Kevin Liao

Various types of Amphibians, including frogs, toads, newts, and salamanders
Various types of Amphibians, including frogs, toads, newts, and salamanders
(LW) (16)

Defining Characteristics
Amphibian means "two lives", a reference to the metamorphosis that all amphibians undergo between their larval and adult stages. This metamorphosis is probably most well known in frogs, which have distinct larval and adult stages. Not all amphibians have such distinct stages of life nor are all amphibians amphibious in terms of living both on land and in water.

The process of metamorphosis differs in many amphibian species. In frogs, the length of time it takes for the process to complete ranges from a few weeks to a few months. (Eggs are laid by the mother in a jelly like substance and then are fertilized by the male amphibian. The eggs are fertilized outside of the mothers body. (LJ) (26)) First, the eggs hatch and give rise to tadpoles, which are small aquatic larvae with external gills. At an early stage, the tadpole grows back and front legs. Other morphological (structural) changes follow, such as the development of lungs, growth of a large mouth and tongue, and loss of gills. In addition, the tadpole's digestive tract changes, which explains the tadpole’s shift in diet from a herbivore to a carnivore. Biochemical changes also occur, such as the synthesis of a visual pigment in the eyes and an oxygen-binding hemoglobin protein in the blood. At the final stage, the tail is resorbed. The developing tadpole must be able to leave the water in order to adjust to its new lungs. This stage is the most challenging and dangerous because the developing frog can easily drown. (MT) (8,9)

There are three modern orders of Amphibia, Urodela ("tailed ones") includes salamanders, Anura ("tail-less") includes frogs, and Apoda ("legless ones").
The oderder of Urodela contains about 250 species of news and scalamanders. The sized of Urodeles ranges greatly, and the order contains the largest amphibian in the world: the giant salamander of Japan (it can grow to by about 5 feet!) Urodeles are characterized as having long tials, and short legs. They live in or near the water, and usually spend their time in the mosit soil near a body of water, under rocks or logs. (RG) (21)
Amphibians can go into a state or dormancy during poor conditions. They can then reemerge when the conditions in the environment improve. During these states of dormancy the amphibians bury themselves in mud or leaves. These states of dormancy are called either hibernation or estivation. Hibernation occurs in the winter and estivation occurs in the summer. (12)(MF)
Most frogs undergo metamorposis, a process that is the namesake of all amphibians. They are the most dispersed of amphibians. (KS)

Acquiring and Digesting food
Most amphibians are carnivorous in their adult forms. Many species of frogs use their long tongues to catch insects. Some amphibians have differing diets when they are in their larval states. For example, the larval tadpole is an herbivore while the adult frog is a carnivore.
Most amphibians hunt during the night, the time of day when food is plenty and predators are at a minimum. (SR) (27)Food enters the mouth, where chemical digestion appears to begin, which sometimes contain small useless teeth present in the upper jaw. However, these teeth are and can be used to grind up food before swallowing. (SR) (27)In frogs for example, the tongue is very specialized, for it’s tip is folded backward toward the throat and is very sticky. Because of this, the frog is able to flick out it’s tongue rapidly to grasp and prey. From the amphibian’s mouth, food passes on into the stomach by way of the esophagus. From the stomach, the food moves into the small intestine, where most of the digestion occurs. Large digestive glands found in the liver and the pancreas are attached to the digestive system by ducts. Solid wastes coming from the large intestine, enter the cloaca, whereas liquid wastes actually come from the kidneys and travel through the ureters to the urinary bladder. But similarly, both the liquid and solid waste material exit the body by way of the cloacal vent.(YA)(1)


(ORS 15)

Sensing the environment
Many species of frog use auditory signals to communicate. This could include mating calls or territorial warnings to other frogs. Frogs develop a pair of external eardrums when they grow to their adult forms. Amphibians have also been found to use chemical messaging and pheromones to send out messages and possibly to mate. They have special receptors that over the course of evolution have allowed them to recognize and process these pheromones. (GR)(11)

The croaks we hear by a pond or on a quiet day are made by male frogs. The two reasons male frogs croak are to attract females, as well as to alert other organisms of territory. They croak by forcing air over their larynx, causing the vocal cords to vibrate and thus create the sound. This sound is amplified by the vocal sacs. (CC) (18)

Amphibian's sense of hearing is similar to that of other vertebrates, but is different and relatively advanced. For amphibians, their tympanic membrane, (or "eardrum"), is located not internally, but on the sides of the head. This is perfect for amphibians, for it allows for hearing in both air and water. Amphibians also have a "second" sense of hearing in terms of their superior system of picking up seismic vibrations. A separate pathway can pick up vibrations through the amphibian's skeletal system- allowing for a useful system useful for the avoidance of large predators. (TM)(13)(14)

