It is widely known that geckos and lizards employ dry adhesion system using a combination of microscopic hairs (setae) on their toe pads as well as other aspects of internal anatomy to climb on vertical walls and run on ceilings. Though adult geckos can weigh as much as 300 g, the ease with they climb vertical walls remains the same.
Geckos and lizards employ a mechanical principle called contact splitting. Each of the microscopic hairs found on their feet split up into hundreds of flat tips. The ends temporarily rearrange electrons on the walking surface, creating an electrodynamic attraction.
The classical view is that as the body size of geckos increases, the size of the climbing structure (toe pad area) also increases to account for the maximum adhesive force required for climbing. However, increasing the surface area of gecko toe pads cannot by itself explain this.
About 22 per cent of maximum adhesive force cannot be accounted for even after taking into account the increased area of the toe pad. Hence, some other critical factor besides pad area is required to provide the additional adhesive force.
In a study published in PLOS ONE, Duncan J. Irschick from the University of Massachusetts Amherst, Massachusetts, U.S.A. and one of the corresponding authors of a paper found that the bodies of geckos act much like springs and that, as geckos become larger, they also become stiffer, thus enabling them to climb as efficiently as smaller geckos.
“In order for maximum adhesive force to increase for a constant or conserved contact area, the entire adhesive system must become stiffer in the direction of loading,” they write.
Thus the adhesive system of larger geckos will be a lot stiffer than those of smaller geckos when they climb a vertical wall, assuming that the ratio of toe pad area to body mass remains the same. “The gecko adhesive system becomes less compliant (stiffer) as geckos become larger,” they write.
The researchers found that as gecko body size increased, their complete adhesive system, that is, the tendons, skin, connective tissue and setae, became stiffer, resulting in the larger animals' legs and feet being far stiffer than those of smaller geckos.
While the relationship between maximum adhesive force and stiffness has been proposed for synthetic adhesives and for only one species ( Gekko gecko ), the authors tested the maximum adhesive force and stiffness in six gecko species.
They found larger geckos had a far stiffer adhesive system compared with smaller geckos. About 92 per cent of the variation in maximum adhesive force could be explained when the ratio of toe pad area and stiffness of the adhesive system was taken into account, compared with only 78 per cent when only toe pad area was taken into account.
In synthetic experiments, they found that when the stiffness of the tendon (spring) was reduced, the maximum adhesive force decreased. They were able to get different values for the maximum adhesive force by using springs of different stiffness.
“This result is novel because it reveals how differences in basic material properties and geometry of living systems enable them to obtain new adhesive properties,” they write.
The maximum adhesive force is “many times greater” than the gecko’s mass and remains so even during dynamic events such as climbing, hanging, running and jumping and reattaching during a fall.
But they point out that the remaining 8 per cent of maximum adhesive force that has not been accounted for could be due to measurement errors, behavioural variation, or other known properties.