Sticky gecko feet: the role of temperature and humidity

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 12 degrees C was nearly double the clinging ability at 32 degrees C. 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.

Figure 1. Experimental trials at variable temperatures without controlling for humidity.

Relationship between temperature and body-size corrected adhesion (clinging force [N]; left axis, bars) or relative humidity (%; right axis, solid line).

Figure 2. Adhesion at constant temperature of 12°C 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 32°C designed to test for an interaction of temperature and humidity; error bars represent ±2 SE. See text for details (materials and methods).

Figure 3. Adhesion during variable temperature, uncontrolled humidity trials (black solid lines), and variable relative humidity constant temperature trials (12°C; red-dotted lines) showing convergence of maximal clinging force with increasing temperature and decreasing humidity.

Error bars are ±2 SEM.

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.