Squamates—legged and legless lizards, including snakes—are among the most successful vertebrates on Earth. Found everywhere but the coldest and highest places on the planet, 8,000 species make squamates more diverse than mammals.

Remarkable adaptations in behavior, shape, movement, and feeding contribute to the success of this huge and ancient group.

BEHAVIOR

An animal's ability to sense and respond to its environment is crucial for survival. Some squamates, like iguanas, rely heavily on vision to locate food, and use their pliable tongues to grab it. Other squamates, like snakes, evolved effective chemoreception and use their smooth hard tongues to transfer molecular clues from the environment to sensory organs in the roof of their mouths. This allows them to detect and track prey (even in complete darkness), which they then seize with their teeth.

Squamates, like all other animals, communicate with members of their own species and other organisms. Except for the distinctive vocalizations of geckos, most squamates are silent, yet they get their message across. For example, like walking billboards, chameleons change the color patterns along their flanks to talk to each other. There are as many "languages" as there are species!

Whether being hunted as prey or threatened for territory, animals have ways to protect themselves. Squamates may react to danger by biting, fleeing, inflating their bodies, or gaping (exposing colored tongues and throats). Some play dead, squirt blood from their eyes, or use venom to keep danger at a distance. In extreme danger, some squamates may sacrifice parts of their bodies—a tail, or even large pieces of skin—to distract and escape from predators.

[back to top]

STRUCTURE & FUNCTION

Species within a group can have diverse forms and sizes. Squamates range in size from the Dwarf Gecko, which can stand on a dime, to the extinct Mosasaurus, which grew to 17 meters (56 feet) long. Some squamates have four limbs while many have no limbs at all; some have only back legs, others only front legs; and many intermediate conditions exist. These diverse body plans enable them to move on the ground, in water, and even in the air. For example, geckos have toe pads with nano-hairs that allow them to cling to and move across many surfaces; sea snakes rely on paddle-like tails to traverse oceans; and some squamates have wing-like structures that enable them to glide.

Crypsis is a phenomenon in which an organism's appearance allows it to blend well into its environment. Chameleons, for example, employ crypsis when they rock like a shaking leaf to blend in with vegetation. Some squamates are mimics, like the harmless Campbell's Milk Snake, which resembles a highly venomous Coral Snake. Many brown and gray forest-dwellers are well camouflaged when they sit motionless on the trunks and limbs of trees.

Over 45O species of snakes (yet only two species of lizards) are considered to be dangerously venomous. Snake venom is a poisonous "soup" of enzymes with harmful effects— including nervous system failure and tissue damage—that subdue prey. The venom also begins to break down the prey from the inside before the snake starts to eat it. Venom is delivered through a wide array of teeth. For example, vipers employ hypodermic needle-style fangs that fold inside the mouth when not in use.

[back to top]

HABITATS, ECOSYSTEMS & ENERGY

Adaptations in form enable animals to live in a variety of environments. Squamate habitats range from deserts to rain forests, treetops to underground burrows, and coral reefs to the open ocean.

Most squamates are ectothermic: they use external heat sources to maintain a relatively constant body temperature. Because this conserves energy, some squamates can go for long periods (more than a year in some cases) between feedings. For snakes, that typically means eating something really big. And they do this with no hands! Flexible skulls, elastic jaw ligaments, and remarkable digestive function make this possible.

[back to top]

EVOLUTION

Over the course of biological evolution, species adapt and change over time. All organisms differ among themselves (variation) and pass traits on to their offspring (inheritance). Over many generations, those better-adapted organisms may give rise to new species (selection).

All of the almost 8,000 living squamates can trace their lineage back to one common ancestor that lived at least 200 million years ago. Since that time, many squamate groups have gone extinct and new groups have evolved. Dozens of squamate groups have undergone limb reduction and loss. Limblessness is an excellent adaptation to life underground, where much food is found and predators are few. Losing limbs may have allowed squamates to take advantage of resources unavailable to limbed vertebrates.

Scientists discover new squamate species all the time, and the more we learn, the more new questions about squamate diversity arise. For example, how many lineages of squamates have lost limbs? How many times has venom evolved? How can answers to questions like these help us protect these amazing animals?

[back to top]