When choosing between life and a limb, many animals quietly sacrifice a limb.
The ability to drop appendages is known as autotomy, or self-amputation. When cornered, spiders shed legs, crabs abandon claws, and some small rodents shed bits of skin. Some sea slugs even decapitate themselves to get rid of parasite-infested bodies.
But lizards are perhaps the best-known users of autotomy. To escape predators, many lizards detach their tails, which keep moving. This behavior confuses the predator, giving the lizard time to escape. While there are downsides to losing your tail—they’re for maneuvering, impressing partners, and storing fat—it’s better than being devoured. Many lizards have the ability to regrow lost tails.
Scientists have studied this antipredatory behavior meticulously, but the structures that allow it to work are intriguing. If a lizard can detach from its tail in an instant, what keeps it connected in non-threatening situations?
Yong-Ak Song, a biochemical engineer at New York University in Abu Dhabi, calls this “the paradox of the tail”: it must be both sticky and detachable. “It needs to get rid of its tail quickly to survive,” Song said of the lizard. “But at the same time you can’t lose her too easily.”
Recently, Song and his colleagues tried to resolve the paradox by examining several newly amputated tails. They had no trouble finding specimens for the test — according to Song, the university campus is full of geckos. Using small loops attached to fishing poles, they captured several lizards of three species: two types of geckos and a desert lizard known as the fringed finger lizard.
Back in the lab, they pulled the lizards’ tails with their fingers, urging them to practice autotomy. They filmed the resulting process at 3,000 frames per second using a high-speed camera. (The lizards were returned to their places of origin.) Then the scientists put the flapping tails under an electron microscope.
On a microscopic scale, they saw that each fracture where the tail had detached from the body was filled with mushroom-shaped pillars. Zooming in even further, they saw that each mushroom was peppered with tiny pores. The team was surprised to find that, rather than parts of the tail intertwined along the fracture planes, the dense pockets of micropillars in each segment only appeared to touch each other slightly. This made the lizard’s tail look like a constellation of loosely connected segments.
However, computer models of the tail fracture planes revealed that the mushroom-shaped microstructures were capable of releasing accumulated energy. One reason is that they are full of tiny gaps, like tiny pores and spaces between each mushroom tip. These voids absorb the energy of a tug, keeping the tail intact.
While these microstructures can withstand tugging, the team found that they were susceptible to cracking with a slight twist. They determined that tails were 17 times more likely to fracture from bending than pulling. In the researchers’ slow-motion videos, the lizards rotated their tails to precisely cut them in two along the fleshy fracture plane.
Their findings, published Thursday in the journal Science, illustrate how these tails strike the perfect balance between firmness and fragility. “It’s a beautiful example of the Goldilocks principle applied to a model in nature,” Song said.
According to chemical engineer Animangsu Ghatak of the Indian Institute of Technology Kanpur, the biomechanics of these lizards’ tails resemble the sticky microstructures found in the sticky fingers of geckos and tree frogs, or tree frogs. “There needs to be the exact balance between adhesion and separation, because that allows these animals to climb steep surfaces,” said Ghatak, who was not involved in the study. He added that the animals’ paws are covered in millions of tiny bristles made up of mushroom-shaped spikes.
This project was entirely driven by curiosity. We simply wanted to know how the geckos around us detach themselves from their tails so quickly
The researchers believe that understanding the process that allows lizards to shed their tails could be useful for applying prostheses, skin grafts or dressings, and could even help robots get rid of defective parts.
However, Song is more excited to finally understand how campus creatures escape predators.
“This project was entirely driven by curiosity,” he said. “We simply wanted to know how the geckos around us separated from their tails so quickly.”
Translated by Luiz Roberto M. Gonçalves
