Skin hydrophobicity as an adaptation for self‐cleaning in geckos

Abstract

Hydrophobicity is common in plants and animals, typically caused by high relief microtexture functioning to keep the surface clean. Although the occurrence and physical causes of hydrophobicity are well understood, ecological factors promoting its evolution are unclear. Geckos have highly hydrophobic integuments. We predicted that, because the ground is dirty and filled with pathogens, high hydrophobicity should coevolve with terrestrial microhabitat use. Advancing contact‐angle (ACA) measurements of water droplets were used to quantify hydrophobicity in 24 species of Australian gecko. We reconstructed the evolution of ACA values, in relation to microhabitat use of geckos. To determine the best set of structural characteristics associated with the evolution of hydrophobicity, we used linear models fitted using phylogenetic generalized least squares (PGLS), and then model averaging based on AICc values. All species were highly hydrophobic (ACA > 132.72°), but terrestrial species had significantly higher ACA values than arboreal ones. The evolution of longer spinules and smaller scales was correlated with high hydrophobicity. These results suggest that hydrophobicity has coevolved with terrestrial microhabitat use in Australian geckos via selection for long spinules and small scales, likely to keep their skin clean and prevent fouling and disease.

Convergent evolution of skin surface microarchitecture and increased skin hydrophobicity in semi-aquatic anole lizards

“…So, the observed negative relationship between contact angle and anole body size in this study is likely a scaling effect, a mere consequence of the microstructure ratio of anoles approaching 10:1 with decreasing body size. Indeed, our statistical models showed that both spinule length and scale size explain a large portion of the variation in contact angle, with longer spinules and smaller scales positively influencing skin surface hydrophobicity; Riedel et al (2020) found the exact same form-function relationship in Australian geckos.…”

“…Earlier functional histological research suggests that a spinulate oberhautchen in the common ancestor of anoles might have played an important role in facilitating ecdysis ( Alibardi and Maderson, 2003 ; Bauer, 2019 ; Irish et al, 1988 ; Maderson, 1970 ). Other hypotheses include functions such as self-cleaning and anti-fouling, reduction of friction and wear protection, as has been shown in geckos ( Riedel et al, 2020 ; Spinner et al, 2013 ; Watson et al, 2015a , b ). To fill this gap, future studies on the functional and ecological significance of spinules should explore the more basal iguanid and gekkotan lineages and focus on species that carry simple spinulate microarchitecture but lack setae and functional toepads.…”

“…This makes sense, as semi-aquatic anoles carry longer spinules, and a higher spinule length can support a higher proportion of trapped gas and a more stable hydrophobic state ( Scarratt et al, 2017 ). Spinule length also strongly determines surface non-wettability of gecko skin ( Riedel et al, 2020 ) as does the height of micro-protrusions on the wings of cicadas ( Sun et al, 2012 ).…”

“…In anoles, the skin surface is covered with microscopic hair-like ornaments (Peterson, 1984), and contingent upon its complexity, organization and length dimensions, these hair-like microstructures may have the potential to generate extreme surface hydrophobicity (Gao et al, 2011;Li and Amirfazli, 2008;Yan et al, 2011). Indeed, similar skin surface microstructures have been found in gekkonid lizards and shown to be responsible for the highly hydrophobic surface of gecko skin (Riedel et al, 2020;Watson et al, 2010Watson et al, , 2015a. The water-resistant properties of anole skin, however, have remained unexamined, but very recent discoveries have provided some insight into this matter.…”

Repeated evolution of underwater rebreathing in diving Anolis lizards

“…Since all anoles we submerged developed a plastron, including many that rarely, if ever, occur near water, the plastron is unlikely to Report be an adaptation for diving. More likely, the anole plastron arose as an evolutionary ''spandrel'' 44 -the byproduct of skin hydrophobicity that evolved for some other purpose (e.g., to shed rainwater or dew, 45 or for anti-fouling via the ''lotus effect'' 46 , as has been shown in geckos [47][48][49] ).…”