Left in the dust: differential effectiveness of the two alternative adhesive pad configurations in geckos (Reptilia: Gekkota)

Abstract

Two distinctive patterns of adhesive toepad are found in the Gekkotaterminal leaf-like pads situated at the distal ends of the digits, and basal pads that encroach more proximally along the digits, are proportionally larger and are associated with more than just the ungual and distal portion of the penultimate phalanges. Although these two configurations have long been recognized, there has been no explanation of whether or not they are functionally different. A small offshore island provided a potential arena for testing the possibility of differential functionality of these two patterns. Euleptes europaea, a leaf-padded taxon, is widespread on Giraglia Island (Corsica), whereas Tarentola mauritanica, a basally padded recent colonizer, has remained restricted to its one concrete building. Challenges to adhesion occur on dusty surfaces, and the self-cleaning mechanism of gecko setal fields can be rendered ineffective if the dust is ubiquitous. Giraglia is composed of prasinite, an inherently friable and dusty schist. The leaf-like terminal pads of E. europaea can be withdrawn from close contact with dusty surfaces, leaving the claws alone to provide traction. The digits of T. mauritanica more fully integrate the claws and adhesive pad, with full-digit hyperextension rendering the claws alone incapable of gripping the surface. Colonization of Giraglia by T. mauritanica has failed because its adhesive system is incompatible with the island's rocky terrain. This has permitted the relictual E. europaea to persist in the face of potentially intense competition. Our findings shed light on the previously noted association of leaf-like pads with rupicolous habits, and indicate that an enhanced understanding of particular surfaces that leaf-padded geckos exploit may further clarify how definitive these associations are.

Nonlinear variation in clinging performance with surface roughness in geckos

“…Shear force exerted on coarse substrates was not significantly different from that on glass in either species; thus, our results showed a nonlinear relationship between peak-to-valley heights and shear forces on the continuum of surfaces we used, consistent with studies by Huber et al (2007; on a scale of single spatula), and Naylor and Higham (2019). Gecko adhesive systems have been well studied on a range of artificial substrates that have revealed the form and function of the adhesive apparatus in this taxon; however, our findings further highlight the need for gecko adhesion studies under more ecologically relevant conditions (Collins et al, 2015;Higham & Russell, 2010;Higham et al, 2019;Russell & Delaugerre, 2017).…”

Population genetic structure and species delimitation of a widespread, Neotropical dwarf gecko

“…The gain and loss of adhesive toepads in geckos has been hypothesized to represent a key innovation (Higham et al, 2017, Losos, 2011, Russell and Delaugerre, 2017. A key innovation is a behavioral or morphological adaptation that has the capacity to enhance competitive ability, relax adaptive trade-offs, or catalyze the exploitation of a novel resource, which, in turn enhances the number or longevity of a species (Hunter, 1998).…”

Repeated evolution of digital adhesion in geckos: a reply to Harrington and Reeder

“…The fully expressed adhesive apparatus includes the transformation of epidermal spinules into elaborate setae that enable adhesion through frictional and van der Waals interactions (Russell, 1975(Russell, , 2002Autumn et al, 2000Autumn et al, , 2002Autumn et al, , 2006, expansion of the subdigital surface into scansors (Russell, 1975(Russell, , 1976(Russell, , 1979 and modification of muscles and tendons to control the scansors (Russell, 1975(Russell, , 1976(Russell, , 1979. Additional, lineage-specific, adaptations include the evolution of specialized phalangeal morphology, blood sinuses and adipose pads to enhance scansor contact with the substrate (Russell, 1981;Russell & Bauer, 1988;, reconfiguration of the form of the autopodium bringing about a symmetrical disposition of the digits (Russell et al, 1997), the elevation of the penultimate and ungual phalanges to segregate these from the underlying scansors (Russell, 1976;Gamble et al, 2012), scansors (with fundamentally different control mechanisms than those exhibited by basally derived toepads) limited to the distal tips of the digit to facilitate movement across dusty substrates (Russell & Delaugerre, 2017) and various modifications of digit I . Indeed, because the ancestral condition in digit I is to possess only two phalanges, it is subject to constraints with regard to how an adhesive apparatus can be accommodated and operated (Russell & Bauer, 2008).…”