Lizards are a highly diverse group of reptiles that have adapted to nearly every terrestrial environment. Their ability to run, climb, burrow, and glide is largely due to the specialized structures at the ends of their limbs. While commonly called “feet,” the complexity of lizard locomotion is revealed in the specialized anatomy of their limbs and digits. This diversity allows different species to navigate substrates ranging from smooth vertical glass to shifting desert sand.
Limb Structure and Terminology
The structures at the end of a lizard’s limb are properly referred to as the manus (forefoot) and pes (hindfoot). Like most four-legged vertebrates, lizards generally possess a pentadactyl structure, meaning they have five digits on each limb. These digits are covered in scales composed of keratin, the same protein that makes up their skin.
At the tip of each digit is a pointed, curved structure known as the claw, or ungual. These claws are made of hard keratin and are used for climbing rough surfaces, digging burrows, or gripping prey. Although many species retain five digits, variations exist depending on the species’ specific locomotor needs.
Specialized Climbing Pads and Scales
A select group of lizards, most famously geckos and anoles, possess specialized adhesive pads on their digits that allow them to adhere to extremely smooth surfaces. These pads are organized into distinct ridges known as lamellae, which are fine, plate-like structures that run across the underside of the digits, significantly increasing the surface area.
The source of this sticking power is found on the lamellae, which are densely covered in microscopic, hair-like projections called setae. Each seta is a tiny, fibrous structure composed of beta-keratin. In geckos, each seta branches further into hundreds of even smaller, flattened tips known as spatulae, which are the final points of contact with a surface.
The Physics of Gecko Adhesion
The mechanism by which geckos achieve adhesion does not involve glue, suction, or sticky chemical secretions. Instead, the process is a physical phenomenon based on weak intermolecular attractions called Van der Waals forces. These forces are temporary attractions that occur between molecules due to fluctuating electrical charges.
The hierarchical structure of the spatulae makes the weak Van der Waals forces powerful. This immense number of tiny tips maximizes the intimate contact area between the lizard’s foot and the climbing surface. When billions of spatulae come into close proximity with a surface, the collective sum of these weak forces generates a powerful adhesive effect.
A single gecko seta can produce a force of about 200 micronewtons, and the total adhesive force generated by millions of setae can support many times the lizard’s body weight. The gecko controls this adhesion through a directional process. It engages the force by pressing its foot down and sliding it slightly backward, and detaches by changing the angle of the setae. By peeling its toes away from the surface, the lizard breaks the Van der Waals bonds with minimal effort.
Foot Adaptations Across Lizard Species
Not all lizard feet are designed for vertical adhesion, and their morphology varies greatly depending on the environment. Lizards living on loose substrates, such as desert sand, have evolved structures to increase surface area and prevent sinking. For example, fringe-toed lizards have specialized, pointed scales on the sides of their toes that act like snowshoes, providing traction over shifting dunes.
Other species have adaptations for burrowing and swimming. Sandfish skinks, which “swim” through the sand, have reduced or lost their limbs and have very smooth scales to minimize friction. Basilisk lizards possess specialized scales on their hind feet that allow them to run across the surface of water for short distances. They achieve this by using a slapping and stroking motion with their feet, which creates a temporary pocket of air and water tension to support their weight. The sheer diversity of these structures highlights how the basic lizard limb has been modified to conquer specific ecological challenges.