Many animals, modern aircraft, and underwater vehicles use fusiform, streamlined body shapes that reduce fluid dynamic drag to achieve fast and effective locomotion in air and water. Similarly, numerous small terrestrial animals move through cluttered terrain where three-dimensional, multicomponent obstacles like grass, shrubs, vines, and leaf litter also resist motion, but it is unknown whether their body shape plays a major role in traversal. Few ground vehicles or terrestrial robots have used body shape to more effectively traverse environments such as cluttered terrain. Here, we challenged forest-floor-dwelling discoid cockroaches (Blaberus discoidalis) possessing a thin, rounded body to traverse tall, narrowly spaced, vertical, grass-like compliant beams. Animals displayed high traversal performance (79 ± 12% probability and 3.4 ± 0.7 s time). Although we observed diverse obstacle traversal strategies, cockroaches primarily (48 ± 9 % probability) used a novel roll maneuver, a form of natural parkour, allowing them to rapidly traverse obstacle gaps narrower than half their body width (2.0 ± 0.5 s traversal time). Reduction of body roundness by addition of artificial shells nearly inhibited roll maneuvers and decreased traversal performance. Inspired by this discovery, we added a thin, rounded exoskeletal shell to a legged robot with a nearly cuboidal body, common to many existing terrestrial robots. Without adding sensory feedback or changing the open-loop control, the rounded shell enabled the robot to traverse beam obstacles with gaps narrower than shell width via body roll. Terradynamically "streamlined" shapes can reduce terrain resistance and enhance traversability by assisting effective body reorientation Bioinspiration & Biomimetics (2015), 10, 046003; https://li.me.jhu.edu 2 via distributed mechanical feedback. Our findings highlight the need to consider body shape to improve robot mobility in real-world terrain often filled with clutter, and to develop better locomotor-ground contact models to understand interaction with 3-D, multi-component terrain. This image entitled 'Giant 'shrooms' has been obtained by the author from the Flickr website where it was made available by wonderferret under a CC BY 2.0 licence.] Here, we propose to advance terradynamics (Li et al. 2013) into three dimensions by going beyond relatively uniform, two-dimensional surfaces with three-dimensional obstacles of diverse, complex topology and mechanics, such as encountered in a forest floor with grass, shrubs, trees, and fungi (figure 1). In particular, small insects, arachnids, and reptiles face considerable challenges traversing such terrain, because these obstacles, which may be negligible for large animals, can be comparable or even much larger in size than themselves (Kaspari and Weiser 1999). Further, these obstacles can be densely cluttered with gaps, slits, and crevices comparable or even smaller than an animal's body, often pushing back against the animals, absorbing energy, and resisting locomotion, similar to surrounding fluids in flying and swimming. However, relative to fluids, the interaction of animals with such three-dimensional, multi-component terrain remains relatively unexplored (Jayne It is well known that animal shape can play an important role during locomotion in air and water . Almost all modern aerial and marine vehicles have adopted the fusiform, streamlined shapes common in large swimmers and fliers to reduce fluiddynamic drag (Etkin 1972 , Newman 1977 . Similarly, for animals that must use their bodies to interact with and traverse three-dimensional, multi-component terrain, body shape might also affect locomotion. Yet, despite many ecological and functional morphology studies which established correlations between body and limb shapes and habitat use in insects, lizards, snakes, and birds (Burnham et al.
Li et al. REF used different shell shapes in legged robot and insect locomotion experiments to reach the conclusion that rounded, more terradynamically streamlined shapes increased the chance of traversal of densely cluttered vertical grass-like beams.
Terradynamically streamlined shapes in animals and robots enhances traversability through densely cluttered terrain
Bioinspiration&Biomimetics (2015), 10, 046003
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