Amphibians are unique life forms in that they have no
single characteristic that sets them apart, such as hair on
mammals and gills on fish. The name, Amphibian means
"double life" (Duellman 1). The term Amphibian can be
interpreted in two ways, either as an animal spending part
of its life in water and then changing to an aquatic adult,
or as an animal spending its life in and out of water.
Amphibians are surprisingly variable. There are three main
types of Amphibians (Cochran 8). They are caecilians,
salamanders, and anurans. Caecilians are legless, wormlike
amphibians (Vogel 79). They normally live in burrows of
tropical regions. All salamanders have scaleless skin, four
legs and a tail. Anurans are either frogs or toads. They
normally move around in hops.
Many amphibians metamorphose, or change, from a gill
breathing aquatic larvae to an air breathing terrestrial
adults. These amphibians have great abilities. A new born
tadpole carries an aquatic life. After it metamorphoses, it
becomes terrestrial. It is like being born again. The
tadpole learns to travel by means of jumps, as well as to
swim with limbs.
This variability of amphibians makes them one of the most
interesting species to study and indeed amphibians have
contributed a great deal to science. Studies on amphibian
metamorphosis provide scientists with knowledge about the
actions of the thyroid and pituitary hormones. Because of
the ease of their breeding in laboratories and their
relatively simple chromosome complements, important
advances in studies of hybridization and speciation have
been made. The study of vocals of frogs have provided
advances in animal sound communication. The poison from
poison-dart frogs shows great medical promise, especially
for patients who do not respond well to pain killers made
from opium seeds. The poison is far stronger and is
two-hundred times more powerful than morphine, in killing
Amphibians are the oldest known land vertebrates and
accordingly have been ideal subjects to study the process
of evolution. The earliest amphibians existed 360 million
years ago (Radinsky 87). The early amphibians known as
ichthyostegids, had a tail fin and canals on the skull.
These two features suggest that they spent most of their
time in the water.
Between 340 and 250 million years ago, the population of
amphibians grew, and several groups emerged. One group of
amphibians, known as labyrinthodonts, grew to extraordinary
size. They could be up to four feet long. They were
short-legged and large headed. Their skulls were deep and
massive, and their jaws were lined with small, sharp,
conical teeth. Also, there was a second row of teeth on the
roof of the mouth. Many labyrinthodonts had a notch on the
back of the skull which acted as a large eardrum and it
transmitted vibrations to the inner ear. Their bulky
skeleton and their short limbs suggest that the majority of
the labyrinthodonts were slow, clumsy walkers on land, and
they probably spent most of their time in shallow water.
The large jaw and sharp teeth suggest that they were
predators and they fed on large prey.
As some labrythodonts strengthened their limbs as
adaptations for life on land, they reduced the size and
strength of their limbs. The weaker ones developed long,
flexible bodies with weak vertebrae columns. Scientist
interpret this to mean that they had secondarily adapted to
full aquatic life. Their heads flatted which suggested that
they spent much time lying on the floor of a lake or pond.
Some developed long and narrow snouts, which was a good
characteristic to have for traveling through water.
Another grouping of early amphibians were the lepospondyls.
They were mostly small, less than a foot, with long,
slender bodies and weak legs. The lepospondyls lacked the
notch on the back of the skull. They had small sharp teeth
which suggested that they fed on small invertebrates.
A later group, known as the nectrideans evolved more long
bodies. Their weaker limbs, and less stiffened vertebrae
columns suggested that they had almost become completely
aquatic. Like the labrinthodonts, the nectrideans had the
notch on the back of their skull which acted as an ear
A last group called the diplocaulids, developed large,
flattened, triangular skulls, with the dorsal eyes and
nostrils that are normally a characteristic of animals that
live on the floor of a body of water. The study of these
early amphibians provide convincing evidence of evolution.
Most amphibians go through annual periods of mating
(Duellman 19). In anurans, rainfall can cause mating.
Amphibians reproduce sexually. Some amphibians lay their
eggs and let them develop by themselves. Others, lay eggs
and guard them. The mother is usually the one who guards
the eggs. A salamander will guard her eggs by wrapping
herself around them. Some anurans carry their eggs imbedded
in their backs.
All amphibian eggs are basically the same in that there are
layers of semipermeable membranes surrounding the ovum.
However, a lot of differences exist in individual eggs,
such as size. Eggs laid in water form into large clumps, or
they are scattered and deposited at different sites.
Normally, the clumps are attached to sticks or vegetation
in the water. This serves to maintain the position of the
clutch. The terrestrial eggs are sometimes in strands
connected by jelly between each of them. They can also be
their eggs in piles and form nest around them.
Amphibians embryos contain all the nutrients for their
development until hatching. Even if the mother bears live
young, all of the nutrients for embryonic development are
provided by the yolk, not the maternal tissues. Amphibian
embryos normally get oxygen from external gills. In
salamanders, three pairs of external gills provide for
oxygen in take. Amphibian embryos generally remove waste in
the form of ammonia.
