From Sharon High School's AP Biology Class

The Red-eyed Tree Frog, Agalychnis callidryas
The Red-eyed Tree Frog, Agalychnis callidryas

Amphibians (class Amphibia) are a sub-division of four-legged vertebrates comprised of about 4,800 species; their common characteristics include undergoing metamorphosis, being cold-blooded, and laying eggs in water. Their ancestors are considered to be the first tetrapods (vertebrates with two pairs of limbs) to spend a considerable amount of time on land, thus forming the crucial link between fishes and terrestrial reptiles. The word "amphibian" means "two lives" and is a reference to the metamorphosis of many frogs. Amphibians are distributed world-wide, but barely exist in Antarctica and Greenland (10 AN). Most species exist in the tropics; the rest are found in token amounts around the world, except in places with no water or permafrost. (JE)

  • Contents
1 Evolutionary history
2 Diagnostic characteristics
3 Acquiring and digesting food
4 Sensing the environment
5 Locomotion
6 Respiration
7 Metabolic waste removal
8 Circulation
9 Self-protection
10 Osmotic balance
11 Temperature balance
12 Conservation

The origin of tetrapods
The origin of tetrapods

Evolutionary history

Amphibians evolved during the Devonian period over a 50-million-year period from 400 to 350 million years ago. By that time, many plants and arthropods already inhabited land, while large vegetation was beginning to evolve. Terrestrial ecosystems were being transformed. Plants at the edges of ponds and swamps provided a new source of food for fish. Over time, certain fish evolved lungs and limbs, enabling them to live in shallower water and paving the way for breathing and walking on land. After tens of millions of years, the transition from water to land resulted in the first amphibians.

During the early Carboniferous period, adaptive radiation gave rise to a great number of amphibians. Because they were the first terrestrial tetrapods, amphibians had plenty of food and little competition. Their proliferation and diversity has earned the Carboniferous period a nickname: the "age of amphibians." In the late Carboniferous period, however, amphibian numbers and diversity began to decline. Today's species of amphibians in fact resemble species dating back to the Triassic period of 245 million years ago.

Diagnostic characteristics

Class Amphibia can be divided into three orders:
Tiger salamander (5RM)
Tiger salamander (5RM)

Order Urodela (meaning "tailed ones") are salamanders. They number about 500 species. Some live only in the water or on land, but many live in water as larvae and on land as adults. Salamanders are primarily a norther temperate group found in North America, Europe, and Asia. They live in rivers, streams, and ponds; on land, they live under rocks, logs, and moist leaf litter (CH 6).

Blue poison dart frog (5RM)
Blue poison dart frog (5RM)

Order Anura (meaning "tail-less ones") are frogs. They number nearly 4,200 species. Frogs are better adapted to moving on land than urodeles. Most frogs undergo metamorphosis, a process that is the namesake of all amphibians. They are the most dispersed of amphibians (10 AN). Frogs range from deserts to rainforests, and they are found everywhere with the exceptions of Greenland and Antarctica (CH 6).

A family of caecilians (5RM)
A family of caecilians (5RM)

Order Apoda (meaning "legless ones") are caecilians. They number only 150 species. Superficially, caecilians resemble earthworms or snakes. They live underground and are the least explored group of amphibians. Caecilians are found in tropical Africa, Southeast Asia, and in the New World from S. Mexico to N. Argentina. They live in streams, under debris, and along root channels (CH 6).

