University of Minnesota
Tokay gecko on glass, showing its adhesive toepads.
Photo: Tony Gamble
Gecko feet don't stick around
The lizards have gained and lost adhesive toes many times
By Deane Morrison
Who wouldn't envy the little gecko as it dashes up a smooth wall or hangs from a ceiling by a toe?
An engineer's dream, gecko feet combine the best of duct tape and Post-It® Notes: They stick, but they don't stay stuck.
The drive to duplicate gecko feats technologically is a hot area of research. Would-be designers of such a technology should note a new study of geckos' evolutionary history that could simplify their task immensely.
In that study, researchers at the University of Minnesota, Villanova University, and the University of Calgary compared DNA sequences from 244 gecko species to draw up the most comprehensive gecko family tree ever. It showed that adhesive toepads have been gained 11 times, producing 11 lineages with the trait. As those lineages diverged into new species, the toepads were subsequently lost nine times. Today, about 60 percent of the 1,400 gecko species have adhesive toepads.
The finding, reported in the journal PLoS ONE, has astonished scientists.
The tokay gecko (Gekko gecko), found across Southeast Asia, is commonly used to study the adhesive properties of gecko toes. Photo: Tony Gamble
"People thought toepads had been gained once or at most twice, and the lack of them was due to the difficulty of gaining the trait versus the ease of loss," says co-author Tony Gamble, a postdoctoral researcher at the U of M. "Therefore, scientists predicted a lot more losses than gains. But we found 11 gains and nine losses, so gaining it is no harder than losing it."
The gain or loss of toepads appears to be tied to a species' habitat: What works on smooth surfaces can be a liability on rough or crumbly ones.
"For example, of 16 species in the analysis that belonged to a group called the Pachydactylus clade, 14 live on boulders and have adhesive pads, and two live on desert dunes and have lost them," Gamble notes.
"The loss of adhesive pads in dune-dwelling species is an excellent example of natural selection in action," adds senior author Aaron Bauer of Villanova.
The secret to stickiness lies in the microscopic anatomy of adhesive toepads. Their surfaces are divided into myriad ridges; on a finer scale, hairy projections called setae carpet the ridges like bristles on a toothbrush. All geckos have setae, but only setae with highly branched tips are adhesive.
Underside of the house gecko (Hemidactylus frenatus) foot, showing expanded adhesive toepads. Photo: Tony Gamble
The branched tips multiply the surface area of the toepads astronomically. This allows for weak forces between molecules, called van der Waals forces and frictional adhesion, to become strong enough to support a lizard's weight.
The tokay gecko has some 6.5 million setae, which together generate enough force "to hold up two skinny guys or one fat guy," Gamble says. "If all the setae had full contact with the substrate, [with that amount of adhesion] you could have a Spider-Man suit."
Engineers who want to emulate the lizard's feats will have to construct precisely designed mechanical feet.
"People making gecko adhesive have just been concerned with stickiness, like flypaper," Gamble explains. "But as soon as you want to make something move in the real world, you must know how to make it stick and unstick on different inclines and different materials and at different speeds."
Since hundreds of gecko species have adhesive toepads, it could be tough choosing one to use as a model.
"If you don't know how they're related, you may do a lot of unnecessary work looking at every species," Gamble continues. "But if you know how they're grouped evolutionarily, you can compare representative species from each of the 11 groups that evolved toepads independently. Then you can figure out what characteristics lineages with toepads have in common."
The crested, or eyelash, gecko (Rhacodactylus ciliatus) is native to New Caledonia. Photo: Tony Gamble
This way, he says, one can glean best practices for assembling an adhesive toepad, filter out lineages that may be redundant, and ignore traits that may be unique to just one lineage.
The family tree also promises to help studies of other traits that have evolved repeatedly and independently, such as mechanisms of sex determination. In some animals, chromosomes determine sex, while in others temperature is the deciding factor. Working with genetics, cell biology and development professor David Zarkower, Gamble hopes to learn how sex determination evolved by tracing the evolution of different methods across evolutionary trees.
"Geckos are the perfect model for answering this and a host of other evolutionary questions," he says. "The family tree from this paper provides the infrastructure for further study."
Tony Gamble is sequencing gecko genomes as part of Genome 10K, an international effort to sequence 10,000 vertebrate species.