functional robot capable of precise sensor deployment and environmental detection. The methodology integrates a biomimetic mechanical design—featuring leg-based crawling, tail-driven swimming, a deployable head mechanism, and buoyancy control—with a multi-sensor control system for navigation and data acquisition. Gait and trajectory planning are optimized using kinematic modeling for both terrestrial and aquatic locomotion. Experimental results demonstrate the robot’s ability to perform accurate underwater sensor deployment, validating its potential for military applications. This work provides a novel approach to underwater deployment robotics, bridging the gap between biological inspiration and functional engineering.