The insect called a froghopper, for instance, is a prodigious jumper even though its legs are much shorter than those of crickets and locusts. Its secret is that it uses its leg muscles to bend the chitinous exoskeleton encasing its thorax, which functions as a spring or bow. A latch holds the stored energy in the bow — as much as 65,000 watts per kilogram. When the latch releases, the exoskeleton springs back to its original shape, unleashing a powerful jump that can hurl the froghopper 100 times its body length.
This springy solution works for more than jumping. The head of the trap-jaw ant is covered with a chitinous exoskeleton that muscles inside the head can bend like an archer’s bow. When the bow releases, it yields as much as 200,000 watts per kilogram, and the mandibles snap closed on unlucky prey at speeds of more than 140 miles per hour. When faced with a predator, the trap-jaw ant can also snap its mandibles against the ground with enough power to propel itself into the air, quickly escaping to safety.
The use of springs is not restricted to insects. When a frog crouches, its leg muscles stretch its long Achilles tendon like a spring and store energy in it. The release of that built-up tension propels the frog’s leap, said Christopher Richards, a paleo-robotics researcher at the Royal Veterinary College at the University of London, who is using a combination of robotics, modeling and anatomy to understand how extinct frogs with diverse pelvic shapes and leg proportions used to jump.
The latch that the frog uses to release the stored power, however, remains a subject of intense debate: “That’s the million-dollar question,” Richards said. “Nobody has found an anatomical latch in frogs. To my knowledge, nobody has found a latch in a vertebrate animal.”
The latches have been figured out for only a handful of insect and crustacean systems. Latches are harder to find than springs because the latch mechanism is usually inside the animal’s body, as opposed to the easily accessible springs made of crustaceans’ outer cuticle or insects’ exoskeleton. Unfortunately, dissection destroys the delicate spring-and-latch systems, making it difficult to determine how they work in living organisms, explained Gregory Sutton, a biomechanics researcher and engineer at the University of Bristol. Usually, researchers end up inferring the existence of a latch from the abrupt release of power from an identified spring. “Something has to switch the system from a mode where the muscles are stretching the springs to a mode where the spring is recoiling and powering, powering that huge motion,” Sutton said.
The first latch to be discovered was the one in grasshoppers, which was described in 1977 by William James Heitler of the University of St Andrews in Scotland. Grasshoppers use what is called a geometric latch: The opposing muscles of the leg work in pairs. First, when the leg is fully flexed, the big muscle of the leg loads the spring while the smaller muscle stabilizes the knee joint. By slightly moving the joint, the small muscle then causes a change in the moment arms of the two muscles and triggers the joint to rotate out of control, setting off the jump.
Moving Past Idealized Springs
Although the spring-and-latch systems that small creatures use have been studied for decades, there has been a serious weakness in scientists’ understanding of them. “Until now we’ve largely treated spring systems as though they’re unlimited in their ability to store energy and release power,” Roberts explained. That’s an acceptable assumption when the mass of the springs involved are negligible relative to body mass. But a mechanical constraint on any spring is that, when it is unloading energy, it has to move its own mass, which inevitably reduces the force output of the spring in a way that is proportional to the velocity of the moving object, Roberts said. For small systems, the mass of the spring frequently becomes an appreciable part of the total mass and has to be taken into account.
Latches were also modeled in an idealized fashion, as if they released springs instantaneously. But the speed at which a latch releases the energy stored in a spring turns out to be important: It determines how quickly the load on the spring can accelerate, Richards said. If animals are limited in how quickly they can release the latches in their biomechanical systems, their performance would also be limited.
In an April 26 Science paper, an interdisciplinary team of engineers, physicists, physiologists, biomechanics researchers and materials scientists presented a new theoretical model that makes explicit use of these real (rather than idealized) properties of muscles, latches and springs, and mathematically describes how the properties of these components must be tuned to one another to optimize performance.
“What to me is the major breakthrough here is, now we can follow the path of power,” said Sheila Patek, the senior author of the paper and an associate professor at Duke University who investigates the mechanics of movement. “We can see which systems should be governed by a spring and a latch and which don’t benefit from that, and we can get a much better handle on classic scaling [problems] in biology that haven’t made sense before this.”
Mantis Shrimp and Robots
Patek has been studying the behavior and biomechanics of mantis shrimp, also known as thumb-splitters, since 2002. They are small crustaceans with hammerlike claws the size of toothpicks that they use to break open snail shells (or to slice open the finger of an unwary human handler).
“We [humans] could seriously be hitting a snail shell with two toothpicks for the rest of our lives and never break a snail shell, right?” Patek said. Yet as she and her colleagues showed, the mantis shrimp can do it because of the latch-and-spring system powering its claws. A muscle loads resistance into the chitinous exoskeleton spring, which is held in place with a yet-uncharacterized latch. Then the latch releases, and the spring accelerates the hammer outward at speeds of up to 30 meters per second. “Their strike is similar to the acceleration of a bullet in the [barrel] of a gun (105 m/s2),” Patek clarified by email.