Nonlinear variation in clinging performance with surface roughness in geckos

Abstract

Understanding the challenges faced by organisms moving within their environment is essential to comprehending the evolution of locomotor morphology and habitat use. Geckos have developed adhesive toe pads that enable exploitation of a wide range of microhabitats. These toe pads, and their adhesive mechanisms, have typically been studied using a range of artificial substrates, usually significantly smoother than those available in nature. Although these studies have been fundamental in understanding the mechanisms of attachment in geckos, it is unclear whether gecko attachment simply gradually declines with increased roughness as some researchers have suggested, or whether the interaction between the gekkotan adhesive system and surface roughness produces nonlinear relationships. To understand ecological challenges faced in their natural habitats, it is essential to use test surfaces that are more like surfaces used by geckos in nature. We tested gecko shear force (i.e., frictional force) generation as a measure of clinging performance on three artificial substrates. We selected substrates that exhibit microtopographies with peak‐to‐valley heights similar to those of substrates used in nature, to investigate performance on a range of smooth surfaces (glass), and fine‐grained (fine sandpaper) to rough (coarse sandpaper). We found that shear force did not decline monotonically with roughness, but varied nonlinearly among substrates. Clinging performance was greater on glass and coarse sandpaper than on fine sandpaper, and clinging performance was not significantly different between glass and coarse sandpaper. Our results demonstrate that performance on different substrates varies, probably depending on the underlying mechanisms of the adhesive apparatus in geckos.

Geckos cling best to, and prefer to use, rough surfaces

“…Our measurements of habitat use were consistent with literature reports for these species [ 42 , 47 , 54 , 57 ], but we report the range of roughnesses used by these geckos in nature (Fig. 1 ).…”

“…The habitat use we observed corresponded well to habitat use reported in the literature for these species, according to which northern velvet geckos ( O. castelnaui ) use arboreal habitats [ 42 , 55 , 57 ]; spotted velvet geckos ( O. coggeri ) use saxicoline habitats, [ 42 , 54 ]; and ocellated velvet geckos ( O. monilis ) use both arboreal and saxicoline habitats [ 42 , 47 , 58 ]. We found Oedura castelnaui ( N = 67) exclusively on arboreal microhabitats and used dead trees and silver-leaf ironbark trees ( Eucalyptus melanophloia) approximately equally (Fig.…”

“…Locomotory capabilities are a consequence of mechanical interactions between animals and the substratum, and the attachment capability of geckos is one such surface interaction [ 56 , 59 , 60 ]. Over the past few decades, studies investigating locomotor performance have mostly focused on very smooth substrates [ 10 , 44 , 54 , 61 , 62 ], however, recently some studies have used naturally rough and ecologically relevant substrates [ 10 , 38 , 62 – 64 ]. We used artificial surfaces (sandpaper) for uniformity, but selected roughnesses (measured as peak-to-valley heights) similar to those of natural substrates (among the roughest and least rough) used by our geckos in nature.…”

“…Thus, the adhesive system of geckos was thought to perform better on smooth and uniform surfaces [ 10 ]. Recently, however, some studies have found that geckos are capable of attachment on rough and undulant substrates, previously thought to provide limited purchase for attachment [ 10 , 54 ]. Clearly, further study of gecko performance on rough and undulant surfaces, similar to those geckos encounter in nature, is required [ 10 , 11 , 38 , 45 , 54 – 56 ].…”

Lizards, toepads, and the ghost of hurricanes past

“…The clinging hypothesis ultimately needs biomechanical measurements to quantify the functional impact of larger toepads and limb-length shifts. Such studies should use perches with different diameters and roughness (12), dry and wet surfaces (13), and turbulent winds. Functional interactions between toepads and claws need to be addressed (2,14).…”