- List reptile traits.
- Describe the general features of lizards and snakes.
- List the characteristics of alligators and crocodiles.
- Describe the traits of turtles.
- Explain the importance of reptiles.
Check Your Understanding
- How have amphibians adapted to living on land?
- What features in amphibians are also useful to reptiles who live in water?
- nictitating membrane
Traits of Reptiles
What reptiles do you know? Snakes, alligators, and crocodiles are all reptiles. Reptiles are tetrapods and amniotes, which means their embryos are surrounded by a thin membrane. Modern reptiles live on every continent except Antarctica. They range in size from the newly-discovered Jaragua Sphaero, at 0.6 inches, to the saltwater crocodile, at up to 23 feet.
There are four living orders of reptiles:
- Squamata, which includes lizards, snakes, and amphisbaenids (or “worm-lizards”).
- Crocodilia, which include crocodiles, gharials (Figure below), caimans, and alligators.
- Testudines, which includes turtles and tortoises.
- Sphenodontia, which includes tuatara (Figure below).
An Indian gharial crocodile.
Reptiles are air-breathing, ectothermic vertebrates that have skin covered in scales. Most reptiles have a closed circulatory system with a three-chambered heart. All reptiles breathe using lungs. They also have two small kidneys. Usually their sense organs, like ears, are well developed, though snakes do not have external ears (middle and inner ears are present). All reptiles have advanced eyesight.
How do Reptiles Reproduce?
The majority of species are egg-laying, although certain species of squamates can give birth to live young. This is achieved either by oviparity (the egg stays in the female until birth), or viviparity (offspring born without eggs). Many of the viviparous species feed their fetuses by a placenta, similar to those of mammals. Some reptiles provide care for their young.
All reptiles have a cloaca, a single exit and entrance for sperm, eggs, and waste, located at the base of the tail. Most reptiles lay amniotic eggs covered with leathery or calcium-containing shells. An amnion (the innermost of the embryonic membranes), chorion (the outermost of the membranes surrounding the embryo), and allantois (a vascular embryonic membrane) are present during embryonic life. There are no larval stages of development.
Most reptiles reproduce sexually, although six families of lizards and one snake are capable of asexual reproduction. In some species of squamates, a population of females is able to produce a nonsexual diploid clone of the mother. This asexual reproduction, called parthenogenesis, also occurs in several species of gecko.
Lizards and Snakes
Lizards and snakes belong to the largest order of reptiles, Squamata. Lizards are a large group of reptiles, with nearly 5,000 species, living on every continent except Antarctica.
Characteristics of Squamata
Members of the order are distinguished by horny scales or shields and movable quadrate bones, which make it possible to open the upper jaw very wide. Quadrate bones are especially visible in snakes, which are able to open their mouths very wide to eat large prey (Figure below).
A corn snake swallowing a mouse.
Characteristics of Lizards
Key features of lizards include:
- Four limbs.
- External ears.
- Movable eyelids.
- A short neck.
- A long tail, which they can shed in order to escape from predators.
- They eat insects.
Vision, including color vision, is well-developed in lizards. You may have seen a lizard camouflaged to blend in with its surroundings. Since they have great vision, lizards communicate by changing the color of their bodies. They also communicate by chemical signals called pheromones.
Adult lizards range from 1 inch in length, like some Caribbean geckos, to nearly 10 feet (Figure below).
A Komodo dragon, the largest of the lizards, attaining a length of 10 feet.
With 40 lizard families, there is an extremely wide range of color, appearance and size of lizards. Many lizards are capable of regenerating lost limbs or tails. Almost all lizards are carnivorous, although most are so small that insects are their primary prey. A few species are omnivorous or herbivorous, and others have reached sizes where they can prey on other vertebrates, such as birds and mammals.
Many lizards are good climbers or fast sprinters. Some can run on two feet, such as the collared lizard. Some, like the basilisk, can even run across the surface of water to escape. Many lizards can change color in response to their environments or in times of stress (Figure below). The most familiar example is the chameleon, but more subtle color changes can occur in other lizard species.
A species of lizard, showing general body form and camouflage against background.
Some lizard species, including the glass lizard and flap-footed lizards, have evolved to lose their legs, or their legs are so small that they no longer work. Legless lizards almost look like snakes, though structures leftover from earlier stages of evolution remain. For example, flap-footed lizards can be distinguished from snakes by their external ears.
Characteristics of Snakes
All snakes are meat-eaters, and are different from legless lizards because they do NOT have eyelids, limbs, external ears, or forelimbs. The more than 2,700 species of snake can be found on every continent except Antarctica and range in size from the tiny, 4-inch-long thread snake to pythons and anacondas that are over 17 feet long (Figure below).
In order to fit inside of snakes’ narrow bodies, paired organs (such as kidneys) appear one in front of the other instead of side by side. Snakes’ eyelids are transparent “spectacle” scales which remain permanently closed. Most snakes are not venomous, but some have venom capable of causing painful injury or death to humans. However, snake venom is primarily used for killing prey rather than for self-defense.
