The evolution of digit form inGonatodes(Gekkota: Sphaerodactylidae) and its bearing on the transition from frictional to adhesive contact in gekkotans

Abstract

Although the phenomenon of adhesion in geckos has been intensively studied for over 200 years, our understanding of how the morphological apparatus associated with this arose is less clear. Indeed, whether or not all of the intricate morphological hierarchy that is implicated in the attachment and removal of the adhesive setae originated at the same time is unknown. To explore whether setae may have arisen prior to the other parts of this structural hierarchy, we undertook morphological observations of Gonatodes, an ancestrally padless, sphaerodatyline genus known to exhibit the expression of incipient subdigital pads in some species. Focusing on this geographically and morphologically well-circumscribed genus, for which intraspecific relationships are adequately known and ecology is quite well documented, allowed us to deduce trends in digit proportions, shape, scalation, and skeletal structure, and associate these with the micro-ornamentation of the subdigital surfaces. Our findings indicate that in Gonatodes, setae capable of inducing adhesion are present without the modifications of the digital musculotendinous, circulatory and skeletal systems that are generally considered to be necessary for the operation of a functional adhesive apparatus. The acquisition of these latter characteristics (independently in many lineages of gekkotans, and incipiently so in Anolis) may have been preceded by a suite of modifications of the digits that enhanced static clinging in relation to sit-and-wait predation and the ability to take refuge on surfaces unavailable to other taxa. These possibilities await further testing.

Nonlinear variation in clinging performance with surface roughness in geckos

“…We found that shear forces exerted by both P. australis and O. coggeri were greater on glass compared to on fine sandpaper. The gekkotan adhesive system is often characterized as most efficient on smooth substrates (Russell, Baskerville, Gamble, & Higham, ). High performance on glass, observed in our study, was consistent with previous studies that have tested clinging ability on artificial smooth substrates (Autumn et al, ; Huber et al, ; Irschick et al, ; Mahendra, ; Naylor & Higham, ).…”

Tempo and mode of performance evolution across multiple independent origins of adhesive toe pads in lizards

“…Epidermal spinules appear to be common across geckos and other lizards, including Chamaeleonidae, Iguanidae, Leiocephalidae, and Polychrotidae (Maderson 1964;Ruibal 1968;Maderson 1970;Stewart and Daniel 1975;Peterson 1984;Bauer and Russell 1988;Irish et al 1988;Peattie 2008;Vucko 2008). Russell et al (2015) provide a stunning example in Gonatodes, highlighting variation in both setal and toe pad morphology suggesting that Gonatodes may represent an example of elongated spinules and enlarged ventral scales performing as a friction-generating pad.…”

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

“…Several lineages have a pedal morphology that is fully consistent with the ancestrally padless condition, which is expressed widely among lizard families in general. Also, it has been known for decades that the morphology of geckos with ancestrally padless digits can be distinguished from secondarily padless forms (Haacke, ; Russell, , , ; Russell et al ., ). Reversion to a secondarily padless condition carries with it evidence of the prior presence of this structural complex and is fully consistent with Dollo's law (Gould, ; Haacke, ; Russell, , , ; Russell et al ., ).…”

“…All of the structural diversity observed across the Gekkota, and the presence of adhesive phenotypes at all stages of transformation (Russell et al ., ), points to multiple origins of this complex trait. Furthermore, as mentioned, many gecko species are secondarily padless but retain clear anatomical indicators of their pad‐bearing ancestry (Haacke, ; Russell, , ; Lamb & Bauer, ).…”