Lizards have scales because scales solve the biggest problem land-dwelling reptiles face: surviving out of water. When the ancestors of modern lizards transitioned from aquatic to terrestrial life, their skin had to develop barriers against water loss, UV radiation, and physical damage. Scales are the result of millions of years of evolutionary pressure to meet all three challenges with a single structure.
Scales Prevent Lizards From Drying Out
The most critical job of a lizard’s scales is keeping moisture inside the body. Unlike amphibians, which have thin, permeable skin and must stay near water, lizards can thrive in deserts and dry grasslands because their scales act as a waterproof barrier. This barrier comes from two layers working together. The inner layer is made of soft keratin (the same protein family found in human skin and hair), which produces specialized lipids and waxes that seal moisture in. The outer layer is made of hard keratin, a tougher protein found only in reptiles and birds, which adds rigidity and further reduces evaporation.
This dual-keratin system is what separates reptile skin from mammal skin. Mammals have only soft keratin in their outer skin layers, relying on sweat glands and other strategies to manage moisture. Lizards traded that flexibility for a nearly watertight seal, which is why they don’t need to drink as frequently as similarly sized mammals and can colonize some of the driest habitats on Earth.
Physical Protection and Armor
Scales are a lizard’s first line of defense against thorns, rough terrain, and predators. The hard keratin in the outer layer gives scales their stiffness, creating a lightweight shield that resists puncture and abrasion. For many lizard families, though, keratin alone isn’t enough. Skinks, girdled lizards, monitor lizards, and several other groups have evolved osteoderms: small bony plates embedded directly beneath or within their scales. These bony deposits dramatically increase skin toughness.
Research on girdled lizards found that skin toughness correlates with osteoderm coverage and thickness. The armadillo girdled lizard, which curls into a ball when threatened, has such dense osteoderm coverage that its skin can withstand simulated bites from several mongoose species. Most other tested lizards weren’t quite that well armored, but even moderate osteoderm coverage adds meaningful protection against scrapes, bites, and crushing pressure. Scales also protect against UV radiation, functioning like a built-in sunscreen for animals that spend hours basking in direct sunlight.
Scales Help Regulate Body Temperature
Because lizards are ectotherms and rely on their environment to warm up or cool down, scale color and reflectance play a direct role in temperature control. Dark-colored scales absorb more solar radiation, helping a lizard heat up faster. Light-colored scales reflect more sunlight, preventing overheating.
Some lizards take this a step further by actively changing scale color. Bearded dragons, for example, shift the reflectance of their dorsal (back) scales depending on body temperature. At cool temperatures around 15°C, their back scales reflect roughly 14% of incoming solar energy. At 40°C, that jumps to about 21%, a 7.4% increase in reflectivity that helps shed excess heat. This color change is concentrated on the dorsal surfaces, the ones most exposed to the sun, while ventral (belly) scales show almost no change. The lizard essentially has a built-in thermostat: darker when it needs to warm up, lighter when it risks overheating.
Importantly, this thermal management involves not just visible light but also near-infrared radiation, which carries a large share of the sun’s heat energy. Bearded dragons adjust reflectance across both wavelength ranges, though visible-light changes are more dramatic (about 7% shift versus 3% in the near-infrared).
Grip and Movement
Scale shape and arrangement directly affect how lizards (and their close relatives, snakes) move across surfaces. Overlapping belly scales, called scutes, are angled so they catch on tiny surface irregularities. This creates directional friction: the scales grip when pushed backward but slide smoothly when pushed forward. On textured surfaces like soil, bark, or rock, this difference is substantial. On perfectly smooth surfaces, the friction advantage nearly disappears, with measurements showing almost identical friction in all directions.
For lizards that climb, scales on the toes and belly may be shaped differently from those on the back or head. Granular scales on toe pads, for instance, provide different gripping properties than the broad, overlapping scales on the trunk. This regional specialization lets a single animal navigate flat ground, rough bark, and vertical rock faces using the same skin.
What Scales Are Made Of
Lizard scales are not separate structures glued onto the skin like fish scales. They are folds and thickenings of the skin itself, made primarily of keratin. The two types of keratin divide the labor cleanly. Soft keratin forms the flexible inner layers, keeping the skin pliable at the joints between scales so the lizard can bend, twist, and run. Hard keratin forms the rigid outer surface, providing the stiffness and durability that make scales useful as armor.
Hard keratin is exclusive to reptiles and birds, which is no coincidence. Birds evolved from reptilian ancestors, and their feathers, beaks, and claws use the same hard keratin found in lizard scales. In fact, the genes responsible for hard keratin in bird scales are closely related to those in lizard scales, and some hard keratin genes originally identified in bird claws turn out to be heavily expressed in scale tissue as well. This shared molecular toolkit is one of the clearest pieces of evidence linking modern birds to their reptilian ancestors.
Shedding and Renewal
Because scales are part of the outer skin, they wear down over time and need to be replaced. Lizards shed their outer layer of skin periodically in a process called ecdysis. As shedding begins, the skin takes on a dull, faded appearance because the old outer layer is separating from a fresh layer forming underneath. A thin layer of lymph-like fluid accumulates between the old and new skin, loosening the bond. In lizards with transparent eye coverings (called spectacles), this fluid makes the eyes appear whitish-blue for a few days before the shed is complete.
Unlike snakes, which typically shed their skin in one continuous piece, most lizards shed in patches and flakes over several days. The frequency depends on the species, age, and growth rate. Young, fast-growing lizards shed more often because their skin needs to keep pace with their expanding body. Adults may shed only a few times a year. Each shed cycle produces a complete new set of scale surfaces, restoring the waterproof barrier and structural integrity that daily wear degrades.