Ecologically-related variation of digit morphology in Cyrtodactylus (Gekkota, Squamata) reveals repeated origins of incipient adhesive toepads

The exploitation of different locomotor substrates in different ecological niches has driven the evolution of specialized morphological structures, and similar ecological demands, such as the structure of the microhabitat, often lead to convergent or parallel evolution. The evolution of adhesive toepads in geckos remains understudied because of the paucity of phylogenetically informed investigations of candidate clades exhibiting purported incipient expression of these (i.e., species having evolved some, but not all, parts of the complex adhesive system of pad-bearing geckos). Using Cyrtodactylus, a speciose genus with well-established ecotypes, we tested the hypothesis that microhabitats that require more climbing will lead to the acquisition of incipient adhesive morphology. We measured subdigital scale area, a proxy for adhesive toepad evolution, and quantified subdigital scale shape for 77 of the 354 described species, including at least one representative of each ecotype. Subdigital scale area increased from terrestrial through generalist and saxicoline (rock-dwelling) to arboreal ecotypes, with subdigital scale shape evolving from ancestral conditions for padless lizards to lateromedially expanded lamella-like scales only in the arboreal ecotypes. This significant link between phenotype and environment supports the contention that scansorial, and particularly arboreal, Cyrtodactylus ecotypes have evolved incipiently expressed adhesive toepads. This highlights the suitability of this genus as a model system for studying the ecology and evolution of adhesive toepads as well as being a promising candidate for research on adaptive radiations.

Methods
We measured the subdigital scale area and quantified the subdigital scale shape of 77 Cyrtodactylus species from museum voucher specimens, measuring 3 to 10 specimens per species. For details of data collection see the methods section of the paper: Ecologically-related variation of digit morphology in Cyrtodactylus (Gekkota, Squamata) reveals repeated origins of incipient adhesive toepads, Riedel et al. 2024, Functional Ecology.