Sticky Gecko Feet: The Role of Temperature and Humidity Peter H. Niewiarowski1*, Stephanie Lopez1, Liehui Ge2, Emily Hagan2, Ali Dhinojwala2 1 Department of Biology, Program in Integrated Bioscience, University of Akron, Akron, Ohio, United States of America, 2 Department of Polymer Science, University of Akron, Akron, Ohio, United States of America Abstract Gecko adhesion is expected to be temperature insensitive over the range of temperatures typically experienced by geckos. Previous work is limited and equivocal on whether this expectation holds. We tested the temperature dependence of adhesion in Tokay and Day geckos and found that clinging ability at 12uC was nearly double the clinging ability at 32uC. However, rather than confirming a simple temperature effect, our data reveal a complex interaction between temperature and humidity that can drive differences in adhesion by as much as two-fold. Our findings have important implications for inferences about the mechanisms underlying the exceptional clinging capabilities of geckos, including whether performance of free-ranging animals is based solely on a dry adhesive model. An understanding of the relative contributions of van der Waals interactions and how humidity and temperature variation affects clinging capacities will be required to test hypotheses about the evolution of gecko toepads and is relevant to the design and manufacture of synthetic mimics. Citation: Niewiarowski PH, Lopez S, Ge L, Hagan E, Dhinojwala A (2008) Sticky Gecko Feet: The Role of Temperature and Humidity. PLoS ONE 3(5): e2192. doi:10.1371/journal.pone.0002192 Editor: Mark Isalan, Center for Genomic Regulation, Spain Received December 5, 2007; Accepted March 27, 2008; Published May 14, 2008 Copyright: 2008 Niewiarowski et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Partial support for this project was provided by the Integrated Bioscience Program, University of Akron (PHN and AD), by E.H. research experience for undergraduates (DMR 0353746) and NSF-NIRT 0609077 (AD). Competing Interests: The authors have declared that no competing interests exist. * E-mail: phn@uakron.edu Introduction An average sized (40–50 g) adult Tokay gecko (Gekko gecko) can generate the equivalent of more than 100 times its own body weight in clinging force (,40 N) using its dry adhesive toepads [1]. Although quantitative measurements of maximal clinging capac- ities, and characterization of the underlying microscopic functional morphology is a relatively recent accomplishment [2], researchers have been proposing hypotheses and debating mechanisms responsible for such remarkable clinging abilities for over 100 years [2]. Despite a burgeoning literature on gecko adhesion and the obvious applications to evolutionary biologists and materials scientists alike [3], there are still significant gaps in our understanding of fundamental aspects of gecko adhesion at multiple scales of analysis. For example, the functional and evolutionary significance of variation in toepad and setal morphology was explicitly raised nearly 50 years ago [4,5], however, studies have only just begun to address such relationships [6,7]. Similarly, locomotor capacities of ectotherms like lizards have been subject to extensive scrutiny focusing on the temperature sensitivity of performance [8], but only recently has the study of the temperature sensitivity of gecko clinging ability joined this literature [9,10]. Studies of gecko adhesion at the setal and spatulae level, suggest that intermolecular interactions (van der Waals forces) are responsible for adhesion. If clinging in geckos is only based on van der Waals forces [11], clinging capacity should be temperature insensitive over the range of body temperatures typically experienced by geckos [10]. Indeed, that a key functional capacity of locomotion in geckos may be free of typical thermal biophysical constraints experienced by ectotherms in thermally heterogeneous environments, suggests that thermal independence of adhesion may have driven the evolution of the gecko adhesive system [10]. Temperature sensitivity of clinging capacity has only been measured in two species (Phelsum dubia and Gekko gecko; [9,10]). Moreover, contrasting results between these two studies are difficult to interpret because different methodologies were employed, leaving the question of the temperature sensitivity of clinging capacity unanswered. In an effort to eliminate the potential contribution of divergent methods used to evaluate the temperature sensitivity of clinging performance, we describe a series of experiments that estimated maximal clinging ability in P. dubia and G. gecko over a range of biologically meaningful temperatures, using a single protocol. During the conduct of our experiments, it became clear that interpreting temperature sensitivity of clinging ability requires knowledge and control of relative humidity, a potential source of variation noted but discounted in previous studies [9]. However, variation in humidity has recently been implicated as a source of variation in adhesive forces generated at the nanoscale between single spatulae and various substrates [12]. Results Temperature effects Tokay geckos were larger (58.1466.8 g) and had greater total toepad areas (4.7460.98 cm2) than Day geckos (5.5860.32 g and 0.7360.06 cm2, respectively; Table 1). Tokays were also able to generate higher maximal total clinging forces (31.2264.39 N and PLoS ONE | www.plosone.org 1 May 2008 | Volume 3 | Issue 5 | e2192