The sense organs of amphibians vary greatly depending on the classification of the amphibian. We know that frogs have well-developed eyes. Most toad's and salamander's vision are adapted to seeing at night or in areas with low light intensity. Often a salamander's vision is assisted by a well-developed sense of smell. Underground amphibians tend to have a reduced eye structure. Another form of sensing the environment that some amphibians have is pores on their bodies, called lateral line organs, which can sense vibrations in the water. (RL)(23,24)

Urodelans can be aquatic or terrestrial. The ones on land walk on four feet with a characteristic side-to-side swagger.

Anurans are more specialized for land travel with powerful hind legs that are used to hop about. These legs are also useful in the water for swimming.

The mode of locomotion varies among amphibians. Some burrow, using their heads like shovels and gaining trust through contraction of muscles to force the body against the soil.The caecilians that swim, and most swimming salamanders, like eels, use body muscle contractions to propel themselves through the water. Salamanders use their limbs very little in swimming. On land, the limbs give propulsion to the body. Frogs make use of their long hind limbs to hop, jump, swim, burrow, and climb. (LW)(3) The burrowing amphibians also often have horny growths on their feet. Many amphibian legs have special adaptations to improve locomotion. These include sucker disks, which secrete sticky substances that are used by the amphibian to attach to certain surfaces, webbed digits, which aid with swimming, and soft pads to reduce the shock that comes from jumping. Leaf-frogs have opposable first and second fingers, which allows them to move by grasping branches, earning them the nickname "monkey frogs." The webbing of amphibians is used in conjunction with disks on the end of their digits by a group of tree frogs known as "flying frogs" to move through sliding jumps. (CSR, 19, 20)

Amphibians, even those that are terrestrial, rely greatly on their skin to conduct gas exchange. The skin of amphibians has many small capillaries located close to the skin to efficiently exchange gas. Many amphibians also have lungs, but they still depend on their skin for a good portion of their gas exchange. In order to conduct gas exchange, the skin is kept moist at all times. Some amphibians, like frogs, have gills while in the larval stage, which they may or may not lose when they are adults depending on their species.


To help increase the efficiency of respiration through skin, many amphibians, like frogs, have two thin layers of skin - an outer epidermis, and an inner dermis, with an extensive network of capillaries running just underneath. When frogs are submerged in water, they can absorb oxygen directly from the water into their bloodstreams. In addition to this, however, they also have lungs that function much like the lungs in humans: air enters through nostrils, passes through a windpipe, and is received by the lungs. Amphibians with lungs differ from humans in the sense that their breathing is not aided by rib or diaphragm muscles. (ZXU)(2)

View of a frog's lungs and gut (AK) (22)

Metabolic waste removal
Most adult amphibians, along with mammals and some bony fish, excrete urea. Urea requires less dilution and this helps the amphibian conserve water. Urea requires more energy to make than ammonia, but it is 10,000 times less toxic to the organism, so it can be stored for a longer period of time. It requires less water to remove than ammonia, but it is still a significant source of water loss for the organism. (SM)(6)

Amphibians have a three-chambered heart. The amphibian heart has two atria, one to receive blood returning from the body and one to receive blood returning from the sites of gas exchange (lung and skin capillaries), and one powerful ventricle to pump blood. The single ventricle pumps blood to the lungs and skin capillaries as well as to the body. This allows for a more vigorous flow of blood to the body organs compared to the two chamber hearts of fish since the blood is pumped again after losing pressure in the lung and skin capillaries. However, some mixing of oxygen poor and oxygen rich blood does occur.
The heart is the only organ within the coelem which has its own protective covering. This is the pericardium. Oxygen-laden blood and oxygen-poor blood containing waste gasses are present together in the Amphibian ventricle at all times. The oxygen-laden bloods and the oxygen-poor bloods do not mix. Such mixing is prevented by a unique arrangment of the Amphibian heart. Instead of being perched on top of the ventricle, the right atrium dips downward into the ventricle. This causes oxygen-poor blood entering the right atrium to pass all the way down to the bottom of the ventricle. The oxygen-laden blood enters the left atrium and enter the same single ventricle. The pool of oxygen-poor blood at the bottom of the ventricle holds the oxygen-laden blood up and prevents it from sinking to the bottom. When the oxygen-poor blood flows from the ventricle into vessels leading to the lungs, the oxygen-laden blood tries to follow it. The lung vessels, however, are filled. This foreces the oxygen-laden blood to enter the arteries and from there it goes to the body tissues. (LW) (17)

