Factors Affecting Adhesion Figure 2. Adhesion at constant temperature of 12uC and variable relative humidity (RH). Adhesion increased significantly with RH, but slopes were not significantly different. Red lines and symbols show results of 35% and 80% trials at 32uC designed to test for an interaction of temperature and humidity; error bars represent 62 SE. See text for details (materials and methods). doi:10.1371/journal.pone.0002192.g002 Irrespective of the mechanism, effects of humidity on otherwise ‘‘dry adhesive’’ biological systems may not be limited to gecko toe pads. For example, members of the spider infraorder Araneomor- phae produce a derived type of silk which adheres to surfaces with greater force as humidity increases [17,18]. It has been hypothesized that regularly spaced nodes in the silk strands are richer in hydrophilic polar or charged amino acids, which under humid conditions should promote hydroscopic interactions between the nodes and substrates [17]. It is unclear whether similar mechanisms might underlie the humidity response of gecko toe pads. However, efforts to characterize the protein structure of setae have revealed that setae have a complex structure of heterogeneous a- and b-keratins [19–21]. Furthermore, there is evidence that the lateral regions of the b-keratins sequenced in gecko setae are relatively rich in hydrophilic and polar amino acids Table 2. Average maximal adjusted adhesive force (N) by species and temperature. Errors are 61 SEM. Temperature (uC) Species 12 17 22 27 32 day tokay 5.5360.51 2.1160.38 1.0560.40 0.7160.30 0.7960.19 31.2264.39 14.3762.20 20.6462.87 20.1963.18 13.0562.96 doi:10.1371/journal.pone.0002192.t002 that could modify adhesive interactions under moderate humidity conditions [22], perhaps in a way analogous to the increased stickiness of nodes in silk of some spiders in the Araneomorphae. In summary, our findings are partially consistent with those of studies examining the effects of humidity on adhesion in individual setae [12,14]. Adhesion increases similarly with humidity at both the nano and whole organism scale, suggesting that van der Waals forces may not provide a complete description of the mechanics responsible for clinging in geckos. Implications are potentially broad-reaching, both for biological hypotheses about the evolution of toe pads, as well as for bioengineers interested in biologically- inspired design of synthetic adhesives. Materials and Methods We obtained 10 Phelsuma dubia (Dull Day Gecko) and 16 Gekko gecko (Tokay Gecko) from California Zoological Supply. Phelsuma. dubia is a diurnal gecko found in Madagascar. Our specimens weighed between 4.7–7.3 g. Gekko gecko, is a nocturnal gecko found in Southeast Asia. Our specimens weighed between 54–80 g. All specimens were housed in individual 10 gallon glass tanks with paper towel substrate, maintained within a dedicated animal research facility at the University of Akron. Each tank was misted with water as well as provided with a fresh bowl of water daily. G. gecko were fed vitamin dusted crickets 5 times a week whereas the P. dubia were fed vitamin dusted crickets 3 times a week and the other 2 days they received a fruit supplement. The room was PLoS ONE | www.plosone.org 4 May 2008 | Volume 3 | Issue 5 | e2192