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ABSTRACT Arboreal animals face potentially substantial torques when reaching across gaps, which may limit the size of the gap they can cross with non-dynamic behaviors. Snakes can experience particularly large torques during gap crossing, because they must counteract both pitching and buckling torques while extending into the gap in a cantilever-like posture. Pitching and buckling limits may impose different constraints on gap-crossing locomotion, but their relative importance remains unclear. To investigate this problem, we analyzed kinematic and morphological data related to gap crossing in Dendrelaphis snakes, complemented by data from their sister taxon, Chrysopelea, to test predictions from a theoretical model of cantilever failure. Specifically, we evaluated whether the existing model of buckling predicts maximum observed cantilever performance in these snakes, and compared those predictions with pitching limits estimated from body mass distributions. The buckling model produced estimates far below observed cantilever extents, even across plausible ranges of tendon ratio and multi-articular span, indicating that the current model does not yet capture the mechanics of buckling in cantilevering snakes. By contrast, snakes switched to dynamic movements close to their theoretical pitching limit, supporting the idea that pitching torques constrain cantilever abilities. These results suggest that pitching torque is a major constraint on cantilever-based gap crossing, while also identifying the need for improved buckling models with higher anatomical fidelity. More broadly, this study shows how testing mechanical models against behavior can reveal both the limits shaping locomotor performance and the limitations of existing models. 

Abstract Snakes are masters of movement through cluttered, confined, and complex habitats, leading to attempts to replicate this mastery in snake robots to improve our access to these challenging environments. While snakes use a variety of locomotor modes, rectilinear allows snakes to move through even the narrowest of spaces. However, while other modes are driven by axial bending, rectilinear relies on motion of the muscular skin, making it challenging to replicate in robots. Prior robotic rectilinear was achieved using secondary actuator systems (e.g. prismatic joints), anteriorly propagating waves, and/or standing waves, which require additional mechanical complexity, high vertical clearance, or or dedicated mechanical parts for symmetry-breaking (e.g. electromagnets, deployable frictional pads), respectively. Here, I present a new algorithm which closely matches the robot to the motion of the snake's ventral skin (rather than the whole body) during rectilinear locomotion. Robot segments are assigned roles of either rigid ventral scales or flexible inter-scale skin. As in biological snakes, scale segments are lifted clear of the substrate and moved forward via contraction and expansion of the adjacent skin motors. This cutaneous rectilinear algorithm allows rectilinear locomotion without the need for additional actuators or other specialized modifications, and achieves similar displacement per cycle and current consumption to propagating vertical wave methods at a lower vertical height, improving access to highly confined spaces. 

ABSTRACT Multiarticular muscle systems are widespread across vertebrates, including in their necks, digits, tails and trunks. In secondarily limbless tetrapods, the multiarticular trunk muscles power nearly all behaviors. Using snakes as a study system, we previously used anatomical measurements and mathematical modeling to derive an equation relating multiarticular trunk muscle shortening to postural change. However, some snake trunk muscles have long, thin tendinous connections, raising the possibility of elastic energy storage, which could lead to a decoupling of muscle length change from joint angle change. The next step, therefore, is to determine whether in vivo muscle shortening produces the postural changes predicted by mathematical modeling. A departure from predictions would implicate elastic energy storage. To test the relationship between muscle strain and posture in vivo, we implanted radio-opaque metal beads in three muscles of interest in four corn snakes (Pantherophis guttatus), then recorded X-ray videos to directly measure muscle shortening and vertebral column curvature during locomotion. Our in vivo results produced evidence that elastic energy storage does not play a substantial role in corn snake lateral undulation or tunnel concertina locomotion. The ability to predict muscle shortening directly from observed posture will facilitate future work. Moreover, the generality of our equation, which uses anatomical values that can be measured in many types of animals, means that our framework for understanding multiarticular muscle function can be applied in numerous study systems to provide a stronger mechanistic understanding of organismal function. 

