The gecko's ability to climb walls and ceilings may seem like a superpower -- a gravity-defying skill that scientists and engineers are trying to re-create in a number of different devices, from sticky tape to spaceship-climbing robots.
But even the gecko knows when to let a superpower go, according to a team led by UC Riverside scientists who tracked geckos that had lost or limited their ability to climb walls. The findings, published in Proceedings of the National Academy of Sciences, shed light on when and how once-useful adaptations are lost when an animal's habitat changes – and what happens when that trait disappears.
The scientists looked at 14 gecko species found in Namibia, a hotbed of gecko evolution in southern Africa. They gathered most of the species themselves (either by hand or with a noose, except for a couple that were obtained through the pet trade). They examined the characteristics of the animals' legs and feet, and used high-speed cameras to film the animals running.
"We were really curious about why they would have lost this spectacular sort of innovation, what might be driving it and what might be the consequences of losing it," said lead author Timothy Higham, a comparative biomechanist at UC Riverside.
The researchers found that while 10 of the 14 species could still climb, four of them had either lost their sticky feet (Chondrodactylus angulifer and Pachydactylus rangei) or had it "simplified" or significantly reduced (Rhoptropus afer and Colopus wahlbergii).
Why give up such a cool ability? It turns out that there are trade-offs for walking on walls, Higham said. The gecko foot uses tiny hair-like structures called setae to cling to surfaces by taking advantage of the Van der Waals force, the electrostatic interaction between two molecules. That's great, except that the gecko must disengage by carefully adjusting the angle of its toes. Doing this with every step takes time – precious fractions of a second that can mean the difference between life and death.
The gecko can also choose to keep the tips of its toes curled up as it runs, keeping the setae from engaging with the surface it's running on, but that reduces the amount of foot it's able to use while running, which also affects its speed. (Imagine trying to run with your toes constantly curled in the air.) That's a fast-track to becoming some other animal's meal.
So as the surfaces in a habitat become less vertical and more flat, sticking to things becomes something of a disadvantage, Higham said. Some of these gecko species probably ditched this fascinating adaptation as they moved from rock-climbing to sand-running and burrowing.
These species also developed new adaptations to their changing environments. Pachydactylus rangei developed webbed feet, all the better for skittering across sand without getting stuck. The African ground gecko (Chondroadactylus angulifer) developed stubbier feet for digging, while fast runners like the common Namib day gecko (Rhoptropus afer) make use of much longer feet.
The geckos that lost their sticky feet didn't just develop different traits for their new environments – they evolved at faster rates, Higham said.
"This is the other side of the coin. … Loss of innovation is an innovation," Higham said. "It allows them to occupy these new kinds of habitats."