When we think of jumping, we think of using our legs to do it. This is true in most mammals and lizards. However, salamanders (and some fish) have limbs that are underdeveloped for jumping (most have limbs that are relatively small and weak). Lungless salamanders (Plethodontidae) have a unique mechanism of jumping that does NOT use their legs for propulsion into the air. My interest is to understand how this mechanism works.
The Hip-Twist Jump:
Plethodontid salamanders have been observed to jump using a unique mechanism in which they bend and unbend their torsos laterally with such force that they hurl themselves into the air (Ryerson, 2013). We have attempted to understand this mechanism by performing a detailed biomechanical analysis. So far, we have learned that the salamander does not really bend and unbend laterally. Instead, the salamander bends its body into a C-shape and then pushes that bend into the hips (think of a wave caused by moving a stretched out slinky from side to side). As the wave moves through the hips, they rotate; this rotation pulls on the ipsilateral (to the bend) hind limb, which is planted in front of the hip. Because the movement of the hip is away from the planted foot and the foot is anchored to the ground, the hip rotates towards the foot at such speed (17 body lengths a second!) that it catapults the salamander over the planted foot, achieving liftoff. (Consider rock climbing: you reach way over your head and grab a ledge. At this point, your shoulders are not lined up because of the arm stretch. If you bring your free hand’s shoulder to the level of the anchored shoulder, you will raise your body. The salamander jump works the same way, except this happens so quickly that it throws the salamander over the planted foot). Due to the importance of hip rotation to the performance of the jump, we call this mechanism “The Hip-Twist Jump”.
-Filmed at 500Hz
Current work: Jumping Energy: Torso Stiffness and Power Analysis
The next question we have begun to tackle is: where is the salamander getting all this energy from?Axial muscle contractions are a likely player. Elastic energy, in the form of stored elastic recoil energy, may also play a big part. In almost all locomotion mechanism muscles, bones and connective tissue stretch, storing energy for use in a latter part of the mechanism. In fact, torso stiffness is of significant importance to most fish locomotion and accounts for its efficiency (Long et al., 2010).
Salamander torso morphology is not too far removed from that of fish ancestors. In fact, the some juvenile forms of salamanders exists in aquatic environments and swims like some fish. As an adult, they metamorphose into a terrestrial form. Can salamanders harness their own torso stiffness to amplify jump power? We believe that they can, and that the lateral bending component of the jumping mechanism exists to take advantage of those morphological characteristics.
Inspired by work conducted by Dr. John Long (Vassar College), we are conducting experiments to measure the torso elastic energy added to the jumping mechanism by the salamander’s axial lateral bend. We built a novel apparatus that has the ability to measure the torso stiffness (the apparent Young’s modulus (E)) which has been proven to give researchers satisfactory insight into the stiffness characteristics of a torso’s lateral bend (Long et al., 2010).
Briefly, the apparatus measures E by making the salamander torso a cantilever beam. Engineers use cantilever beams principals to find E values and work (W) needed to bend the material. The hip of the salamander is secured at the edge of a ledge with the torso hanging over the edge, freely. The salamander is laterally oriented, so that the lateral elastic work would be calculated when bent. A force transducer is place directly under the salamander’s pectoral girdle and then attached via a string. As we lower the force transducer down a ring stand, the torso is bent and the passive elastic energy is captured by the transducer as the bent torso attempts to straighten itself back out. From this data, we can calculate the apparent E.
As an analogy, secure one end of a pencil to the edge of a desk, allowing the other end to stick over the edge. Then push down on that end. Can you feel the “push back”, from the pencil to be straight again? Knowing that force, along with some other data, allows us to know the total work exerted by that pencil as it is released and returns to its normal position.
We compared this to the total Work output (described above) to see how much it contributed to the total energy output of the jump (around 10-15% with our preliminary results).
Some fish can actively modulate torso stiffness with their axial muscles… can salamanders?
We work with a genus of Plethodontidae that live most of their life on the ground under rocks and debris. Some Plethdontidae live in trees, and thus have very different limb and tail structures…do these guys jump, and in the same way? We are currently exploring this with UC Santa Barbara.
Lateral stiffness properties are well studied in fish and are highly specialized to maximize swimming performance. The salamander jumping model may show an evolutionary re-use of an old aquatic function for terrestrial movement. How the function is maintained morphologically and how it allows the salamander to jump without the aid of its legs is of interest to vertebrate morphologists, evolutionary-developmental biologists and biomechanics.
If this is a re-use of old machinery, it is possible that this it is also run by the same neural mechanisms fish use. Mauthner cells run an escape mechanism in fish called the startle response. Our work has shown that this is very similar to our salamander’s jumping escape mechanism. It’s not clear whether plethodontids have Mauthner neurons. In the anatomically sense of an escape neuron with a really big axon, they do not. But, they may still have a derivative of that old system. These control cells are almost never found in vertebrates above fish, so finding them in use within salamanders would be exciting for amphibian neurobiologist and Herpetologists. This work is ongoing as we speak.
There are also some interesting biomimetic applications to consider. Salamander jumping shows us that there are alternative ways to get into the air, even with an object that lies flat on the ground and has no means of “pushing” directly down into the ground. A piece of equipment could be placed covertly in the ground, and when triggered, flung into the air. We are just beginning to consider designs for a “flat catapult”.