Integrative architecture and structural boundaries of the subdigital adhesive toe pads of Gekko gecko (Linnaeus, 1758) and guidelines for toe pad demarcation in other geckos.

Many species of gecko and all but one species of anole possess enlarged, serially arranged scales on the ventral surfaces of their digits that collectively are referred to as toe pads. These are commonly considered key innovations and have been the subject of many studies within and beyond the biological sciences. The definition of squamate toe pads, however, remains vague. This is particularly problematic for comparative morphological and biomechanical studies wherein toe pad area is commonly employed as a dependent or independent variable or covariate. Lack of a standardized means of delimitation of toe pads impedes accurate measurement of these ecologically and evolutionarily important features and compromises effective comparison between species and studies. Herein, we take a comprehensive approach to the delimitation of the toe pads of the Tokay gecko (Gekko gecko), the species of gecko most frequently studied in relation to the phenomenon of gecko adhesion. In addition to consideration of the broadly adopted, but somewhat arbitrarily delineated, circumscription of toe pads based on characteristics of scale dimensions alone, we also investigate their boundaries by examining the relationship between subdigital scales and subcutaneous anatomical features (employing osteological and tendon-based [TB] criteria). We find that the methods employing only external dimensions of subdigital scales to delimit toe pads overestimate their area, thereby resulting in functional studies that likely underestimate toe pad adhesive stress (force per unit area). Of the two approaches that anatomically delimit toe pads, that based upon tendinous connections (demarcating what we refer to as the LDT region of the digits) encompasses all epidermal filaments that are true setae (those bearing spatulate tips), the only structures empirically validated to facilitate whole-animal adhesion. As such, we encourage future studies to employ the TB approach to accurately delimit gekkotan (and anoline) toe pads in a functionally and anatomically meaningful (and standardized) fashion and provide guidelines about how this can be achieved.