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13.3: Amphibians

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

  • Define amphibians and identify their traits.
  • Describe how amphibians reproduce.
  • Outline how amphibians are classified.
  • Explain the roles of amphibians in ecosystem and why they are at risk of extinction.

Lesson Vocabulary

  • amphibian
  • cloaca
  • keratin
  • tadpole


Golden toads, like the one pictured in Figure below, haven’t been seen since 1989. It’s feared that this beautiful and unusual amphibian has gone extinct. It’s just one example of a much larger problem. As many as one-third of all amphibian species are presently at risk of extinction. Reasons include loss of habitat, pollution, and climate change. Amphibians are especially sensitive to such changes. In this lesson, you’ll learn why. You’ll also learn what you can do to help save them.

Golden toad

What Are Amphibians?

Amphibians are vertebrates that live part of the time in fresh water and part of the time on land. They were the first vertebrates to evolve four legs and colonize the land. They most likely evolved from lobe-finned fish. Modern amphibians include frogs, toads, salamanders, newts, and caecilians. They are ectotherms, so they have little control over their body temperature. This allows them to be active in warm weather, but they become sluggish when the temperature cools.

Amphibian Skin

Amphibians have moist skin without scales. The skin is kept moist by mucus, which is secreted by mucous glands. In some species, the mucous glands also secrete toxins that make the animal poisonous to predators. The blue poison-dart frogs in Figure below are a good example. The toxin in their mucus is used by native people in South America to poison the tips of their hunting arrows.

Blue poison-dart frogs

Amphibian skin contains keratin, a protein that is also found in the outer covering of most other four-legged vertebrates. The keratin in amphibians is not too tough to allow gases and water to pass through their skin. Most amphibians breathe with gills as larvae and with lungs as adults. However, extra oxygen is absorbed through the skin.

Organ Systems in Amphibians

All amphibians have digestive, excretory, and reproductive systems. All three of these organ systems use a single body cavity, called the cloaca. Wastes enter the cloaca from the digestive and excretory systems. Gametes enter the cloaca from the reproductive system. A single external opening in the cloaca allows the wastes and gametes to exit the body. (Many other four legged vertebrates also have a cloaca.)

Amphibians have relatively complex circulatory and nervous systems. They have sensory organs for smelling and tasting, as well as eyes and ears. Frogs also have a larynx, or voice box, that allows them to make sounds. The purpose of frog calls varies. Some calls are used to attract mates, some are used to scare off other frogs, and some are signals of distress.You can hear a collection of frog calls at this link: http://animaldiversity.ummz.umich.edu/collections/frog_calls/

How Amphibians Reproduce

Amphibians reproduce sexually. Fertilization may take place inside or outside the body. Amphibians are oviparous. Embryos develop in eggs outside the mother’s body.

Laying Eggs

Amphibians do not produce amniotic eggs with waterproof membranes. Therefore, they must lay their eggs in water. The eggs are usually covered with a jelly-like substance that helps keep them moist and offers some projection from predators. You can see a mass of frog eggs in jelly in Figure below. Amphibians generally lay large numbers of eggs. Often, many adults lay eggs in the same place at the same time. This helps ensure that the eggs will be fertilized. Once eggs are laid, amphibian parents typically provide no parental care.

Frog eggs in a pond

Larvae and Metamorphosis

Most amphibians go through a larval stage that is different from the adult form. In frogs, for example, the early larval stage resembles a fish, as you can see in Figure below. Frogs at this stage of development are called tadpoles. Tadpoles live in the water. They lack legs and have a long tail that helps them swim. They also have gills, which absorb oxygen from the water.

Tadpoles swimming in shallow water

During metamorphosis, the tadpole changes to the form of an adult frog. It grows legs, loses its tail, and develops lungs. All of these changes prepare it to live on the land. In Figure below, you can see how a frog larva looks as it changes to the adult form.

Metamorphosis of a frog larva

How Amphibians are Classified

There are only about 6200 known species of amphibians. They are placed in three orders: frogs, salamanders, and caecilians. Table below shows a picture of an amphibian in each order. It also provides additional information about the orders.

