title: Experimental evidence for friction-enhancing integumentary modifications of chameleons and associated functional and evolutionary implications.
abstract: The striking morphological convergence of hair-like integumentary derivatives of lizards and arthropods (spiders and insects) demonstrates the importance of such features for enhancing purchase on the locomotor substrate. These pilose structures are responsible for the unique tractive abilities of these groups of animals, enabling them to move with seeming ease on overhanging and inverted surfaces, and to traverse inclined smooth substrates. Three groups of lizards are well known for bearing adhesion-promoting setae on their digits: geckos, anoles and skinks. Similar features are also found on the ventral subdigital and distal caudal skin of chameleons. These have only recently been described in any detail, and structurally and functionally are much less well understood than are the setae of geckos and anoles. The seta-like structures of chameleons are not branched (a characteristic of many geckos), nor do they terminate in spatulate tips (which is characteristic of geckos, anoles and skinks). They are densely packed and have attenuated blunt, globose tips or broad, blade-like shafts that are flattened for much of their length. Using a force transducer, we tested the hypothesis that these structures enhance friction and demonstrate that the pilose skin has a greater frictional coefficient than does the smooth skin of these animals. Our results are consistent with friction being generated as a result of side contact of the integumentary filaments. We discuss the evolutionary and functional implications of these seta-like structures in comparison with those typical of other lizard groups and with the properties of seta-mimicking synthetic structures.
authors.affiliation: Faculty of Science, Department of Zoology, Fayoum University, , Fayoum 63514, Egypt, Centre for Cell Engineering, University of Glasgow, , Joseph Black Building, University Avenue, Glasgow G12 8QQ, UK, Department of Biological Sciences, University of Calgary, , 2500 University Drive NW, Calgary, Alberta, Canada , T2N 1 N4, Department of Biology, Lewis and Clark College, , 0615 Palatine Hill Road, Portland, OR 97219-7899, USA.
authors.forename: Eraqi R
authors.initials: ER
authors.lastname: Khannoon
authors.forename: Thomas
authors.initials: T
authors.lastname: Endlein
authors.forename: Anthony P
authors.initials: AP
authors.lastname: Russell
authors.forename: Kellar
authors.initials: K
authors.lastname: Autumn
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001696:Biomechanical Phenomena; D017276:Friction; D008116:Lizards / Q000033:anatomy & histology* / Q000145:classification; D008855:Microscopy, Electron, Scanning; D058618:Sensilla / Q000033:anatomy & histology / Q000648:ultrastructure; D012867:Skin / Q000033:anatomy & histology / Q000648:ultrastructure
keywords: biomechanics; chameleon; convergent evolution; friction; setae