From swimming to walking with a salamander robot driven by a spinal cord model

The transition from aquatic to terrestrial locomotion was a key development in vertebrate evolution. We present a spinal cord model and its implementation in an amphibious salamander robot that demonstrates how a primitive neural circuit for swimming can be extended by phylogenetically more recent limb oscillatory centers to explain the ability of salamanders to switch between swimming and walking. The model suggests neural mechanisms for modulation of velocity, direction, and type of gait that are relevant for all tetrapods. It predicts that limb oscillatory centers have lower intrinsic frequencies than body oscillatory centers, and we present biological data supporting this. The salamander, an amphibian, is regardedas the tetrapod most closely resemblingthe first terrestrial vertebrates and repre-sents, therefore, a key animal from which the evolutionary changes from aquatic to terrestrial locomotion can be inferred (1, 2). It is capable of rapidly switching between two locomotion modes: swimming andwalking (3–5). The swim-ming mode is similar to that of the lamprey, a primitive fish, with fast axial undulations bein