Geckos are master climbers, and can scurry up smooth vertical surfaces and even across ceilings thanks to their hairy toes. They’re not hairy in the same way that yours or mine might be, with a tuft of fuzz on the top. Rather, the bottoms of their toe pads are covered in millions of tiny hair-like bristles called setae, and each setae is tipped with thousands of even smaller structures called spatulae. All these setae and spatulae give a gecko’s toe a lot of contact with the surface underneath it and Van der Waals forces—weak attractions between molecules over short distances—act on all of them, allowing them to adhere to most anything.
While scientists have a pretty good grip on how this amazing sticking ability works, one thing they weren’t sure of until recently is whether the geckos have to actively control it—say, by making certain movements or flexing certain muscles to make the setae stick—or if the toes just grip on their own as soon as they touch a surface.
To find out, Timothy Highman and William Stewart, biologists at the University of California, tested the sticking ability of five geckos before and just after death. They found that the lizards’ post-mortem grips were just as strong as when they were alive, suggesting that the toes stick just fine on their own without any input or action from the lizard.
Their study started with building a custom gecko-pulling machine that allowed them to drag the lizards across an acrylic sheet with carefully controlled, steadily increasing force while measuring their toes’ holding power and recording toe position and surface contact area. The live lizards were dragged across the sheet several times and euthanized. Then, their bodies were dragged a few times.
“Death surprisingly did not affect the strength of shear adhesion,” the researchers wrote. “We found no difference in the peak adhesive force produced by animals while living and during the 30 minute period following euthanasia.” Whether the lizards were alive or dead, their feet could have nearly 20 times their body weight dragging on them and still cling to the sheet, suggesting that the toe pads don’t need any help from the rest of a gecko to hang on.
This passive adhesion is a cost-effective way to literally hang around, Highman and Stewart say, and allows geckos to cling to a perch and grab some sleep or hide from a predator without expending energy.
This isn’t to say that the geckos have no control over their grip, though. It’s just more of an “opt-out” situation than “opt-in.” While a gecko doesn’t need to do anything to make a toe pad stick, they can control how much of a pad sticks by hyperextending the toe, which lessens the strength of the grip. This allows them to trade some stickiness for speed when moving along a surface and quickly “de-stick” themselves to avoid injury.