A species of anaconda, one of the largest snakes, which can be as long as 17 feet.
Most snakes use specialized belly scales, which grip surfaces, to move. The body scales may be smooth, keeled or granular (Figure below). In the shedding of scales, known as molting, the complete outer layer of skin is shed in one layer (Figure below). Molting replaces old and worn skin, allows the snake to grow, and helps it get rid of parasites such as mites and ticks.
A close-up of scales on a scarlet kingsnake, showing a tricolored pattern of red, black, and white bands. Notice the distinction between the belly scales and the rest of the snake's scales.
A California snake shedding its skin.
Although different snake species reproduce in different ways, all snakes use internal fertilization. The male uses sex organs stored in its tail to transfer sperm to the female. Most species of snakes lay eggs, and most species abandon these eggs shortly after laying.
How do Snakes Eat?
All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insects. Because snakes cannot bite or tear their food to pieces, prey must be swallowed whole. The body size of a snake has a major influence on its eating habits. The snake’s jaw is unique in the animal kingdom. Snakes have a very flexible lower jaw, the two halves of which are not rigidly attached. They also have many other joints in their skull, allowing them to open their mouths wide enough to swallow their prey whole.
Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by strangling them, and still others swallow their prey whole and alive. After eating, snakes enter a resting stage, while the process of digestion takes place. The process is highly efficient, with the snake’s digestive enzymes dissolving and absorbing everything but the prey’s hair and claws, even when the prey is swallowed whole!
Alligators and Crocodiles
Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to Crocodilia are the closest living relatives of birds. Reptiles and birds are the only known living descendants of the dinosaurs. Think about how organisms with the same ancestors can evolve to be so different.
The basic crocodilian body plan (Figure below) is a very successful one, and has changed little over time. Modern species actually look very similar to their Cretaceous ancestors of 84 million years ago.
Nile crocodiles display the basic crocodilian body plan.
Characteristics of Crocodiles
Crocodilians have a flexible, semi-erect posture. They can walk in low, sprawled “belly walk,” or hold their legs more directly underneath them to perform the “high walk.” Most other reptiles can only walk in a sprawled position.
All crocodilians have, like humans, teeth set in bony sockets, but unlike mammals, they replace their teeth throughout life. Crocodilians also have a secondary bony palate that enables them to breathe when under water, even if the mouth is full of water. Their internal nostrils open in the back of their throat, where a special part of the tongue called the palatal valve closes off their respiratory system when they are underwater, allowing them to breathe.
Crocodiles and gharials (large crocodilians with longer jaws) have salivary glands on their tongue, which are used to remove salt from their bodies. Crocodilians are often seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them down, but it may also have a social function.
Crocodilians are known to swallow stones, known as gastroliths, which help digest their prey. The crocodilian stomach is divided into two chambers. The first is powerful and muscular. The other stomach is the most acidic digestive system of any animal. It can digest mostly everything from their prey, including bones, feathers, and horns!
The sex of developing crocodilians is determined by the temperature of the eggs during incubation (eggs are kept warm before they hatch). This means that the sex of crocodilians is not determined genetically. If the eggs are kept at a cold or a hot temperature, then their offspring may be all male or all female. To get both male and female offspring, the temperature must be kept within a narrow range.
Evolving More Complex Structures
Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than those of other reptiles. As in many other aquatic or amphibian tetrapods, the eyes, ears, and nostrils are all located on the same "face" in a line one after the other. They see well during the day and may even have color vision, but they have excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater.
While birds and most reptiles have a ring of bones around each eye which supports the eyeball, crocodiles lack these bones, just like mammals and snakes. The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive, preventing water from entering their external head openings. The middle ear cavity has a complex of bony air-filled passages and a branching tube.
The upper and lower jaws are covered with "sensory pits," which hold bundles of nerve fibers that respond to the slightest disturbance in surface water. Crocodiles can detect vibrations and small pressure changes in water, making it possible for them to sense prey and danger even in total darkness.
Like mammals and birds, and unlike other reptiles, crocodiles have a four-chambered heart. But, unlike mammals, blood with and without oxygen can be mixed.
Turtles are reptiles in the order Testudines. If you have seen turtles before, what is the most noticeable thing about them? Their shells. Most turtle bodies are covered by a special bony or cartilaginous shell developed from their ribs. About 300 species are alive today, and some are highly endangered. Like other reptiles, turtles are poikilothermic, meaning their temperature changes in response to their environment.
Turtles are broken down into two groups, based on how they bring their neck back into their shell:
- Cryptodira, which can draw their neck inside and under their spine.
- Pleurodira, which fold their necks to one side.
Characteristics of Turtles
Although many turtles spend large amounts of their lives underwater, they can also spend much of their lives on dry land and breathe air. Turtles cannot breathe in water, but can hold their breath for long periods of time. Turtles must surface at regular intervals to refill their lungs.
Turtles don’t lay eggs underwater. Turtles lay slightly soft and leathery eggs, like other reptiles. The eggs of the largest species are spherical, while the eggs of the rest are longer in shape (Figure below).