Class Distinguishing Traits Example
Frogs The frog order also includes toads. Unlike other amphibians, frogs and toads lack a tail by adulthood. Their back legs are also longer because they are specialized for jumping. Frogs can jump as far as 20 times their body length. That’s like you jumping more than the length of a basketball court!

red-eyed tree frog

Salamanders The salamander order also includes newts. Salamanders and newts keep their tails as adults. They have a long body with short legs. They are adapted for walking and swimming rather than jumping. Unlike other vertebrates, salamanders can regrow legs or other body parts if they are bitten off by a predator.

smooth newt

Caecilians The caecilian order is the amphibian order with the fewest species. Caecilians are closely related to salamanders. They have a long, worm-like body. They are the only amphibians without legs. Caecilians evolved from a four-legged ancestor but lost their legs later in their evolution. As adults, they often burrow into the soil. That’s one reason why Caecilians tend to be less well known than other amphibians.


Ecology of Amphibians

Amphibians live in freshwater and moist-soil habitats throughout the world. The only continent that lacks amphibians is Antarctica. Amphibians are especially common in temperate lakes and ponds and in tropical rainforests.

Predator and Prey

Amphibians are the prey of many other vertebrates, including birds, snakes, raccoons, and fish. Amphibians are also important predators. As larvae, they may eat water insects and algae. As adults, they typically eat invertebrates, including worms, snails, and insects. You can watch a frog catching an invertebrate in the slow-motion video at the following link. At its real speed, you would barely see it because it happens so quickly.


The Threat of Extinction

Why are so many amphibian species threatened by extinction, and why should you care? The second question is easy. Amphibians control pests, may be a source of new medicines, and help feed many other animals. The nature of amphibian skin may help explain why so many amphibian species are at risk. Their skin easily absorbs substances from the environment, such as pollutants in water or air. Therefore, they may suffer from poor environmental quality before other animals do. As such, they may provide an early-warning system of environmental damage.

What can you do to help save amphibians?

  • Protect the natural environment. For example, reduce your use of energy to curb greenhouse gases and global warming.
  • Avoid the use of garden pesticides. Poisoned insects may be eaten by amphibians that are also harmed by the poison.
  • Make a backyard habitat. A small pond surrounded by native vegetation provides a place for amphibians to live.
  • Help raise awareness. Start a letter-writing campaign to politicians, asking them to support conservation activities for amphibians.
  • For more ideas about what you can do to help save amphibians, check out this website: http://www.amphibianark.org/the-crisis/what-can-i-do-to-help/

Lesson Summary

  • Amphibians are vertebrates that live part of the time in fresh water and part of the time on land. They were the first vertebrates to evolve four legs and colonize the land. They are ectothermic. They have permeable skin and several organ systems.
  • Amphibians reproduce sexually. Fertilization may take place inside or outside the body. Eggs are laid in water. They hatch into larvae that live in the water until they undergo metamorphosis to the adult form.
  • There are only about 6200 known species of amphibians. They are placed in three orders: frogs, salamanders, and caecilians. Frogs include toads and have the most species. Caecilians have the fewest species.
  • Amphibians are the prey of many other vertebrates. They are also important predators. Many amphibian species are threatened by extinction. There are many steps individuals can take to help protect amphibians.

Lesson Review Questions


  1. What are amphibians?
  2. Describe the skin of amphibians.
  3. State how amphibians reproduce and develop into adults.

Apply Concepts

  1. Create a public serve announcement to help raise awareness about the importance of amphibians, their risk of extinction, and what individuals can do to help.

Think Critically

  1. Compare and contrast the three living orders of amphibians.
  2. Explain the ecological roles of amphibians, including both larval and adult stages.

Points to Consider

Amphibians were the dominant land vertebrates until they gave rise to reptiles.

  1. What are some examples of modern reptiles?
  2. How do you think reptiles differ from amphibians?

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