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 surroun...

STATEMENTComparative study of cockroach self-righting reveals performance advantages of using rotational kinetic energy to overcome potential energy barrier and rolling more to lower it, while maintaining diverse strategies. ABSTRACTTerrestrial animals often must self-right from an upside-down orientation on the ground to survive. Here, we compared self-righting strategies of the Madagascar hissing, American, and discoid cockroaches on a challenging flat, rigid, low-friction surface to quantify the mechanical principles. All three species almost always self-righted (97% probability) when given time (30 seconds), frequently self-righted (63%) on the first attempt, and on that attempt did so in one second or less. When successful, two of the three species gained and used pitch and/or roll rotational kinetic energy to overcome potential energy barriers (American 63% of all attempts and discoid 78%). By contrast, the largest, heaviest, wingless cockroach (Madagascar hissing) relied far less on the energy of motion and was the slowest to self-right. Two of the three species used rolling strategies to overcome low potential energy barriers. Successful righting attempts had greater rolling rotation than failed attempts as the center of mass rose to the highest position. Madagascar hissing cockroaches rolled using body deformation (98% of all trials) and the American cockroach relied on leg forces (93%). By contrast, the discoid cockroach overcame higher and a wider range of potential energy barriers with simultaneous pitching and rolling using wings (46% of all trials) and legs (49%) equally to self-right. Our quantification revealed the performance advantages of using Journal of Experimental Biology (2019), 222, jeb186080; https://li.me.jhu.edu 2 rotational kinetic energy to overcome the potential energy barrier and rolling more to lower it, while maintaining diverse strategies for ground-based self-righting.