Thanks to millions of microscopic hair-like structures called setae, geckos have the amazing ability to cling tenaciously to smooth surfaces like windows or ceilings. The adhesion is so strong—even in death—that people have been trying to mimic it for the last several decades. So why did some geckos give up their sticky feet over evolutionary time? Because it’s expensive, according to a study published in Proceedings of the National Academy of Sciences last month.
Evolution can remove previously acquired morphological adaptations in order to help confer survival advantages for a species. Geckos are a diverse group of lizards that have spread throughout a variety of microhabitats, and how complex their innovative sticky feet are can vary wildly. This complex adaptation not only depends on setae, but also on the integration of expanded toe scales and modified skeletal elements of the foot. The system originated independently at least 11 times among the Gekkota, and it’s also been lost a minimum of nine times.
To understand how complex adaptations are simplified—or lost completely—a team led by Timothy Higham from the University of California, Riverside, studied two to four specimens from 14 species of geckos. Some were obtained through the pet trade, and most of them were collected from the wild in Namibia, Africa, where they like to occupy sheet rock, hard-packed sand, or rocky outcrops. The team combined high-speed photography with biomechanical and phylogenetic analyses.
“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,” Higham tells the Los Angeles Times. Two of the 14 species they studied have lost their sticky feet completely (Chondrodactylus angulifer and Pachydactylus rangei), another two showed reduced stickiness (Rhoptropus afer and Colopus wahlbergii), while the other 10 could still climb rocks with ease.
Turns out, that awesome innovation can also be costly. The deployment and disengagement of their adhesive system is a highly choreographed movement that takes time—a process that limits how fast a gecko can run and escape. Tarentola mauritanica, for example, suffers a 37 percent decrease in running speed when using its adhesive system on a 10-degree incline compared with a level surface where it wouldn’t be using its adhesive system.
The species who have lost or simplified their adhesive system evolved at faster rates when it comes to morphology and locomotion, the team found. “This is the other side of the coin...Loss of innovation is an innovation,” Higham says. “It allows them to occupy these new kinds of habitats.” Removing the constraints that are associated with adhesion allowed the geckos to either run faster or burrow better. The two species who got rid of their sticky system altogether also developed new adaptations that make them better suited to their environments, Los Angeles Times explains: P. rangei has webbed feet for running on sand, and C. angulifer has stubby feet for digging.