Figure 4. Force measuring apparatus. Geckos could be pulled at a constant rate selectable over a wide range of values. All pulls were accomplished with the substrate in a vertical orientation. Maximum force was the highest value recorded between the start of a pull and the point at which all four feet began to slide on the substrate. doi:10.1371/journal.pone.0002192.g004 humidity was maintained at one of four levels, presented in a References 1. Autumn K (2006) How gecko toes stick-The powerful, fantastic adhesive used by geckos is made of nanoscale hairs that engage tiny forces, inspiring envy among human imitators. American Scientist 94: 124–132. 2. Russell AP (2002) Integrative functional morphology of the gekkotan adhesive system (Reptilia : Gekkota). Integrative and Comparative Biology 42: 1154–1163. 3. Pianka ER, Sweet SS (2005) Integrative biology of sticky feet in geckos. Bioessays 27: 647–652. 4. 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Losos (1990) Thermal sensitivity of sprinting and clinging performance in the tokay gecko (Gekko gecko). Asiatic Herpetological Research 3: 54. 10. Bergmann PJ, Irschick DJ (2005) Effects of temperature on maximum clinging ability in a diurnal gecko: Evidence for a passive clinging mechanism? Journal of Factors Affecting Adhesion humidity was maintained at one of four levels, presented in a random order (30, 55, 70, 80%). Humidity trials at 12uC were followed by two trials at 32uC (at 35 and 80% RH) to test whether temperature and humidity interact. At the completion of every trial, geckos were weighed to the nearest 0.1 g. Before every trial and after the very last trial, the feet of each gecko were scanned with a flat bed scanner so we could estimate the total adhesive area of the toe pads. Statistical analyses Our analyses include data from several separate experiments, each with slightly different sample sizes and overall designs. Whenever an analysis involved data from geckos exposed to all combinations of experiment factors (i.e., humidity and tempera- ture), we used a univariate repeated measures ANOVA to test for the statistical significance of effects in the model (assumption of sphericity was never violated). Clinging force is reported in two forms, raw and ‘‘adjusted,’’ facilitating comparisons using both standardized and raw values. Adjusted clinging forces are the residuals from a linear regression of raw clinging force on toe pad area, providing a measure of clinging force that accounts for variation among individual lizards in toe pad size. Throughout the manuscript, whenever clinging forces are presented as ‘adjusted’ they are standardized as described above; values without the ‘adjusted’ descriptor represent the raw values. Finally, when means are reported in the text or tables they are accompanied by their respective standard errors (6 1 SEM), unless otherwise noted. Acknowledgments We thank M. 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Alibardi L (2003) Ultrastructural autoradiographic and immunocytochemical analysis of setae formation and keratinization in the digital pads of the gecko Hemidactylus turcicus (Gekkonidae, Reptilia). Tissue & Cell 35: 288–296. PLoS ONE | www.plosone.org 6 May 2008 | Volume 3 | Issue 5 | e2192