A surprisingly diverse array of animals, from spiders to mammals, have adhesive structures on their feet that allow them to run along vertical surfaces, climb high in the canopy, or hang upside down to await their prey. Lizards, particularly geckos, have received a lot of attention throughout the last century because of the incredible weight-bearing ability of their adhesive pads. This ability is thanks in part to remarkable, tiny, hair-like projections on these pads, called setae. Similar structures have been found, and only recently described in detail, on the feet and tail of chameleons. Chameleons are a morphologically unique group of lizards, with tightly grasping, grouped, opposable toes and prehensile tails. The hair-like structures on their feet and tail do not confer adhesive advantages upon the slow and careful chameleon as they do on their cousins the geckos, but Eraqi Khannoon at the University of Glasgow and his international team of colleagues suspected that they might provide some frictional advantages – an additional boost to maintain purchase on the substrate that would likely be advantageous on slippery and narrow branches. In a recent study, they tested whether these tiny structures were the arboreal chameleon's key to locomotor success.
Khannoon and his colleagues first examined the general morphology of the toe pads and tail of five species of arboreal chameleons and then used histology and scanning electron microscopy to compare the morphology of the setae-like structures of these species with the setae of other lizards. They then tested the frictional forces generated by the setae-like structures on the top and bottom surfaces of the front and hind feet of the veiled chameleon, Chamaeleo calyptratus, using a specially designed force-measurement probe.
The researchers discovered that, unlike true setae, the chameleon's setae-like structures form a continuous carpet along the bottom surface of the digits and end of the tail; areas that regularly come in direct contact with the locomotor substrate. In addition, the structures are not branched, like setae, and the ends of these structures do not terminate in tiny spatulas, but rather tiny globes, flat plates or pointy tips. As predicted, the frictional forces generated by the surface of the skin were significantly greater on the bottom of the feet than on the top. And once Khannoon and his colleagues removed the seta-like structures and re-tested the skin, the frictional forces of the bottom surface of the foot decreased. It appears that these friction-enhancing filaments work in conjunction with the tightly gripping feet and prehensile tail to form one integrated, arboreal-navigating machine.
Khannoon and his colleagues suggest that the friction-enhancing filaments on the feet and tail of arboreal chameleons have been derived independently of those of the adhesion-enabling true setae found on geckos, anoles and skinks, making it the second derivation of this type of structure in lizards. They propose that the friction-enhancing setae-like structures not only impart a locomotor advantage to arboreal chameleons but also may provide insight into how the adhesion-inducing structures of other lizards have arisen. These results are exciting for biologists and biomimeticists alike. Although much research in the past has focused on adhesion, the evolution of friction-enhancing structures in animals and the functional integration of this trait with other morphological features may provide great new insights into the way we think about whole-animal performance and locomotion in general.
