Geckos are famed for their extraordinary ability to scale walls, run across ceilings and even hang upside down from apparently smooth materials like glass.
Microscopic hairs allow the lizards to employ dry adhesion – meaning they can stick to surfaces without the use of liquids or surface tension – through the creation of so-called van der Waals forces which draw materials together.
Their amazing climbing skills have long been a source of fascination for scientists, and have even led to the invention of adhesive tape that mimics the properties of their specialised toe pads to easily attach and detach.
But some elements of their abilities have remained a mystery, including how some of the heavier species (weighing up to 250g) can still stick to things so effectively. The assumption was that their adhesive ability was related to the size of their toe pads, allowing larger geckos to climb just as well as the smaller ones (which weigh as little as 2g).
These results certainly challenge the prevailing view
But now a team of scientists at the University of Massachusetts Amherst, US, have shown that additional factors are also at play. They've discovered that their bodies become stiffer as they grow bigger, acting like a spring, giving their adhesion the increased power needed to support more weight.
“This is an exciting result because it shows how simple mechanical changes in the adhesive system explains how large geckos can climb effectively," explains Professor Duncan J. Irschick, co-author of the study.
They based their hypothesis on recent work which showed that man-made adhesives inspired by gecko's adaptations get stronger as they are made stiffer, or less compliant.
"Prior theory has shown that synthetic adhesive systems become more powerful if they are stiffer, and we wanted to see if this theory was upheld in living animals," Prof Irschick says.
Adhesion tests were conducted on both live geckos and on synthetic adhesives, to establish their clinging force as well as changes in the stiffness of gecko anatomy.
They found that as gecko body size increased, the tendons, skin, connective tissue and tiny hairs (known as setae) became stiffer, resulting in the larger animals' legs and feet being far stiffer than those of smaller geckos.
This increased stiffness allows larger geckos to produce sufficient attractive forces to climb, the authors say.
"These results certainly challenge the prevailing view that as geckos become larger they achieve higher adhesive forces simply through having larger toepads," Prof Irschick explains.
"While larger geckos did obtain additional adhesive ability from larger toepads, the changes in compliance with body size is a major contributing factor as well, and this is a novel result."
The increased stiffness improves their stickiness by enabling the van der Waals forces to be stored and distributed efficiently.
Not only do the findings increase our understanding of climbing animals but they may also allow engineers to create better adhesives.
“We believe that our results will open new doors towards understanding how animals varying greatly in size can still adhere to surfaces," Professor Irschick says.
“Our data also confirm prior synthetic data which shows that stiffer adhesives produce higher forces, and this principle has important implications for adhesives for human use.”
The findings are published in the journal PLOS ONE.
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