Scales are small, rigid plates that grow from an animal’s skin, forming part of the integumentary system. They provide an outer layer of protection, essential for survival in diverse environments. These structures are a successful evolutionary adaptation found across multiple classes of vertebrates. The composition and origin of scales vary significantly between different animal groups, illustrating convergent evolution.
Scales of Aquatic Vertebrates
The scales found on fish and other aquatic animals primarily originate from the dermis, the inner layer of the skin, and often contain bone. Cartilaginous fish, like sharks and rays, possess placoid scales, which are structurally similar to tiny teeth. These scales are also known as dermal denticles. They are composed of dentine surrounding a pulp cavity, covered by a hard, enamel-like substance called vitrodentine.
The majority of modern bony fish have either cycloid or ctenoid scales, which are thin, flexible, and overlap to allow for greater body movement. Cycloid scales have a smooth outer edge, while ctenoid scales possess small teeth or spines, called ctenii, along the outer margin. These scales grow throughout the fish’s life, adding concentric rings that can sometimes be used to estimate the animal’s age.
Scales of Reptiles
Reptilian scales, found on animals like snakes, lizards, and crocodilians, are formed from the epidermis, the outermost skin layer, unlike fish scales. They are composed primarily of keratin, a tough structural protein that provides a hard, protective surface. This includes both pliable alpha-keratin and the much harder beta-keratin, which forms the stiff outer layer.
This keratinized outer layer is a key adaptation for terrestrial life, acting as a barrier against water loss and preventing desiccation. The scales on snakes and lizards are overlapping folds of skin with a hinge region between them to allow for flexibility. Snakes and many lizards periodically shed their entire outer layer of skin, including the scales, through a process known as ecdysis, which replaces worn skin and helps to remove external parasites.
Specialized and Modified Scales
The integument of some animals features structures that resemble scales but have different compositions or origins. The most striking example is the pangolin, the only mammal with large, protective, overlapping scales. These scales are made of keratin, similar to human fingernails, and are structurally distinct from the scales of reptiles. In birds, scales are limited to the legs and feet, providing a link back to their reptilian ancestry.
These structures are often technically termed scutes, which are large, plate-like epidermal structures. Crocodilians and turtles also possess scutes, which are large plates often reinforced by bony plates underneath called osteoderms, originating in the dermis. The term scute differentiates these structures from the epidermal scales of snakes and lizards because scutes are formed deeper in the skin and the outer keratin layer is shed piecemeal. The armadillo’s bony armor is another example of dermal armor. The tails of some rodents, such as rats and beavers, also feature small, non-bony scales.
Biological Roles of Scales
Scales serve multiple biological functions that extend beyond simple protection. Protection is a primary role, shielding the animal from predators and physical abrasion. Scales are also instrumental in regulating the movement of water across the skin, a function described as osmoregulation or water retention. This is important for fish maintaining internal salt balances and for terrestrial reptiles preventing desiccation. Furthermore, specialized ventral scales in snakes provide traction for locomotion, and scales can contribute to thermal regulation and camouflage.