All amphibians are ectothermic vertebrates with three-chambered hearts. Most respire through lungs, skin, or a combination of both. Their skin is generally smooth and moist, devoid of fur, feathers, and scales. With the exception of Apodans, amphibians are four-legged. Often frogs are distinguished from the group of species commonly called toads, but in fact all toads are actually frogs: members of the family Bufonidae, generally having short hind legs more suitable for walking and drier, warty skin; toads usually prefer drier climates than most frogs. (AR 16)

As their name suggests, one of the most striking characteristics of amphibians is that they live both in water and on land. Unlike other tetrapods such as birds and reptiles, amphibians do not have terrestrially-adapted eggs; that is, their eggs lack a shell and dehydrate quickly in air. As a result, most amphibians live close to water and lay their eggs in ponds, swamps, or other moist environments. Reproduction involves external fertilization in water: the male grasps the female, the female lays her eggs, and the male releases his sperm over them. Because amphibians often lay a huge number of eggs and invest little parental care, mortality is high for their young. They can be said to exhibit a Type III survivorship curve.

Amphibians have a double-channeled hearing system, green rods in their retinas (for differentiating between hues) and two-part (pedicellate) teeth (EG 8).

Another, well-known attribute of frogs is metamorphosis. The tadpole is its legless larval stage; over time, legs develop while gills and the lateral line system disappear. The adult frog possesses lungs, external eardrums, and a digestive system adapted to a carnivorous diet. Take a look, for example, at the life cycle of the North American toad. (6DC)

There are exceptions: some amphibians live only in water or on land, and many do not undergo metamorphosis. Some species show paedomorphosis instead; adults still have gills and other larval qualities.

Acquiring and digesting food

Most tadpoles are aquatic herbivores, but by the time all amphibians have reached their adult stages, their digestive systems are adapted to a carnivorous diet. Amphibians regularly consume insects and other arthropods available in their respective environments, but they are not picky about what they eat. Usually, if something is alive and can fit in their mouths, they will eat it; many frogs and salamanders are known to eat rodents, fish, snakes, birds, and even their own species. Caecilians mostly eat earthworms and other soft-bodied organisms because those are found abundantly in their environments.

Most amphibians hunt at night. Almost all of them lack claws and sharp teeth; in order to catch their prey, frogs and salamanders flick out long, sticky tongues that are attached to the front of their mouths. They then swallow their prey whole, and digestion takes place in the stomach and small intestines. Frogs are an exception. They have teeth along their upper jaw called maxillary teeth, which are used to grind down food before swallowing. (17 AL)

Bullfrogs, like many other amphibians, are not picky eaters.

Sensing the environment

Amphibians, like all organisms, must be aware of the environment in order to obtain food and water and maintain homeostasis. Compared with most organisms, amphibians have a highly developed nervous system. Frogs and salamanders are able to see shapes and colors, although not in great detail. To obtain food, frogs rely on sensing the movement of their prey rather than their shapes or colors. Because caecilians live underground, their sight has become limited to dark-light perception. All caecilians have a pair of tentacles between the eyes and the nostrils, probably to aid in their sense of smell. Amphibians do not have external ears, but they are able to perceive sound because they have well-developed internal ears.


Members of the order Urodela (meaning "tailed ones") have tails, and those that live on land walk with a side-to-side bending of the body that may be a remnant of the habits of early tetrapods.

Tadpoles lack legs altogether, so they swim around by undulating their long tails. Adult anurans (meaning "tail-less ones") lack tails and hop around on land using their powerful hind legs. Anurans are more specialized than urodeles for walking on land.

The frog's hind legs are specialized for jumping (CS 7)

Organisms of the order Apoda (meaning "legless ones") do not have legs and are nearly blind. They resemble earthworms, but they evolved from a legged ancestor. Apodans move through concertina locomotion, much like the way snakes do.

Modes of locomotion varies greatly among amphibians. Limbless caecilians burrow, some salamanders swim, and frogs can jump, hop, burrow, and climb. (15 SC)


Compared to other vertebrates, amphibians have small lungs that do not have a large surface area for gas exchange. Lungs provide a surface for atmospheric oxygen to diffuse into the circulatory system. In addition to or instead of lungs, the moist skin of many amphibians also contributes to respiration, as gases simply diffuse across it into the capillaries under the skin. Modern frogs breathe air through a mechanism called buccal pumping. Rather than using the rib cage to inhale air, frogs use their mouths. First, the nostrils open and the floor of the mouth drops, sucking air in. Then, the nostrils close and the floor of the mouth rises, forcing air into the lungs.

