Many geckos can cling tenaciously to smooth surfaces like ceillings and glass windows thanks to the adhesive pads on their toes. A single foot can support over twenty times the lizard’s body weight. Now, it turns out that a dead gecko can cling with as much adhesion strength as a live one, according to a study published in Biology Letters this week. The attachment is passive.
On the underside of gecko toes are tiny structures called setae -- millions of very fine, hair-like structures -- that provide an enhanced surface area for really close contact between their feet and their resting surface. These work through frictional forces as well as those between molecules, called van der Waals forces. The setae are curved inward toward the center of the foot, and when the gecko pulls back a toe, the setae straighten. The gecko's adhesion is so strong, people have been trying to mimic it for years.
To see if geckos actively control this steadfast attachment, William Stewart and Timothy Higham from the University of California, Riverside, compared the clings of five tokay geckos (Gekko gecko) before and within 30 minutes after their death. They developed a device that measures shear adhesion strength while steadily pulling on gecko feet along a vertical acrylic sheet.
Dead geckos, they found, maintain the ability to adhere with the exact same force as living geckos. The strength of their “stickiness” is intrinsic to the adhesive system. Previous work have suggested that muscle recruitment or neural activity are required to push the foot and toes onto a surface. “With regards to geckos, being sticky doesn’t require effort,” Higham says in a news release. “Death affects neither the motion nor the posture of clinging gecko feet. We found no difference in the adhesive force or the motion of clinging digits between our before- and after-death experiments.”
To the right, you can see a tokay gecko clinging to a smooth surface. Passive adhesion is an excellent, cost-effective way of remaining stationary in a habitat, Higham adds. “For example, geckos could perch on a smooth vertical surface and sleep for the night -- or day -- without using any energy.”
The “active” component is a reduction of adhesion force when the gecko hyperextends its digits. That's when the lizard quickly stops or reduces the adhesion by only curling up the digit tips while the rest of the foot stays on the surface. “In other words, when the forces become too high, the gecko likely releases the system using its muscles,” Stewart explains. That means dead animals are more likely to experience damage to their adhesive system, suggesting how the active control portion works to prevent injury.
Images: Emily Kane, UC Riverside (top, middle), William Stewart (bottom)