This project will use electromyography, three-dimensional kinematics and force measurements to study the arboreal locomotion of snakes as a model system to better understand axial function of vertebrates and very general features of musculoskeletal systems and motor control. The narrow and cylindrical surfaces, inclines and gaps between branches in complex three-dimensional arboreal environments pose several demands well suited for determining: the limits and tradeoffs for musculoskeletal performance, proximate factors influencing gait selection, and the role of sensory information for modifying motor output and choosing different routes in complex environments. The study species have considerable variation in musculoskeletal anatomy that is universally important for function including: the ratio of tendon to contractile tissue length within muscles, the cross-sectional areas of muscle, total body length to weight, and the numbers and mobility of skeletal joints. This anatomical variation likely reflect tradeoffs between being flexible vs. stiff, strong vs. weak, and heavy vs. light, all of which are generally important as well as specifically relevant for snakes performing the different ecologically relevant tasks of actively bending to conform and grip cylindrical surfaces vs. supporting a body draped loosely over widely spaced branches. One study species is the brown tree snake, which causes costly power outages in Guam, but determining the surfaces that are impassable for this species and obtaining information on route choice and gap-bridging should facilitate managing this destructive invasive species. Interdisciplinary collaborators include a cognitive scientist specializing in the integration of perception and motor control and applied biologists who are attempting to decrease the economic costs of the brown tree snake. The project emphasizes providing opportunities for early career development of biologists from diverse backgrounds, disseminating scientific results to a wide audience including web materials, and providing lectures with an emphasis on evolution for K-12 students and the local community.
Despite the considerable diversity of animals that live and move in trees, arboreal locomotion is poorly understood compared to terrestrial locomotion. The narrow and cylindrical surfaces, inclines and gaps between branches in complex three-dimensional arboreal environments pose several demands well suited for determining: the limits and trade-offs for musculoskeletal performance, proximate factors influencing gait selection, and the role of sensory information for modifying motor output and choosing different routes. The project quantified whole animal performance, behavioral choice, three-dimensional kinematics and force measurements to study the arboreal locomotion of snakes as a model system to better understand axial function of vertebrates and very general features of musculoskeletal systems and motor control. Comparisons were also made with a few species of limbed vertebrates. Most data were for three species of snakes: 1) a stout species with short axial muscle segments (boa constrictors), 2) a morphologically intermediate species (corn snake), and 3) an attenuate arboreal specialist with long axial muscle segments (brown tree snakes). Both the structure of the surfaces on which the animals moved on and the three dimensional direction of movement were manipulated. The attributes of the animals and their environment combined to yield a remarkable amount of functional and behavioral diversity, and many consequences of environmental structure were species-dependent. Much of the observed diversity of snake gaits was associated with surface structure rather than speed, where this is the reverse for many limbed species of animals. We discovered more distinct types of arboreal snake locomotion than had been described previously for all types of terrestrial snake locomotion. This diversity runs counter to the frequent suggestion that axial function and motor pattern of vertebrates is relatively conservative, but such suggestions have arisen largely from work on the straight-line swimming of fishes. Many studies on the limits of locomotor performance of animals emphasize factors such as the contractile kinetics of muscle or the ability of muscle to produce mechanical power as the primary determinants of maximal speed. However, an under-appreciated aspect of locomotion in arboreal environments is the need to maintain balance. Many of the observed changes in gait and trends in performance emphasize the importance of balance rather than only the ability to produce forces. A useful analogy is the extent to which poor balance can limit the speed of a novice ice skater regardless of leg strength. For different environmental structure we detected trade-offs between morphology and performance between boa constrictors and corn snakes as the former was better at climbing smooth cylinders by strongly gripping them and the latter was superior at using sliding contact during arboreal lateral undulation past pegs that simulated secondary branches. The locomotion of the brown tree snakes was superior to both of these other species in all regards, and thus its specialized axial anatomy, including a prehensile tail, seems to lack any apparent trade-offs for different types of locomotion. Our results emphasize the importance of changing the orientation of the animal or its three-dimensional direction of movement. Moving horizontally on narrow cylinders was challenging because of the tendency to roll about the long axis of the surface. By contrast, long-axis rolling was rarely problematic for snakes going straight up, and despite having to lift the weight of the animal against the direction of gravity, some speeds of climbing vertically greatly exceeded those of moving horizontally on a narrow surface. In addition, when brown tree snakes moved straight up to bridge a gap, they greatly exceeded their ability to bridge a gap in a horizontal plane, presumably as a result of a reduced tendency for the body to buckle or pitch downward. The project supported the education, training and career development of one post-doc, six graduate students and 22 undergraduate students. By meeting with fewer than 30 students per individual class, the PI’s outreach regarding evolution and the biology of snakes has totaled more than 700 K-12 students from nearby public schools. In addition to the contributions of the project to basic concepts regarding animal movement, we may have discovered a practical application useful for preventing the invasive brown trees in Guam from causing electrical outages. Our laboratory and field tests suggest that smooth 3-4 inch diameter pipes covering the cables that are anchored to the ground and support utility poles would make this very difficult for snakes to traverse and hence prevent the most common way they gain access to power lines. Another design of a pipe plus a vertical sheet of material may be even more effective. In summary, our work emphasizes how relating form to function can benefit greatly by studying some of the diversity of form and behavior produced by evolution as well as attempting to better simulate some of the diverse of tasks animals regularly encounter while traversing their complex three-dimensional natural habitats.
