Under its thick skin, an iguana has many different organs, including a three-chambered heart. (The human heart has four chambers.) Some of the iguana’s other organs are a well-developed pair of lungs and a pair of kidneys.

Unlike many animals, some iguanas have no urinary bladder. These iguanas turn waste liquid into a whitish substance, which passes out of the body in a solid form. This helps iguanas survive on less water.

Iguanas and other lizards have a special organ within the roof of their mouth. It is called the Jacobson’s organ. This organ may help them tell whether another iguana is male or female. It also helps them test possible foods.

Iguanas also have an opening in the top of the skull covered only by skin and tissue. Beneath this opening is a structure called the “third eye.” This structure seems to send signals to the brain. Scientists think the third eye may help the animal sense how long the day is and figure out how long to sunbathe.

Iguanas eat insects, fruit, flowers, and leaves. Some iguanas will even eat cactus plants.

Most lizards have trouble digesting plants—that is, breaking the plants down into nutrients that their bodies can use. Bacteria, tiny single-celled organisms, in the iguana’s digestive system help it to digest plants. Iguanas do not have these bacteria when they hatch. Scientists think iguanas may get these bacteria by eating the feces (FEE seez, meaning digestive wastes) of adult iguanas.

Iguanas are plant-eaters, but many lizards, such as the Komodo dragon, are meat-eaters. The Komodo dragon even eats animals as large as deer and water buffaloes.

Green iguanas, also known as common iguanas, are found in a range from Mexico to southern Brazil and Paraguay. They may grow up to 6 feet (1.8 meters) long.

Green iguanas live in trees, especially near water. To avoid being seen, they stay very still in tree branches. Their coloring blends in with the branches. If an enemy does attack, these lizards leap out of the trees and hide underwater.

People often eat these iguanas and their eggs. In fact, the green iguana’s Spanish name, gallina de palo, means “chicken of the tree”—a name given to them because they are so tasty.

Also, people often capture these lizards to sell as pets. As a result, common iguanas are in danger of becoming uncommon in some of their wild habitats.

The Galapagos marine iguana lives in the sea. It is the only lizard that does so. This iguana lives on the Galapagos Islands, located off the coast of Ecuador.

Marine iguanas are found along coastal rocks, where they dive underwater to search for algae (AL jee) to eat. Algae are plantlike organisms that do not have stems, roots, or leaves. Another name for algae that grow in the ocean is seaweed.

Marine iguanas end up taking in a lot of salt because the algae they eat grow in salt water. To clear the salt out of their noses, marine iguanas sneeze and snort often. The salt they sneeze out glands atop their heads.

Marine iguanas, which are expert swimmers, must go into the sea for long periods to get enough food. They use their flattened tails to help them swim.

Galapagos marine iguanas can shrink in size by as much as one-fifth when food is scarce. If a change in climate reduces the supply of the lizards’ favorite foods, the lizards may respond by becoming smaller. That way, they do not need as much food. These iguanas can become as much as 2 3⁄4 inches (7 centimeters) shorter “from snout to vent” (a way that scientists usually measure them) when necessary. The animals’ bones actually become smaller during these periods. Marine iguanas can grow back to their larger size again when there is plenty of food.

The marine iguana is the first vertebrate species known to reduce the size of its bones to adapt to changing conditions. Scientists are interested in studying this process because they hope it will help them solve such human health problems as osteoporosis (os tee oh puh ROH sis), a condition in which bones become very fragile.

Iguanas and other lizards are cold-blooded, which means they do not produce their own body heat. They need outside sources of heat to keep warm. The most important of these sources is the sun. So, iguanas spend much of their time bathing in the sunlight.

If it is a cold time of year and their temperature falls too low, the lizards become sluggish. Their body systems begin to slow down, and they must find shelter and hibernate.

On the other hand, if a lizard becomes too hot, it cannot function properly either. It may even die from being overheated. During extremely hot periods, iguanas will seek cooler, shaded places.

Iguanas and other lizards shed their skin as they grow. The outer layer of skin is not expandable, so it cannot grow with the animal.

Many reptiles, including snakes, shed their skin. Most snakes shed their skin in one long piece. But lizards shed their skin in patches and pieces. Some lizards speed the shedding process by pulling on the loose outer skin with their mouth or their claws.

Young iguanas shed their skin more often than older iguanas. The young first shed soon after hatching. They shed again at other times depending on their growth rate, what they eat, the temperature of the air, the amount of moisture in the air, and their health.

When an iguana sheds, it does not lose all of its skin. Like other vertebrates, iguanas actually have layered skin. As the iguana’s outer layer of skin is shed, there is a new layer beneath that is ready to become the outer layer.

When an iguana sticks out its tongue, it is learning about its environment. Taste buds on the iguana’s tongue and in its mouth give the animal clues about its surroundings.

An iguana also learns about its environment by using two tiny cavities (holes) that are located in the roof of its mouth. Together, these cavities are called Jacobson’s organ. The iguana picks up particles from the air and the ground with its tongue and puts them into this organ. The cavities are lined with sensitive cells that aid the sense of smell. Nerves in the cavities send information about the particles directly to the iguana’s brain.

Other lizards and snakes also have the Jacobson’s organ. All these animals use this organ and their tongue to track prey, test food, and identify if another animal is male or female.

Chuckwallas (CHUHK wol uhs) are members of the iguana family. These large lizards are found in rocky deserts in the southwestern United States and in Mexico. Chuckwallas get their name from the word chacahuala, the Spanish name for this lizard.

When chased by a predator, a chuckwalla will run to a crack in a rock or some other narrow opening and wedge itself in tightly by filling its lungs with air. This makes it almost impossible for the predator to pry it loose.

Unlike most lizards, but like other iguanas, chuckwallas are plant-eaters. The chuckwalla’s favorite foods are leaves and flowers.

Chuckwallas are the second-largest lizard in North America. Only the Gila monster is larger. The largest type of chuckwalla can grow to about 24 inches (61 centimeters) long and weigh about 2 pounds (0.9 kilograms).

Anoles (uh NOH leez) are the smallest members of the iguana family. Most grow to about 8 inches (20 centimeters) long. Because of their size and many other differences, many scientists classify anoles into their own separate family.

Most anoles are green or brown and may have brightly colored patterns on their skin. Some can change their color quickly, as a chameleon can.

Like some other lizards, such as green iguanas, male anoles have a large, colorful throat flap called a dewlap. They fan their dewlap when they want to attract females or scare off rival males. Each species of anole fans its flap in a particular pattern.

The smallest lizard species is the dwarf gecko. This tiny lizard was discovered in 2001 on Isla Beata, off the coast of the Dominican Republic. The dwarf gecko is around 1⁄2 inch (some 1.25 centimeters) long.

The common iguana is Iguana iguana; rhinoceros, Cyclura cornuta; marine, Amblyrhynchus cristatus. Iguanas belong to the family Iguanidae.