Abstract Organismal solutions to natural challenges can spark creative engineering applications. However, most engineers are not experts in organismal biology, creating a potential barrier to maximally effective bioinspired design. In this review, we aim to reduce that barrier with respect to a group of organisms that hold particular promise for a variety of applications: snakes. Representing >10% of tetrapod vertebrates, snakes inhabit nearly every imaginable terrestrial environment, moving with ease under many conditions that would thwart other animals. To do so, they employ over a dozen different types of locomotion (perhaps well over). Lacking limbs, they have evolved axial musculoskeletal features that enable their vast functional diversity, which can vary across species. Different species also have various skin features that provide numerous functional benefits, including frictional anisotropy or isotropy (as their locomotor habits demand), waterproofing, dirt shedding, antimicrobial properties, structural colors, and wear resistance. Snakes clearly have much to offer to the fields of robotics and materials science. We aim for this review to increase knowledge of snake functional diversity by facilitating access to the relevant literature. 

Abstract Muscles spanning multiple joints play important functional roles in a wide range of systems across tetrapods; however, their fundamental mechanics are poorly understood, particularly the consequences of anatomical position on mechanical advantage. Snakes provide an excellent study system for advancing this topic. They rely on the axial muscles for many activities, including striking, constriction, defensive displays, and locomotion. Moreover, those muscles span from one or a few vertebrae to over 30, and anatomy varies among muscles and among species. We characterized the anatomy of major epaxial muscles in a size series of corn snakes (Pantherophis guttatus) using diceCT scans, and then took several approaches to calculating contributions of each muscle to force and motion generated during body bending, starting from a highly simplistic model and moving to increasingly complex and realistic models. Only the most realistic model yielded equations that included the consequence of muscle span on torque‐displacement trade‐offs, as well as resolving ambiguities that arose from simpler models. We also tested whether muscle cross‐sectional areas or lever arms (total magnitude or pitch/yaw/roll components) were related to snake mass, longitudinal body region (anterior, middle, posterior), and/or muscle group (semispinalis‐spinalis, multifidus, longissimus dorsi, iliocostalis, and levator costae). Muscle cross‐sectional areas generally scaled with positive allometry, and most lever arms did not depart significantly from geometric similarity (isometry). The levator costae had lower cross‐sectional area than the four epaxial muscles, which did not differ significantly from each other in cross‐sectional area. Lever arm total magnitudes and components differed among muscles. We found some evidence for regional variation, indicating that functional regionalization merits further investigation. Our results contribute to knowledge of snake muscles specifically and multiarticular muscle systems generally, providing a foundation for future comparisons across species and bioinspired multiarticular systems. 

Outreach events are an excellent way to expose science disciplines to the public beyond the ivory tower of academics. In this paper, we describe how a zoo-academia collaborative team designed and hosted an interdisciplinary outreach event exposing thousands to the wonders of bio-inspired design and biomechanics. Over the past four years, we have run such a collaborative outreach event which has attracted 26,750 people, allowing children and adults to be exposed to diverse research fields through our annual Zoo Biomechanics Day. In partnering with labs, companies, and student organizations, we have hosted 17 unique booths highlighting human, plant, and animal biomechanics and bio-inspiration to showcase engaging demonstrations of science, technology, engineering, arts, and medicine (STEAM) principles. This event is low-cost ( < $300 annually) while increasing attendance at the zoo by over 65% compared to similar non-event days. Furthermore, participating researchers get connected to the zoo, allowing discussions of new collaborative research between the zoo and universities. This paper will serve as a framework for others to host events which highlight the zoo-university collaborations that enhance the dual objectives of outreach and research at modern zoos. 

(no formal abstract for Short Notes) This paper provides the first description of reverse concertina locomotion in snakes, elicited by challenging the snakes to move into and then escape a dead-end tunnel. This discovery shows that, despite behavioral reluctance, snakes are capable of generating coordinated locomotion in a posterior-to-anterior fashion, which has implications for the neural control of locomotion.