The paradox of the lizard tail, solved
When choosing between life and limb, many animals willingly sacrifice the limb. The ability to drop appendages is known as autotomy, or self-amputation.
Lizards may be the bestknown users of autotomy. To evade predators, many lizards ditch their still-wiggling tails. This behavior confounds the predator, buying the rest of the lizard time to scurry away.
Scientists have studied this anti-predator behavior meticulously, but the structures that make it work remain puzzling. If a lizard can shed its tail in an instant, what keeps it attached in non-life-threatening situations?
Yong-ak Song, a biomechanical engineer at New York University Abu Dhabi, calls this the “paradox of the tail.”
“It has to detach its tail quickly in order to survive,” Song said of the lizard. “But at the same time, it cannot lose its tail too easily.”
Recently, Song and his colleagues sought to solve the paradox. They rounded up several lizards from three species: two types of geckos and a desert lizard known as Schmidt’s fringetoed lizard.
Back at the lab, they pulled the lizards’ tails, coaxing them into acts of autotomy. They filmed the resulting process at 3,000 frames per second using a high-speed camera. Then the scientists stuck the squirming tails under an electron microscope.
At a microscopic scale, they could see that each fracture where the tail had detached from the body was brimming with mushroom-shaped pillars. Zooming in even more, they saw that each mushroom cap was dotted with tiny pores. The team was surprised to find that instead of parts of the tail interlocking along the fracture planes, the dense pockets of micropillars on each segment appeared to touch only lightly. This made the lizard tail seem like a brittle constellation of loosely connected segments.
However, computer modeling of the tail fracture planes revealed that the mushroomlike microstructures were adept at releasing built-up energy. One reason is that they are filled with minuscule gaps, like tiny pores and spaces between each mushroom cap. These voids absorb the energy from a tug, keeping the tail intact.
While these microstructures can withstand pulling, the team found that they were susceptible to splintering from a slight twist. They determined that the tails were 17 times more likely to fracture from bending than from being pulled. In the slow-motion videos the researchers took, the lizards twisted their tails to cleanly cleave them in two along the fleshy fracture plane.
Their findings, published in the journal Science, illustrate how these tails hit the perfect balance between firm and fragile. “It’s a beautiful example of the Goldilocks principle applied to a model in nature,” Song said.