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.
Highlights
Patterns of terrestrial locomotion differ considerably among taxonomic, temporal, and spatial scales
We examined the application of the gecko adhesive apparatus on complex arboreal substrates in order to understand how habitat structure constrains and facilitates locomotion. We examined both forelimb and hind limb kinematics in response to changes in perch diameter and incline, with a focus on the foot kinematics in a clawless arboreal specialist, Phelsuma madagascariensis
Higher values on PC3 corresponded with decreased hind foot alignment at footfall and mid stance, greater ankle joint angles at footfall and midstance, a decreased angle between digits III and IV at end stance and greater stride frequency
Summary
Patterns of terrestrial locomotion differ considerably among taxonomic, temporal, and spatial scales. The locomotor behavior of lizards on a variety of substrates including sand, water, and arboreal branches/ trunks has been examined in several taxa [1, 9,10,11], and differences among these taxa suggest that lizard locomotion is variable [12]. These dynamic changes in locomotion are constrained by the morphology underlying the locomotor system, especially the foot, which transfers force from the animal to the substrate. The variation in kinematics of the more proximal joints has been described, detailed foot kinematics remain relatively understudied
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