Propelling and perturbing appendages together facilitate strenuous ground self-righting

Abstract

Terrestrial animals must self-right when overturned on the ground. To do so, the discoid cockroach often pushes its wings against the ground to begin a somersault but rarely succeeds in completing it. As it repeatedly attempts this, it probabilistically rolls to the side to self-right. Here, we studied whether seemingly wasteful leg flailing in this process helps. Adding mass to increase hind leg flailing kinetic energy fluctuation increased the animal's self-righting probability. We then developed a robot with similar, strenuous self-righting behavior and used it as a physical model for systematic experiments. As legs flailed more vigorously and wings opened more, self-righting became more probable. A potential energy landscape model revealed that, although wing opening did not generate sufficient kinetic energy to overcome the high pitch potential energy barrier, it reduced barriers for rolling, facilitating the small kinetic energy fluctuation from leg flailing to probabilistically overcome roll barriers to self-right.

Locomotor transitions in the potential energy landscape-dominated regime

“…To enable systematic experiments (as in a wind or water tunnel), for each model terrain, we created a testbed that allowed controlled, systematic variation of obstacle properties such as stiffness [50], geometry [52] and size [53,54] (figure 3b). In addition, because animals and robots often flip over when traversing large obstacles [4,52,55], we studied strenuous ground self-righting in which existing appendages must be co-opted [55][56][57][58][59]. Furthermore, we developed tools to address technical challenges in measuring locomotor transitions and locomotor-terrain interaction in complex three-dimensional terrain (figure 3b-d; electronic supplementary material, Text S1).…”

“…Using robotic physical models, we discovered several principles of locomotor transitions with feed-forward selfpropulsion. First, locomotor kinetic energy fluctuation from self-propulsion helps the system stochastically cross potential energy barriers to make transitions [50,57]. In addition, escape from a basin is more likely in directions on the landscape along which the barriers are lower [50,57].…”

“…First, locomotor kinetic energy fluctuation from self-propulsion helps the system stochastically cross potential energy barriers to make transitions [50,57]. In addition, escape from a basin is more likely in directions on the landscape along which the barriers are lower [50,57]. Finally, during a transition, the system tends to transition to more favourable modes attracted to lower basins [50,52,57].…”

“…Not surprisingly, the animal can make active adjustments to facilitate or enable desired transitions when feed-forward self-propulsion is insufficient. For example, even when (iv) beams (ii) gap fast, pitch-up, head-on approach to cross [53] (i) bump fast, pitched-up, head-on approach to climb [54] (iii) pillars shape change and turning with leg [52] co-opted wing opening and leg oscillation to roll [56][57][58][59][60] deflect S1 and text S3-S7 for detail. We renamed some modes/basins in this review from in the original papers to better distinguish them across model systems.…”

“…Transitions between modes occur stochastically, with large trial-to-trial variation[4,50,51,53,54,56,57]. The probability of using or transitioning to a mode strongly depends on locomotor and terrain parameters that affect physical interaction[4,50,[52][53][54]56,57] ( §4f ). The robot's locomotor modes are also stereotyped and transitions stochastic[4,[50][51][52][53][54][55]57].…”