General Diagram of an Amphibian Three Chambered Heart

Self Protection
To avoid predation, many amphibians have skin colorations that camouflage. Others secrete distasteful or even poisonous mucus from the skin. Usually those with poison also have bright coloration to warn predators to keep away.
Amphibiams have some cool ways of surviving. One is flash coloring. Like an animal that is brightly colored to warn of poison, this animal too is brightly colored, but not until it is in danger. An amphibian that has the ability to flash will quickly change colors to confuse its predator and make a quick escape. Some amphibians play dead, scream, and bite. Frogs, toads, and some salamanders can change colors to blend in with their surroundings. Just as some can change their color to blend in, some animals choose to stand out as a warning that they are posionous. (NG)(4)
"Amphibians also use cryptic coloration, aposematic coloration, batesian mimicry, and mullerian mimicry to avoid predation. Many use cryptic coloration to blend in with their environments and make themselves harder to be noticed. Others use aposematic coloration to warn off predators. Aposematic coloration is used in the two types of mimicry by many amphibians to help "teach" predators quickly to avoid brighly colored organisms or to "trick" predators in the case of batesian mimicry." (SD)

Osmotic balance
Amphibians, even those that live only on land, are sensitive to moisture levels since they rely on their damp skin to carry out some or all of their gas exchange. Because of this, amphibians spend a good deal of time either in water or in a damp environment to maintain their required moisture. When in dry environments amphibian kidneys will try to accommodate by reabsorbing more water. They will also excrete more water when in an excess of water.
Animals whose internal osmotic concentration is the same as the surrounding environment are considered osmoconformers, whereas those that maintain an osmotic difference between their body fluid and the surrounding environment are osmoregulators. Freshwater animals (all osmoregulators) include invertebrates, fishes, amphibians, reptiles, and mammals. The freshwater animals are generally hyperosmotic, containing a higher concentration of salts or other dissolved materials than normal tissues, to their environment. The problems that they face because of this are that they are subject to swelling by movement of water into their bodies owing to the osmotic gradient, and they are subject to the continual loss of body salts to the surrounding environment (which has a low salt content). They way these animals deal with these problems is to produce a large volume of dilute urine. The kidney absorbs the salts that are needed, and the rest of the water is excreted. Another way these animals deal with lack of salt is by obtaining it from the food they ingest. A key salt replacement mechanism for freshwater animals is active transport of salt from the external dilute medium across the epithelium into the interstitial fluid and blood. Amphibian’s skin and fish gills are active in this process. Freshwater animals tend to take in water passively and to remove it actively through osmotic work of kidneys (in vertebrates) or kidney-like organs (in invertebrates). (10)(ZS)

Temperature Balance
All amphibians are cold blooded or ectothermic. Due to their moist skin, amphibians lose heat rapidly to evaporation. Although some species of bullfrog can regulate the thickness of their skin mucus to conserve heat, most amphibians can only compensate by moving to a warmer location when needed

Because amphibians are ectothermic, their body temperatures change with the temperature of their surroundings. In cold weather, amphibians become sluggish and reduce their level of activity. Much amphibian activity is built around preventing its body temperature from becoming too hot or too cold. Some species lay in the sun to maintain a warmer body temperature. In warmer climates, amphibians are more active at night, rather than during the day, to prevent excessive heat or water loss. These amphibians, during the day, will hide under rocks or moist places underground. Some amphibians, during extreme hot or cold temperatures, will go into a hibernation-like sleep. For example, frogs remain dormant underground when the temperature gets extremely hot. (SI) (5)

Review Questions:

Explain the three-chambered heart of Amphibians? (SR)
Explain the process of metamorphosis. [MS]
Explain how the structure of the amphibian tongue relates to its function. (NI)
How does amphibian respiration compare and contrast to human respiration? (AK)
Describe and compare 2-3 ways amphibians protect themselves. (AC)

Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings. (1,2) (3) (4) (5) (8) (9) (10) (11) (12)(MF) (13) (14) (15) (ORS) (16) (LW) (17) (LW) (18) (CSR, 19) (CSR, 20) (RHG, 21) (AK, 22) (RL, 23)
Stebbins, Robert C., and Nathan W. Cohen. A Natural History of Amphibians. Princeton, NJ: Princeton UP, 1995. Print. (RL, 24) (25) (26) (27) (SR)