For amphibians that undergo metamorphosis, the larvae that live in water initially have thin, gas-permeable skin (3 VK) and gills, which are outfoldings of the body surface suspended in water. Gills are the surface at which oxygen diffuses from the water into the organism's bloodstream. In addition to these structures, frog-tadpoles use their large tail fins for respiration because the tail fins contain blood vessels and have a large surface area aiding the gas exchange. (3 VK) As the larvae become adults, they lose their gills and develop lungs, enabling them to live on land.

A step-by-step guide to buccal pumping
A step-by-step guide to buccal pumping

Metabolic waste removal

Most metabolic waste must be dissolved in water when removed from the body; therefore, how waste products are removed affect an animal's water balance. The form that waste takes depends on both an organism's evolutionary history and environment.

Ammonia is water-soluble and very toxic; organisms whose wastes take this form must have access to lots of water. It is no wonder, then, that tadpoles and other aquatic amphibian larvae excrete ammonia. Adult amphibians, however, generally excrete urea. Urea is produced in the liver by a metabolic cycle that combines ammonia with carbon dioxide. It is much less toxic than ammonia and can be stored in high concentrations, meaning adult amphibians require less water to dispose of it. To eliminate wastes, amphibians use excretory organs that are found on either side of their spines (2 JSun). For example, the kidneys remove nitrogenous wastes from the blood (2JS).

The amphibian circulatory system; blue represents oxygen-poor blood and red represents oxygen-rich blood


Amphibians have a three-chambered heart, with two atria and one ventricle. One pump of the ventricle delivers blood through a forked artery into two circuits: the pulmocutaneous circuit and the systemic circuit. The pulmocutaneous circuit delivers blood to the gas-exchange organs of the amphibian. In a frog, these are the lungs and skin (hence "pulmocutaneous": "pulmo" for lung and "cut" for skin). Blood in these organs picks up oxygen and releases carbon dioxide, then returns to the left atrium. In the systemic circuit, blood nourishes the body organs and returns to the right atrium. It takes two pumps of the ventricle, then, to circulate blood throughout the organism. The second pump of this double circulation setup ensures that blood flowing through the brain, muscles, and other organs has sufficient pressure.

Having two atria supply blood to one ventricle means that there is mixing of oxygen-rich and oxygen-poor blood from the two circuits. The mixing of blood, however, is not complete; a ridge in the ventricle diverts most of the oxygen-rich blood from the left atrium to the systemic circuit and the oxygen-poor blood from the right atrium to the pulmocutaneous circuit.

The amphibian circulatory system is less efficient than that of mammals, which has a four-chambered heart as well as double circulation. Double circulation can also be contrasted with the single circulation of fish, where blood flows from respiratory organs straight to systemic organs despite losing pressure in the gills.

The poison dart frog, family Dendrobatidae
The poison dart frog, family Dendrobatidae


A variety of carnivores consume amphibians; aside from the usual ways of evading predators, such as fleeing or hiding, amphibians have some rather unique ways of protecting themselves. One such method is through the secretion of toxins onto their skin. The poison dart frogs of Central and South America are one example of an entire family of frogs that produces noxious chemicals. Often accompanying these chemical defenses is aposematic coloration, which involves bright hues that stand out to a predator. Over time, predators learn to associate bright colors with a painful experience and subsequently avoid the frogs. This painful experience can either come from a bitter taste, or poison (7T2).

Also, some salamanders are also known to be able to lose their tails when threatened by a predator. That way, the predator is distracted by the tail and the animal is able to escape.

