When selecting between life and limb, many animals readily sacrifice the limb. The capacity to shed appendages is known as autotomy, or self-amputation. When forced into a corner, spiders let go of legs, crabs lose claws and certain tiny rodents shed clumps of skin. Some sea slugs would even decapitate themselves to rid themselves of their parasite-infested bodies.
But lizards may be the best-known uses of autotomy. To dodge predators, several lizards discard their still-wiggling tails. This action confounds the predator, affording the remainder of the lizard time to scuttle away. While there are downsides to losing a tail — they come in useful for moving, pleasing mates and storing fat — it beats getting eaten. Many lizards are even capable of recovering lost tails.
Scientists have examined this anti-predator behaviour thoroughly, but the components that make it function remain mysterious. If a lizard can lose its tail in a second, what keeps it connected in non-life-threatening situations?
Yong-Ak Song, a biomechanical engineer at New York University Abu Dhabi, labels this the “paradox of the tail”: It must be simultaneously adhesive and removable. “It needs to remove its tail swiftly in order to survive,” Dr. Song said of the lizard. “But at the same time, it cannot lose its tail too easily.”
Recently, Dr. Song and his colleagues tried to address the conundrum by investigating six newly severed tails. They did not want for test subjects – according to Dr. Song, the N.Y.U. Abu Dhabi campus is teeming with geckos. Using small loops linked to fishing poles, they picked up multiple lizards from three species: two kinds of geckos and a desert lizard known as Schmidt’s fringe-toed lizard.
Back in the lab, they tugged the lizards’ tails with their fingers, coaxing them into acts of autotomy. They captured the resultant process at 3,000 frames per second using a high-speed camera.
At a microscopic scale, scientists could observe that each crack where the tail had disengaged from the body was filled with mushroom-shaped pillars. Zooming in even farther, scientists observed that each mushroom cap was dotted with tiny pores. The team was shocked to discover that instead of portions of the tail interlocking along the fracture planes, the dense pockets of micropillars on each segment seemed to contact very weakly. This made the lizard tail look like a fragile constellation of loosely linked pieces.
However, computer modelling of the tail fracture planes indicated that the mushroomlike microstructures were effective at releasing built-up energy. One explanation is because they are loaded with minute gaps, like tiny pores and holes between each mushroom cap. These spaces absorb the energy from a pull, keeping the tail unbroken.
While these microstructures can endure tugging, the scientists observed that they were prone to splintering with a minor twist. They discovered that the tails were 17 times more likely to shatter from bending than from being pulled. In the slow-motion movies the researchers captured, the lizards twisted their tails to neatly slice them in two along the fleshy fracture plane.
Their results, published Thursday in the journal Science, demonstrate how these tails reach the optimum balance between hard and frail. “It’s a fantastic illustration of the Goldilocks principle applied to a model in nature,” Dr. Song added.
According to Animangsu Ghatak, a chemical engineer at the Indian Institute of Technology Kanpur, the biomechanics of these lizard’s tails are similar of the sticky microstructures seen on the tacky toes of geckos and tree frogs. “It needs to be exactly the perfect balance between adhesion and dissociation, because that permits these creatures to mount steep surfaces,” said Dr. Ghatak, who was not involved with the research. He stated that the animals’ feet were coated in billions of tiny bristles which themselves were formed of mushroom-shaped caps.
The researchers think that knowing the mechanism that allows lizards discard their tails might have benefits for attaching prosthesis, skin grafts or bandages, and may even assist robots remove defective components.
However, Dr. Song is most delighted to finally grasp how the species on campus elude predators.
“This endeavour was purely curiosity-driven,” he remarked. “We just simply wanted to know how the lizards around us cut their tails off so quickly.”