March 8, 2007 — Salamanders swim with the slippery ease of an eel. But as soon as their toes touch bottom, they switch rapidly to the four-legged gait typical of lizards.
Because the critters haven't changed much in about 150 million years, most researchers think that salamanders may resemble the first land-based vertebrates. But how so-called tetrapods evolved from swimmers to walkers remains a secret locked away in the complex circuitry of their spinal cords.
Now researchers have replicated that circuitry with a computer model and a salamander-like robot — Salamandra Robotica — that may help tease out some answers.
"We hypothesize that the evolutionary transition was made by reusing and extending a previous circuit designed for swimming, rather than developing a completely new locomotion circuit," said Auke Jan Ijspeert, a professor at the Swiss Federal Institute of Technology in Lausanne.
Ijspeert and his team reported their results in this week's issue of Science.
The researchers started with a computer model for the spinal cord of a lamprey eel-like animal and then added functions for limbs. They used the model along with a laptop and a wireless signal to control the walking and swimming of Salamandra Robotica.
They found that although the spinal circuitry that controls locomotion is fairly complex and sophisticated, the stimulations needed to change the robot's gait were simple. A variation in just two simple signals sent from the brain stem did the trick. Applying asymmetrical signals to neurons along the spine caused the robot to turn left or right, whether swimming or walking.
"This exactly replicates how gait transitions can be induced by electrical stimulation in a real salamander," said Ijspeert.
