A Matter of Scale

One of the characteristic features of the reptiles is that their bodies are covered with scales. The scales of a reptile may cover the entire body. Reptilian skin, with its waterproof scales, is one of the features of reptiles that makes life on land possible. The scales form a continuous barrier to evaporation. The bodies of reptiles are not wet and slimy like the bodies of fish or amphibians. Reptile skin is dry because reptiles have no skin glands.

Reptile scales are epidermal scales, that is, they arise from the skin. Like feathers, scales are composed of the protein keratin, which is light, flexible, and strong, like the keratin that makes up fingernails.

Reptiles actually have two kinds of scales -- one type more typical of snakes and lizards and the other of crocodiles and turtles. These are different both in their arrangement over the animal's body and in the way they develop.

How do scales develop? They begin as a projection from the skin. Skin has two layers, called the epidermis and the dermis. The epidermis is the top layer.

In snakes and lizards, a scale typically begins as a fold of epidermis, into which part of the dermis projects. See the diagram on the left. As the scale continues to develop, the upper layer of the epidermis becomes hardened with the protein keratin, and the dermis withdraws, leaving a series of tough overlapping scales.

In turtles and crocodiles, most scales do not overlap. In these animals, scales begin as local thickening in the epidermis. Bony plates may also develop in the dermis under the scales. These plates do not necessarily form in the same pattern as the scales above them.

All reptiles grow continuously, although they grow more slowly as they get older. As reptiles grow, their scales get larger. Rather than adding to their old scales, reptiles shed them and replace them with larger scales. The thin surface layer also gets worn by abrasion. Reptiles may shed the whole skin, or shed the skin in large patches, or individual scales may flake off. In turtles, the old scale may remain stuck to the new larger scale below it, so that turtle shells display a series of growth rings (but you can't determine a turtle's age by counting them). In older turtles the smaller inner rings will gradually wear off. When you go to the zoo, look for reptiles that are shedding. What kind of shedding patterns can you observe?

McGregor's Pit Viper with recently shed skin.

Panther chameleon, shedding in patches.

Egyptian tortoise, showing growth rings.

You might be surprised to learn that you shed your skin, too. You don't shed it all at once, although you may shed it in patches if you get sunburned. Your skin is constantly replaced, and the keratinized cells on the surface are shed. Much of the dust that accumulates in your house is shed skin!

Reptiles exhibit as much variety in their scale pattern as birds do in their feathers. Many reptiles have beautiful colors and patterns produced by pigment cells in their skins. Black and brown pigments are synthesized in pigment cells called melanophores. In some reptiles these dark pigments can be moved within the cell from the deep layers of the skin to a position closer to the surface and the animal can change color. What kinds of reptiles can change color? In addition to the melanophores, reptiles also have xanthophores, which store other pigments -- yellow or red. Finally, iridophores contain reflective crystals that produce iridescent blues or metallic colors. Microscopic ridges in the scale itself may also produce a rainbow-like iridescence.

Below are some examples of the different colors that can be seen in reptiles. Can you tell on what kind of animal each of these scales would be found? Look closely at some reptiles with bright colors or strong patterns. Do individual scales have more than one color? Look at shed reptile skins. Are they the same color as the new scales on the animal? If not, what color is the shed skin? What does this tell you about scale colors?

Click here when you think you know who these scales belong to.