Another way that some amphibians protect themselves is through cryptic coloration, or camouflage. In this video below, you can see a Leopard frog camouflaged into it's surroundings, making it nearly invisible to predators. (EK)

Osmotic balance

Because water is essential for life to exist, dehydration is one of the greatest challenges that terrestrial organisms face. Amphibians are especially vulnerable to drying out on land due to their smooth, porous skins that lack adaptations like fur and scales. Because of these porous skins, they can take up water three times their body weight per day (7 J. Stein). To retain moisture, amphibians tend to live in damp environments. They modify their behavior to maintain a healthy water balance; if they are losing too much moisture, they will seek out shade and water. Some amphibians, especially frogs, are actually able to secrete a mucus layer to protect the skin from drying out.

When amphibians are in water, they are able to obtain water through their skin due to osmosis, or diffusion of water. Therefore, most amphibians do not actively drink water; they just soak it up.

Temperature balance

All amphibians are ectothermic, meaning they have such low metabolic rates that the amount of heat they generate has little effect on body temperature. In air, most amphibians lose heat very quickly due to evaporation from their moist skin, so maintaining a warm temperature is difficult and crucial to survival. Because they cannot regulate body temperature internally, amphibians modify their behaviors to achieve optimal temperatures, much like they do to achieve optimal water balance. If surroundings are too warm, they may seek out shade; if they are too cool, they may bask in the sun. Species that can secrete mucus from their skin to control water loss also use that mechanism to control evaporative cooling.

Optimum temperature ranges from species to species. Even closely related species of salamanders, for example, prefer very different temperatures, anywhere from 7°C to 25°C.

For amphibians that live in cooler environments, in the winter it is nearly impossible to maintain optimal temperature. Many amphibians enter a period of hibernation underground. Likewise, in the summer, high temperatures and scarce water supplies cause amphibians to enter estivation, a state of dormancy like hibernation.


Worldwide amphibian populations are declining rapidly. One of the reasons is the fatal disease chytridiomycosis, a skin infection caused by the fungus Batrachochytrium dendrobatidis, which can infect many different species of amphibians in all three orders. (DPOD 9) Wetland destruction/degradation is another obvious factor in the decline of amphibian populations, but there are also less obvious factors to this decline. New highways have been built over where frogs and salamanders need to cross to reach ancestral breeding grounds. These creatures are often nighttime mortalities, left on the road as food for scavengers. These losses are not obvious to most people because scavengers often consume the dead amphibians before sunrise. (AW 8)

Review Questions

1. Describe the way in which amphibians eat prey. (10 HL)
2. Through associative learning, predators of amphibians have learned to associate the bright colors of their prey (aposematic coloration) with what amphibious characteristic (MB)?
3. Based on the survivorship curves we have seen during class discussion, what type do amphibians correspond to? Explain. (AS 17)
4. Although amphibians are capable of living on land, why do most live in moist areas or near water? (KA)
5. What do amphibians mainly rely on to sense their enviornment and hunt for prey? (EK19)
6. How do amphibians that live in different temperatures stay at a desired body temperature? (DO)


1. Campbell, Neil A, and Jane B. Reece. Biology. Sixth Edition. San Francisco: Pearson Education, Inc, 2002.
2. "Amphibian (animal)." Encarta. MSN, 1993. Web. 15 Oct. 2009. <>.
3. "Amphibians." Cmassengale. Web. 23 Oct. 2009. <>
4. "Amphibian (animal) -- Britannica Online Encyclopedia." Encyclopedia - Britannica Online Encyclopedia. Web. 25 Oct. 2009.
5. Reaser, Jamie K. "Under Fire." Reptiles Magazine. 25 Oct. 2009 <>.
6. "Amphibians." 2002. 31 Oct. 2009 <>
7. "Osmotic Balance." 1 Nov. 2009 <>.
8. "Reptile and Amphibian Population Decline Report." Grizzly Run. 2005 2 Nov 2009 <>.
10. "Amphibians." Dogs, Cats, Pets, Horses, Pets for Sale, Animal Care Jobs. Web. 04 Nov. 2009. <>.
11. "Frogs and Toads." 4 Nov. 2009. <>.
Sherrie Wang