Reptiles are a diverse group of animals distinguished by the tough, dry covering that protects their bodies. This outer layer is a complex arrangement of specialized, hardened folds of the epidermis, known as scales. They form an overlapping or mosaic pattern over the entire body, creating a continuous, protective barrier. The composition and structure of these scales enable reptiles to thrive and endure harsh conditions.
The Primary Building Block: Keratin
The material that gives reptile scales their strength is keratin, a fibrous structural protein. This protein is also found in human hair and fingernails, but reptiles utilize two primary types within their scales: alpha-keratin and beta-keratin.
Alpha-keratin is the softer, more flexible version, common to the skin of all vertebrates. This pliable material is typically found in the deeper layers of the epidermis and in the flexible hinge regions between the scales. Beta-keratin is a distinct protein found exclusively in reptiles and birds, and it is much harder and less extensible. This rigid form provides the strength and durability for the outer plate of the scale.
Layered Design and Structure
Reptile scales are formed from folds of the epidermis. The outer surface, known as the scale plate, is reinforced with hard beta-keratin, creating a strong external shield. This rigid plate transitions into a thinner, more flexible hinge region between adjacent scales, which is rich in softer alpha-keratin. This alternating hard-and-soft design allows the reptile’s body to bend and move.
In snakes and most lizards, scales overlap one another in a pattern called imbricate, similar to shingles on a roof. In crocodiles and turtles, the scales, often called scutes, are large, non-overlapping plates. These scutes often have bony plates called osteoderms embedded beneath them in the dermis layer, providing extra structural reinforcement. Scale arrangement and texture vary widely, ranging from smooth surfaces that reduce friction to keeled scales that offer rigidity or better camouflage.
Essential Functions of the Scales
One important role of reptile scales is preventing water loss, a significant adaptation for life on land. The thick stratum corneum, bolstered by keratin and waxy layers, creates a nearly waterproof seal that prevents desiccation, especially in dry habitats. The scales also provide robust physical protection against injury and abrasion, acting as armor against potential predators. This defense can be passive, like the interlocking strength of the scales, or active, such as when certain species puff up their bodies to make their spiny scales more formidable.
Scales also assist with movement across various surfaces. The ventral, or belly, scales on snakes are often specialized to grip tiny imperfections on the ground, creating traction for forward propulsion. Modified scale structures, such as the lamellae on gecko feet, allow for powerful adhesion and climbing on smooth surfaces. The scales also play a role in thermoregulation, as their color and texture influence how much solar radiation is absorbed or reflected, helping the reptile maintain its body temperature.
The Cycle of Scale Replacement
Reptiles must periodically replace their entire outer layer in a process known as ecdysis, or shedding, because they cannot simply grow larger scales or repair damaged ones in place. Shedding is necessary to accommodate growth, repair damage, and remove accumulated external parasites. Before the process begins, a new layer of skin forms underneath the old one, and a layer of fluid containing enzymes is secreted to separate the two layers.
The mechanics of ecdysis differ between groups of reptiles. Snakes typically shed their skin in a single, continuous piece, peeling backward from the mouth. Lizards usually shed their skin in patches or flakes over a period of time. Turtles and crocodilians shed their scales and scutes continuously in small pieces rather than in a single event.