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13.4: Reptiles

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • List terrestrial adaptations of reptiles.
  • Describe how reptiles reproduce.
  • Explain how reptiles are classified.
  • Identify where reptiles live and what they eat.

Lesson Vocabulary

  • carnivore
  • crocodilian
  • diaphragm
  • herbivore
  • omnivore
  • reptile


Loggerhead turtles spend most of their life in the ocean. Adult female loggerheads go ashore briefly to lay their eggs in the sand. Then they return to the water and leave the eggs to hatch on their own. Figure below shows baby loggerheads on a beach shortly after hatching. The baby turtles must make their way back to the water, hopefully without being snatched up by a predator. Loggerhead turtles are reptiles. Unlike amphibians, turtles and other reptiles can lay their eggs on dry land. That’s because they produce amniotic eggs. Amniotic eggs have waterproof membranes to prevent them from drying out.

Newly hatched loggerhead turtles start crossing the sand to the ocean.

What Are Reptiles?

Reptiles are ectothermic, four-legged vertebrates that produce amniotic eggs. The reptile class is one of the largest classes of vertebrates. Besides turtles, it includes crocodiles, alligators, lizards, and snakes. Although some turtles and other reptiles now live mainly in the water, reptiles evolved many adaptations for life on land. For an amusing overview of reptiles, watch this Bill Nye the Science Guy reptile video: http://www.youtube.com/watch?v=DCLvwk3zhh8.

Adaptations for Life on Land

Reptiles were the first vertebrates to lay amniotic eggs. This freed them from returning to the water to reproduce. In addition to amniotic eggs, reptiles have several other adaptations for living on land. For example, reptile skin is covered with scales. You can see how the scales overlap and cover the snake in Figure below. Reptile scales are made of very tough keratin. They help protect reptiles from injury as well as loss of water.

Like other reptiles, this tree viper snake is covered with overlapping, waterproof scales.

Because of their tough scales, reptiles can’t absorb oxygen through their skin as amphibians can. However, reptiles have more efficient lungs for breathing air. They also have various ways of moving air into and out of their lungs. For example, their chest muscles contract to push air out of the lungs. The muscles relax to allow air to rush into the lungs. Another muscle, called the diaphragm, which lies below the lungs, also helps move air into and out of the lungs. (Mammals also have a diaphragm for breathing air.)

Reptile Organ Systems

Reptiles have a circulatory system with a heart that pumps blood. Reptiles also have a centralized nervous system with a brain. Their brain is relatively small, but the parts of the brain that control the senses and learning are larger than in amphibians. Reptiles have good senses of sight and smell. They use their tongue to smell scents. That’s what the blue-tongued lizard in Figure below is doing. Some reptiles also have a heat-sensing organ that helps them locate the warm bodies of prey animals such as birds and small mammals.

This lizard, called a skink, is flicking out its blue tongue to sniff the air.

How Reptiles Reproduce

Most reptiles have sexual reproduction with internal fertilization. Reptiles have a body cavity called a cloaca that is involved in reproduction. Sperm or eggs are released into an adult reptile’s cloaca. Males have one or two penises that pass sperm from their cloaca to the eggs in the cloaca of a female, where fertilization takes place. In most reptile species, once fertilized the eggs leave the body through an opening in the cloaca. These reptiles are oviparous. Eggs develop and hatch outside the mother’s body.

Young reptiles, like the baby alligator in Figure below, look like smaller versions of the adults. They don’t have a larval stage as most amphibians do. Baby reptiles are able to move and search for food but are at high risk of predation. Adult reptiles rarely provide any care for their offspring once the eggs are laid. The only exceptions are female alligators and crocodiles. They defend their eggs and hatchlings from predators and help them reach the water.

This baby alligator, being held gently by a game warden, looks just like an adult alligator but on a much smaller scale.

