The Evolution Of Sticky Feet

Brett Smith for – Your Universe Online

Aside from having a knack for selling auto insurance, geckos have also developed ℠sticky´ foot pads several times throughout their evolution that give them another ability, the scaling of smooth walls and ceilings to escape predators and find prey.

A new study published in the most recent edition of PLoS ONE said that changes in habitat have caused geckos to gain and lost these exceptional adhesive structures several times over the course of their evolution.

“Scientists have long thought that adhesive toepads originated just once in geckos, twice at the most,” said study co-author University of Minnesota researcher Tony Gamble. “To discover that geckos evolved sticky toepads again and again is amazing.”

In order to understand the mechanism that develops adhesive pads in about 60 percent of geckos, the research team said they produced the most complete gecko family tree ever constructed. In addition to aiding their research, this family tree can improve gecko taxonomy and serve as the basis for answering many other questions surrounding the evolution of these lizards.

During their study, researchers found that the sticky toes evolved separately in about 11 different gecko groups. In addition, at least nine different gecko groups were found to have lost these structures. The gecko´s habitat appeared to be the main factor in determining the gain or loss of adhesive toepads. For example, lizards that lived on boulders and in trees developed the feature, while those living in sand dunes did not.

“The loss of adhesive pads in dune-dwelling species is an excellent example of natural selection in action,” said the study’s senior author Aaron Bauer, a professor at Villanova University.

The adhesive structures work through a combination of weak intermolecular forces and frictional adhesion. A tiny forest of hair-like bristles, called setae, lines the underside of a gecko’s toes. The increased surface area created by the bristles generates enough weak intermolecular forces to support the weight of the entire animal.

Another study published this month in Interface, the Journal of the Royal Society showed a dynamic self-cleaning mechanism for the gecko´s adhesive pads is achieved through the hyperextension of their toes.

Yet, for all that scientists know about the adhesive structures, they are still learning how gecko toepads have evolved to move in a natural environment. Researchers said they hope to eventually develop robotic technologies that can do similar things.

“It’s one thing to stick and unstick a piece of ‘gecko tape’ to a smooth surface in a lab, but something else altogether to get a robotic gecko to move across a complicated landscape in the real world and stick to all the different shapes and textures it will encounter,” says Gamble.

“Through biomimicry, a gecko-inspired adhesive can function under conditions where traditional adhesives do not, such as in a vacuum, outer space or under water,” said Peter Niewiarowski a biology professor at the University of Akron who worked on the self-cleaning study.

“More broadly, a gecko-inspired adhesive would be able to bind materials together very strongly yet also release very easily.  Imagine a tape that binds things together securely like duct tape yet can also be removed and reused over and over again like a post-it note.”

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