Arboreal Day Geckos (Phelsuma madagascariensis) Differentially Modulate Fore- and Hind Limb Kinematics in Response to Changes in Habitat Structure

Abstract

By using adhesion, geckos can move through incredibly challenging habitats. However, continually changing terrain may necessitate modulation of the adhesive apparatus in order to maximize its effectiveness over a range of challenges. Behaviorally modulating how the adhesive system is applied can occur by altering the alignment of the foot relative to the long axis of the body and/or the angles between the digits (interdigital angle). Given the directionality of the adhesive system, geckos likely vary the application of the system via these mechanisms as they run. We quantified 3D movements (using high-speed video) of the day gecko, Phelsuma madagascariensis, running on a range of ecologically relevant inclines (0°, 45°, 90°) and perch diameters (1.5 cm, 10 cm and broad). We measured the instantaneous sum of interdigital angles and foot alignment relative to the body, as well as other kinematic variables, throughout each stride and across treatments. Modulation of foot alignment at 45° and 90° was similar between the forelimb and hind limb, but differed at 0°, suggesting that P. madagascariensis is able to exert an adhesive force using multiple strategies. Both the sum of interdigital angles and alignment in the fore- and hind foot were modulated. Differences in modulation between the limbs are likely related to the underlying morphology. The modulation of interdigital angle and foot alignment suggests that aspects other than the mechanism of adhesion, such as joint morphology, are important for arboreal movement in geckos. Our study of foot usage in arboreal locomotion reveals patterns that may be widespread across pad-bearing lizards. In addition to understanding the constraints exerted by the adhesive apparatus, we highlight how biomechanical traits may respond to the evolution of novel adaptations and morphologies.

There’s more than one way to climb a tree: Limb length and microhabitat use in lizards with toe pads

“…In particular, while both geckos and anoles have adhesive pads ( Draco and Tropidurus lack adhesive pads), geckos generally generate greater frictional and adhesive forces (negative normal forces) compared to anoles [ 26 , 68 , 69 ], possibly allowing geckos to resist better lateral forces and cling to narrow perches. Previous locomotor studies of arboreal pad-bearing lizards have found that lizards tend to lower their center of mass on narrower perches [ 70 – 74 ]. Future studies incorporating species limb length, adhesive capabilities, and perch diameter would be very informative.…”

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

“…The performance of the adhesive pads does not solely depend on the adhesive setae layer itself, but also on the way it is used for locomotion at the toe, footpad and whole animal level. Geckos alter foot orientation to facilitate adhesion in response to changes in habitat structure [20,21]. Here, we provide evidence that the adhesive system is engaged and disengaged behaviorally through the modulation of the foot alignment.…”

“…Multiple toes differing in orientation appear to play an important role during gecko locomotion, as the animal can avoid detachment by aligning some of their toes opposite to the force vector. It is likely that employment of the adhesive system is associated with modulation of inter-digital angle and foot alignment [20,21]. However, much less is known about adhesion force of single toes during locomotion.…”

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

“…This secondary asymmetry in digit length is different from that evident in ancestrally padless lizards in that it is associated with interdigital angles that are much greater than those exemplified by the latter and may also be associated with the vestigialization of the first digit (Russell et al, ; Russell & Bauer, ). Asymmetry resulting from the postaxialmost interdigital angle being the largest of the interdigital angles and the fourth digit being elongated, has several possible functional correlates, including the permitting of grasping between the first four digits and the fifth and/or facilitating propulsion and positioning the fifth metatarsal such that it can function like the calcaneal tuber of mammals (Robinson, ; Russell & Rewcastle, ; Zhuang & Higham, ). In the case of Phelsuma , grasping small perch diameters does not appear to function as described by Robinson ().…”

“…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).…”