Most turtles that spend most of their life on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtles can hide from predators in shallow water, where they lie entirely submerged in water except for their eyes and nostrils.
Sea turtles (Figure below) have glands near their eyes that produce salty tears, which remove excess salt taken in from the water they drink.
A species of sea turtle, showing placement of eyes, shell shape, and flippers.
Turtles have exceptional night vision due to the unusually large number of cells that sense light in their eyes. Turtles have color vision.
In some species, temperature determines whether an egg develops into a male or female. Large numbers of eggs are placed by the female in holes dug into mud or sand. They are then covered and left to grow and develop by themselves. When the turtles hatch, they squirm their way to the surface and head toward the water.
How do Turtles Eat?
Turtles have a rigid beak and use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated ridges that help them cut through tough plants.
How Big Are Turtles?
The largest turtle is the great leatherback sea turtle (Figure below), which can have a shell length of 7 feet and can weigh more than 2,000 pounds. The only surviving giant tortoises are on the Seychelles and Galapagos Islands and can grow to over 4 feet in length and weigh about 670 pounds (Figure below). The smallest turtle is the speckled padloper tortoise of South Africa, measuring no more than 3 inches in length, and weighing about 5 ounces.
The leatherback turtle can reach up to 7 feet in length and weigh over 2,000 pounds.
A Galapagos giant tortoise can grow to over 4 ft in length and weigh about 670 lb.
Importance of Reptiles
The chief impact of reptiles on humans is their role as predators of pest species. For example, in many different countries, like India, snakes kill rats that can be pests and carriers of disease. Also, since turtles live for hundreds of years, genetic researchers are examining the turtle's DNA for possible genes involved in a long life.
Reptiles as Food
Reptiles are also important as food sources:
- Green iguanas are eaten in Central America.
- The tribals of Irulas from Andhra Pradesh and Tamil Nadu in India are known to eat some of the snakes they catch.
- Cantonese snake soup is consumed by local people in the fall to prevent colds.
- Cooked rattlesnake meat is commonly consumed in parts of the Midwestern United States.
- Turtle soup is consumed throughout the world.
Reptiles as Pets
Reptiles also make good pets. In the Western world, some snakes, especially less aggressive species, like the ball python or corn snake, are kept as pets. Turtles, particularly small land-dwelling and freshwater turtles, are also common pets. Among the most popular are Russian tortoises, Greek spur-thighed tortoises and terrapins.
Reptiles in Art and Culture
Finally, reptiles play a significant role in folklore, religion and popular culture. The Moche people of ancient Peru worshipped reptiles and often put lizards in their art. Snakes or serpents are connected to healing and to the Devil. Since snake's shed and then heal again, they are a symbol of healing and medicine, as shown in the Rod of Asclepius (Figure below).
In Egyptian history, the Nile cobra is found on the crown of the pharaoh. This snake was worshipped as one of the gods.
The Rod of Asclepius, where the snake is a symbol of healing and medicine.
- Reptiles are air-breathing, cold-blooded vertebrates characterized by a scaly skin.
- Reptiles have a variety of reproductive systems, with different strategies for providing nutrition to developing young.
- Lizards and snakes are distinguished by a unique type of scaly skin and movable quadrate bones.
- Lizards have some unique adaptations, including regeneration of lost limbs or tails and changing color.
- Snakes are distinguished by lack of eyelids, limbs, external ears and vestiges of forelimbs.
- Snakes have various adaptations for killing and eating their prey.
- Crocodilia have a flexible semi-erect posture, lifelong replacement of teeth, and a secondary bony palate.
- The sex of developing crocodilians is determined by the incubation temperature of the eggs.
- Other crocodilian traits, such as salt glands, nictitating membranes, ear flaps, and sensory pits, are adaptations for aquatic living.
- Turtles are characterized by a special bony or cartilaginous shell. They have specialized adaptations for aquatic living, such as eye placement and salt glands, and adaptations for terrestrial living as well, like placement of eyes and protection of eggs.
- Reptiles play important roles as predators of pest species, food sources, pets, in medical and scientific research, and in folklore, religion and popular culture.
1. Describe three general traits of reptiles.
2. Describe two different types of reproduction in reptiles.
3. How are snakes different from legless lizards?
4. Name two adaptations of a crocodilian stomach which help it in digestion.
5. Name two organs that a turtle or a snake have that closely resemble organs in humans. Why are they similar?
6. The shape and structure of a turtle’s shell can give its inhabitant advantages for avoiding predators, aid in swimming and diving, and for walking on land. Given what you know about a turtle’s shell, explain how the structure and shape could help the turtle in the above situations.
Further Reading / Supplemental Links
Points to Consider
Next, we continue our discussions with birds and mammals
- What colorful displays do you think are used to attract mates in birds and mammals?
- How do you think the hearts of fish, amphibians, and reptiles compare to that of birds and mammals?
- How do birds or mammals reproduce? By eggs like fish and amphibians? Or do they have live births?