Lizards are covered in both skin and scales, as the two are not separate structures. Scales are specialized, highly modified structures that form the visible outer layer of the lizard’s skin, known as the integument. This durable covering allowed reptiles to successfully transition to terrestrial life, differentiating them from the moist skin of amphibians or the hair-covered skin of mammals.
The Composition of Lizard Skin
Lizard skin, or the integument, is composed of two primary layers: the outer epidermis and the inner dermis. The epidermis is the superficial layer that produces the scales and is largely made up of a tough, fibrous protein called keratin. Keratin is produced in two distinct forms: flexible alpha-keratin found in the hinge regions between scales, and harder beta-keratin that forms the rigid plate of the scale itself. This dual-keratin structure provides the necessary balance between armor-like protection and flexibility for movement.
The deeper dermis layer contains connective tissues, blood vessels, nerves, and specialized pigment cells called chromatophores. These pigment cells, including melanophores, xanthophores, and iridophores, give lizards their coloration and pattern. By expanding or contracting these cells, some species can rapidly change their skin color for camouflage or social display. The scales are simply the highly fortified surface of the epidermis, making the skin structure a unified system.
Scales: Specialized Keratin Structures
Lizard scales are not separate, detached plates like fish scales, which are derived from the dermis. Instead, reptilian scales are folds or modifications of the epidermis. These scales are continuous with the rest of the skin, connected by thin, flexible hinge regions containing the softer alpha-keratin. This structure permits the entire body to flex despite the rigidity of the individual scale plates.
The morphology of these scales is highly varied across different lizard species, depending on their habitat and lifestyle.
Scale Morphology
Scales can be smooth and plate-like, often overlapping in an imbricate pattern like roof tiles. They may also be granular, giving the skin a bumpy texture. Some species have keeled scales, which possess a central ridge that may aid in water collection or provide a rougher surface. In some lizards, bony plates called osteoderms develop within the dermis beneath the epidermal scales, providing an additional layer of armor.
Essential Functions of the Outer Covering
The lizard’s tough, keratinized outer covering is responsible for several functions fundamental to its survival in terrestrial environments. A primary function is the prevention of water loss, or desiccation, which was a major evolutionary hurdle for vertebrates moving onto land. The thick stratum corneum, the outermost layer of the epidermis, contains waxes and lipids that create an effective, waterproof barrier against evaporative water loss. This adaptation is particularly important for species living in arid environments.
The scales also serve as physical armor, protecting the lizard from abrasion as it moves through rough terrain and providing defense against predators. Furthermore, the skin plays an indirect role in thermoregulation, or body temperature control. Lizards are ectotherms and rely on external heat sources, often using behavioral methods like basking in the sun to warm up. The color and texture of the scales influence this process, as darker scales absorb heat more quickly than lighter, reflective scales.
The Process of Ecdysis (Shedding)
The durable nature of the lizard’s skin means it cannot continuously grow and expand like mammalian skin. This necessitates a periodic replacement process known as ecdysis, or shedding. Shedding occurs to accommodate growth, repair damage, and remove external parasites. A new layer of epidermis begins to form underneath the old layer before the process begins.
Just before ecdysis, the lizard’s skin may appear dull or cloudy as a fluid-filled layer develops between the old and new skin layers. Unlike snakes, which typically shed their entire skin in one piece, most lizards shed their skin in patches or flakes over days or weeks. Retained shed, especially around the digits or tail tip, can be a concern because the old, dry skin can constrict blood flow and lead to tissue damage. The new skin revealed after shedding is often brighter and more vibrant.