Limb and digit orientation during vertical clinging in Bibron's gecko, Chondrodactylus bibronii (A. Smith, 1846) and its bearing on the adhesive capabilities of geckos

Abstract

Russell, A.P. and Oetelaar, G.S. 2016. Limb and digit orientation during vertical clinging in Bibron's gecko, Chondrodactylus bibronii (A. Smith, 1846) and its bearing on the adhesive capabilities of geckos. -Acta Zoologica (Stockholm) 97: 345-360.Geckos with subdigital adhesive pads can scale smooth vertical surfaces in defiance of gravity. The deployment of the adhesive system is activated by the musculoskeletal system during active traverses of such surfaces, but adhesion on such substrata can also be achieved by passive means, with the body weight of the gecko applying tensile loading to the adhesive setae, maintaining prolonged, static contact with the surface. To investigate whether passively induced adhesion is employed by geckos holding station on smooth vertical surfaces, we investigated the magnitude of shear force generation for the manus and pes, and the positioning of the limb segments and digits in Chondrodactylus bibronii in freely selected resting postures (head-up, head-down and facing laterally to the left and right). Our results indicate that different subsets of digits occupy positions consistent with them being passively loaded in different body orientations. Limb segment and digit orientation are consistent within, and differ between, the resting postures, and relatively few of the 20 digits are positioned to take advantage of gravitationally induced loading in any posture. The pedal digits have greater adhesive potential than the manual ones and, more frequently, capitalize on passive loading than do manual digits. This is especially evident in the commonly adopted head-down resting posture.

Control strategies of gecko’s toe in response to reduced gravity

“…Furthermore, the modulation of foot orientation is also an important biological principle that promises to be very useful for technical application [33]. The characteristic structure of the pes and manus of the gecko, allowing the feet to rotate freely and the digits to be spread widely [47], leads to the animal's ability to adhere in the head up and head down orientation as well as on vertical surfaces [48]. Observation in tree frogs showed that they are unable to adhere to smooth surfaces in the head-down orientation [49], probably due to the constraint in digit spread and foot alignment.…”

Evolution of pedal digit orientation and morphology in relation to acquisition and secondary loss of the adhesive system in geckos

“…Thus, digital spreading permits the ability to increase the size of individual toepads (Webster et al, 2009). The importance of the spread of the digits for padbearing lineages is apparent, because it permits the engagement of the adhesive apparatus in a variety of orientations simultaneously (Birn-Jeffery & Higham, 2014;Russell & Oetelaar, 2015;Zhuang & Higham, 2016). Larger species may benefit from greater support provided by toepads applied in opposing directions, which may also facilitate changing directions quickly (Imburgia et al, 2019;Russell & Oetelaar, 2015).…”

“…A study similar to that of Powell et al (2018), examining ontogenetic growth in such species, with the simultaneous measurement of metatarsal head shape and interdigital angle measurement, could help determine the mechanisms underlying changes in interdigital angles. In addition, behavioral analyses, similar to that of Russell and Oetelaar (2015), would reveal how such species use their claws in combination with the broad digital spread that they possess (Naylor & Higham, 2019).…”

“…Previous studies have suggested how gecko foot morphology is expected to change with the acquisition of an adhesive system (Russell, 1975(Russell, , 1979Russell, Bauer, & Laroiya, 1997). For padbearing lineages, in addition to changes in phalangeal form and proportions, digits are likely to be shorter and the arc subtended by them should increase to attain a fan-like configuration (Russell et al, 1997), facilitating the ability of at least some of the digits to always be aligned with the vector of gravitational acceleration while moving on vertical surfaces (Russell & Oetelaar, 2015). Such enhanced digital spreading also provides increased interdigital space for the laterally expanded individual toepads, preventing overlap (Russell & Oetelaar, 2015).…”

Light level impacts locomotor biomechanics in a secondarily diurnal gecko,Rhoptropus afer

“…Geckos move in a variety of complex habitats, and often do so by employing an adhesive apparatus located on the ventral surface of the toes Higham, 2014, 2016;Russell, 2002;Russell and Higham, 2009;Russell and Oetelaar, 2015;Zhuang and Higham, 2016). The combination of visual and locomotor specializations makes geckos an ideal group for quantifying the relationship between the two.…”