Geckos can weigh as little as two grams (0.07 ounces) and more than 300 grams (10.5 ounces), yet even these latter behemoths can scale walls and walk across ceilings with ease. Researchers have long assumed that the larger lizards climb as well as little ones (which can be 100 times smaller) thanks to the increased surface area of larger toe pads. Turns out, it’s a bit more complicated. According to a new PLOS ONE study, a spring-like mechanism in gecko bodies enhances adhesion as they and their toe pads become larger.
A team of researchers from the University of Massachusetts Amherst measured the ability of six gecko species – ranging in body size from around two to 100 grams (0.07 to 3.5 ounces) – to adhere to glass on a device that’s designed to measure their clinging force. They also measured changes in the stiffness of gecko anatomy.
They found that the bodies of geckos act like a spring, and that as these lizards get bigger, their legs and feet become stiffer. Specifically, what stiffened was their entire adhesive system – their tendons, skin, connective tissue, and the tiny hair-like projections on their toe pads called setae. This increased stiffness enabled larger geckos to produce enough adhesive force to climb. In particular, they efficiently stored and distributed surface forces produced by van der Waals bonds. “Simple mechanical changes in geckos explains a large portion of the adhesive ability of geckos,” Duncan Irschick of UMass Amherst said in a statement.
Additionally, the researchers created synthetic three-toed gecko feet from fabrics and soft elastomers, and then they attached a synthetic tendon (or mechanical spring) to model their adhesive performance. Synthetic geckos showed the same pattern: the adhesive system becomes less compliant (that is, stiffer) as they become larger.