How Reptiles are Classified

There are over 8200 living species of reptiles. They are classified in four orders, called Crocodilia, Sphenodontia, Squamata, and Testudines. Table below shows a picture of a reptile in each order. It also provides additional information about the orders. For an online gallery of amazing photos of reptiles, go to this link: http://video.nationalgeographic.com/video/magazine/my-shot-minute/ngm-reptiles-msm

Class Distinguishing Traits Example
Crocodilia Reptiles in the Crocodilia Order are called crocodilians. They include crocodiles, alligators, caimans, and gharils. They have four sprawling legs that allow them to run surprisingly fast. They have strong jaws and replace their teeth throughout life. Crocodilians have relatively complex brains and greater intelligence than other reptiles.


Sphenodontia The Sphenodontia Order includes only tuataras like the one in this photo. They resemble lizards but are the least specialized of all living reptiles. Their brain is similar to the amphibian brain.


Squamata The Squamata order includes lizards and snakes. Lizards have four legs for running or climbing, and they can also swim. Many change their color when threatened. Snakes do not have legs, although they evolved from a four-legged ancestor. They have a very flexible jaw for swallowing large prey whole. Some inject poison into their prey through fangs.


Testudines The Testudines Order includes turtles, tortoises, and terrapins. They have four legs for walking. They have a hard shell covering most of their body.


Reptile Ecology

Modern reptiles live in many different habitats. They can be found on every continent except Antarctica.

Reptile Habitats

Many turtles are aquatic. They may live in the ocean or in fresh water. Other turtles are terrestrial and live on land. All lizards are terrestrial. Their habitats may range from deserts to rainforests. They may live in a range of places, from underground burrows to the tops of trees. Most snakes are terrestrial, but some are aquatic. Crocodilians live in and around swamps or bodies of water. The water may be fresh or salty, depending on the species of crocodilian.

What Reptiles Eat

All reptiles are heterotrophs, and the majority eats other animals. Heterotrophs that eat only or mainly animals are called carnivores. Large carnivorous reptiles such as crocodilians are the top predators in their ecosystems. They prey on large birds, fish, deer, turtles, and sometimes farm livestock. Their powerful jaws are strong enough to crush bones and turtle shells. Smaller carnivorous reptiles—including tuataras, snakes, and many lizards—are lower-level predators. They prey on small animals such as insects, frogs, birds, and mice.

Most terrestrial turtles eat plants. Heterotrophs that eat only or mainly plants are called herbivores. Herbivorous turtles graze on grasses, leaves, flowers, and fruits. Marine turtles and some lizards feed on both plants and animals. Heterotrophs that eat a variety of foods including both plants and animals are called omnivores.

Lesson Summary

  • Reptiles are a class of ectothermic, four-legged vertebrates that produce amniotic eggs. They include turtles, crocodiles, lizards, and snakes. Reptiles were the first vertebrates to live full time on land, and they evolved many terrestrial adaptations.
  • Most reptiles reproduce sexually and have internal fertilization. They lay eggs on land and generally do not provide parental care. Reptile hatchlings look like miniature adults. They lack a larval stage and do not go through metamorphosis.
  • There are over 8200 living species of reptiles. They are classified in four orders: Crocodilia, Sphenodontia, Squamata, and Testudines.
  • Reptiles live in a wide range of habitats. Some are aquatic, but most are terrestrial. Most reptiles are carnivores, but some are herbivores or omnivores.

Lesson Review Questions


  1. What are reptiles?
  2. Identify some of the terrestrial adaptations of reptiles.
  3. Describe how reptiles reproduce.

Apply Concepts

  1. In many parts of the world, beaches are being used for new homes and condominiums. Such beach development may put aquatic turtle populations at risk. Explain why.

Think Critically

  1. Compare and contrast the Crocodilia and Squamata Orders of reptiles.
  2. Explain the ecosystem roles of carnivorous reptiles.

Points to Consider

Birds evolved from a reptile ancestor, but modern birds and reptiles are very different. Birds are now the most numerous four-limbed reptiles on Earth.

  1. How do modern birds differ from reptiles?
  2. Why do you think birds have been so successful?

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Date Created:
Feb 29, 2012
Last Modified:
Aug 16, 2016
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