Walking Fish Robots

The Undulating Tripod Gait

As a step to studying how ancient fish may have started developing the ability to walk on land hundreds of millions of years ago, we studied a number of fish species alive today that are capable of a very rudimentary gait, which we call the "undulating tripod gait". We created a very simple model of that gait from observations of animals and designed simulated and physical robotic models to explore the properties of this gait. 

We found that:

1) the simplest model of this gait are three rigid segments for the body and two pectoral fins that alternate contact with the ground

2) it produces forward motion for a range of gait parameters and body morphologies

3) the fastest motion produced by the robot is with gait parameters that closely match those tracked from our exemplar species Polypterus senegalus, suggesting that there is a mechanics-based reason the fish walk they way they do!

Ultimately, we found that very little anatomical specialization is necessary for fish to create this walking gait. If the requirements for this type of walking are minimal, then it's plausible that the earliest fish to walk on land used some variation of this gait as well!

Ishida M., Berio F., Po T., Khadem Hosseini N., Di Santo V., Shubin N. H., Iida F. (2026) "The undulating tripod gait as a model of the locomotion of walking fish", Nature Communications, 17, 4596 (2026). https://doi.org/10.1038/s41467-026-73111-2 . 

Fish-like Walking with 1 DoF Robot

Building on the single actuator wave-like robot design, this robot takes inspiration from the walking motion of fish (Polypterus senegalus) in the simplest way possible.

The robot creates sinusoidal body waves when a motor in the robot's head rotates an intenal helix. As the helix rotates, it forces the body elements to move in a pattern defined by the helix's geometry. This emulates the undulation of the fish's body while the legs and head create stabilizing points against the ground.

While a sinusoidal body shape doesn't mimic the fish's motion perfectly, this work shows the absolute minimum robotic requirements this type of locomotion. All it takes is body undulation creating thrust with a coupled motion of the legs that provides roll stability!

Khadem Hosseini N., Ishida M., Berio F., Di Santo V., Iida F. (2026) "A minimalistic walking fish robot twin based on the single actuator wave-like mechanism", Bioinspiration & Biomimetics, 21(1), 016002.