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

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.