Amphibious and Sprawling Locomotion: From Biology to Robotics and Back

A milestone in vertebrate evolution, the transition from water to land, owes its success to the development of a sprawling body plan that enabled an amphibious lifestyle. The body, originally adapted for swimming, evolved to benefit from limbs that enhanced its locomotion capabilities on submerged and dry ground. The first terrestrial animals used sprawling locomotion, a type of legged locomotion in which limbs extend laterally from the body (as opposed to erect locomotion, in which limbs extend vertically below the body). This type of locomotion-exhibited, for instance, by salamanders, lizards, and crocodiles-has been studied in a variety of fields, including neuroscience, biomechanics, evolution, and paleontology. Robotics can benefit from these studies to design amphibious robots capable of swimming and walking, with interesting applications in field robotics, in particular for search and rescue, inspection, and environmental monitoring. In return, robotics can provide useful scientific tools to test hypotheses in neuroscience, biomechanics, and paleontology. For instance, robots have been used to test hypotheses about the organization of neural circuits that can switch between swimming and walking under the control of simple modulation signals, as well as to identify the most likely gaits of extinct sprawling animals. Here, I review different aspects of amphibious and sprawling locomotion, namely gait characteristics, neurobiology, numerical models, and sprawling robots, and discuss fruitful interactions between robotics and other scientific fields.