PEATTIE, A M.*; CORDER, A. B.; FULL, R. J.; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley: Effect of Morphological Variation on Single Seta Force in Geckos

Studies on Gekko gecko (the Tokay) have detailed the morphology of their subdigital adhesive hairs or �setae� as well as their function. The Tokay is fast becoming a model species for setal adhesion. Yet, setal morphology varies greatly across the gekkonids. To test the effect of this variation on adhesion, we measured single-seta force in two species with different setal morphology. Thecadactylus rapicauda setae are shorter, wider and have about 3-4 times as many branched tips or spatulae than typical Tokay setae, whereas Ptyodactylus hasselquistii setae are longer and have a similar number of spatulae to the Tokay. We measured normal and shear forces at velocities ranging from 0.15 – 1.5 mm/s using a silicon surface attached to a wire functioning as a cantilever force gauge. A threshold normal preload was required to engage each seta, but increasing preload further did not increase shear force. Velocity did not affect shear force. The average Thecadactylus shear force was 2.5 times greater than the Tokay, and 2.2 times greater than Ptyodactylus. Detachment angle in Thecadactylus averaged 39.9°, Ptyodactylus 44.1°, both greater than reported for the Tokay (30.6°). The van der Waals hypothesis of gecko adhesion predicts that an increased number of spatulae should lead to the increased shear force seen in Thecadactylus. Further comparative data will be required to determine the effect of setal length and width on shear force. Research groups currently modeling gecko setae mathematically, as well as physically by fabrication, should consider the effect of morphological variation on performance. Future evolutionary analyses will assist in the identification of key functional parameters amongst the diversity of setae in nature.