The lizard tail paradox is finally resolved

(Trilobite)

When forced to choose between their life and a limb, many animals are willing to sacrifice the limb. The ability to detach appendages is known as autotomy, or self-amputation. When cornered, spiders shed their legs, crabs drop their claws, and some small rodents shed segments of their skin. Some sea slugs even decapitate themselves to separate themselves from their parasite-infested bodies.

However, lizards are perhaps the creatures best known for resorting to autotomy. To evade predators, many lizards shed their tails, which keep moving even after the fact. This behavior confuses the predator, giving the rest of the lizard time to flee. Although losing a tail has its downsides—they’re useful for maneuvering, impressing mates, and storing fat—it’s preferable to being eaten. Many lizards are even capable of regenerating their lost tails.

Scientists have carefully studied this defense system against predation, but the structures that make up its operation are unclear. If a lizard can do without its tail in an instant, what keeps it in place in non-life-threatening situations?

Yong-Ak Song, a biomechanical engineer at New York University in Abu Dhabi, calls this the “tail paradox”: It must be sticky and removable at the same time. “It must be able to detach from its tail quickly to survive,” Song said of the lizard. “But at the same time, the tail can’t come off too easily.”

Song and his colleagues recently set out to resolve the paradox by analyzing several newly severed tails. They didn’t lack for research subjects: According to Song, the Abu Dhabi campus is teeming with geckos. Using tiny clips attached to fishing poles, they rounded up several lizards from three species: two types of geckos and a desert lizard known as Schmidt’s fringe-toed lizard.

Once in the laboratory, they pulled the lizards’ tails with their fingers to persuade them to resort to autotomy. They filmed the resulting process at 3,000 frames per second with a high-speed camera. (Shortly after, the lizards were returned to the place where they were found.) The scientists then put the still-squirming tails under an electron microscope.

On a microscopic scale, they were able to see that each fracture where the tail broke off from the body was studded with mushroom-shaped pillars. Zooming in closer, they saw that each mushroom cap had multiple tiny pores. The team was surprised to discover that the tail parts did not interlock along the fracture zones, but instead the dense arrays of micropyles in each segment barely seemed to touch. This made the lizard tail look like a fragile constellation of loosely connected segments.

However, computer modeling of the fracture zones in the tail revealed that the mushroom-shaped microstructures were capable of releasing the pent-up energy. One reason behind this is that they are full of minuscule gaps, like pores and tiny spaces between each mushroom cap. These voids absorb the energy generated by a pull, allowing the tail to remain intact.

While these microstructures can resist stress, the team found that they were susceptible to breaking with a slight twist. They determined that the tails were 17 times more likely to break when bent than when pulled. In the slow-motion videos the researchers took, the lizards twisted their tails to precisely snap them in two along the fleshy fracture zone.

Their findings, published Thursday in the scientific journal Science, illustrate how these glues strike the perfect balance between firmness and brittleness. “It’s a beautiful example of the Goldilocks principle applied to a model in nature,” Song said.

According to Animangsu Ghatak, a chemical engineer at the Indian Institute of Technology in Kanpur, the biomechanics of these lizard tails are reminiscent of the sticky microstructures seen on the sticky toes of geckos and tree frogs. “They have to have just the right balance between stickiness and detachment, because that allows these animals to scale steep surfaces,” explained Ghatak, who was not involved in the study. He added that the animals’ feet were covered with billions of tiny bristles, which, in turn, were made up of mushroom-like caps.

The researchers believe that understanding the process that allows lizards to shed their tails could be useful for attaching prosthetics, skin grafts or bandages, and perhaps even help robots separate from damaged parts.

However, Song is more excited to finally understand how the creatures on campus escape predators.

“This project was completely based on curiosity,” he said. “We just wanted to know how the lizards around us cut off their tails so quickly.” (I)