The majority of amphibian larvae are aquatic. Unlike the
embryos, amphibian larvae obtain nutrients from the
environment for development and growth. Most larvae feed on
small aquatic invertebrates. Some amphibians eat algae and
continue to feed and grow until there is no more algae
left. Larvae growth rates are dependent mostly on
temperature and food availability. Some chemicals left by
previous larvae can slow growth down. Anurans tadpoles are
mostly short and develop length through growth, in
salamanders the larvae are long.
Most species of salamander and anurans stay in schools as
larvae. They stay in schools to avoid predators and to have
bigger food supply. An organism that is slightly bigger
than the larvae could not attack a school. Some species of
tadpoles stir up the bottom, releasing mixtures of
particles of food.
A metamorphosis is a series of complex physiological,
biochemical, and behavioral transformations. Three major
kinds of changes occur during an amphibian's metamorphosis
(Vogel 83). The first one is the removal of structures and
functions that are significant only to the larvae. The
second kind is transformation of larval structures into a
form suitable for adult use. The last kind is development
of new structures and functions that are essential to the
There are many fundamental as well as subtle changes
throughout the changes of metamorphosis. For example,
during metamorphosis, larger larval red blood cells are
replaced with by smaller adult blood cells. Also, during
metamorphosis, the amphibian gills are completely removed.
In anurans, the intestine reduces in its size during
metamorphosis. Even the vitamins in the eye change. In
larvae, the eye contains vitamin A2, while the adults have
vitamin A1 in the eye. Perhaps, the greatest changes occur
in the respiratory mechanisms. Lungs are formed when the
flow of amino acids and thymidine into the lung tissues is
increased. In the gill tissues, the flow of amino acids and
thymidine is decreased causing them to eventually
Some of the more obvious changes are the removal of the
tail and development of eyelids. Initially, the fins reduce
in size. Then the tail becomes smaller and smaller until it
is removed. The formation of the mouth is very important to
an amphibian. During the development of the mouth and jaw,
the animal is unable to feed. In preparation for this
phase, large quantities of foods are stored.
Metamorphosis is a fascinating transformation, providing
scientists with a greater understanding of the biochemical
and physiological processes.
Most amphibians avoid daytime temperatures and low
humidity. During the day, they usually stay in areas with
high moisture content, and they stay in insulated areas
away from air currents. Inside a log and in mounds of soils
serve as good places to spend the day. Amphibians may come
out during the day, but only if there is a sufficient
amount of moisture. However, they might risk water loss or
even death to accomplish some goal such as feeding or
mating. Amphibians can reduce water loss by reducing the
amount of surface area exposed to evaporation. Some
salamanders coil their bodies tightly to prevent
evaporative water loss. Tree frogs reduce surface area by
selecting a shaded site and tucking limbs close to the
body. Some amphibians dig deep burrows and stay there for
up to nine months at a time. Terrestrial amphibians are
generally nocturnal, with the exception of some species of
The skin of amphibians is highly permeable. Most
salamanders and all frogs that live in aquatic areas have
smooth skin on the belly and sides. Most terrestrial
anurans have rough skin on the belly and the thighs. The
rough stomach surfaces provide a great surface for water
absorption. Even here, the amphibians have proven to be
capable of "double life". For example, sometimes, the
amphibian may not want water to enter the body. For such
occasions, some amphibians have developed ways of
waterproofing the skin. They form a cocoon that encases the
body. They make these cocoons during long periods of
dormancy and during the day. An example would be the
salamander Siren intermedia. Siren intermedia burrows into
the mud at the bottom of drying ponds (Duellman 198). They
make the cocoons to prevent too much moisture from entering
the body. In frogs, the cocoon is made of a dry substance
called statum corneum. It encases the entire frog leaving
openings in the nostrils.
Amphibians generally have temperatures close to that of
their immediate surroundings, and are therefore,
categorized as cold blooded. Amphibians are not capable of
internal heat-production so the body temperature and
environment temperature are about the same. Therefore,
amphibians are tolerant to a wide range of temperature. For
example, some Central American salamanders can stand
temperature between -2.0șC and 30șC. Some anurans can stand
temperatures between 3.0șC and 35.7șC. Also, to raise body
temperature, some anurans lay in the sun; however, this
could create water loss problems, requiring them to balance
The feeding strategies of amphibians include their choice
of prey and the ways they locate, capture, and eat the
prey. All adult amphibians are carnivores. They feed mostly
on insects and, but some eat a wide variety of
invertebrates. Some large anurans, such as Ceratophrys
ornata feed on large prey, such as birds, turtles, snakes,
and other anurans (Vogel 83).
As amphibians grow larger, the kinds of prey they select
may change. As larvae develop teeth, they capture larger
prey. Hylid frogs eat increasingly larger preys, even
Seasonal difference in diets have been reported for various
species of amphibians The diet of anurans living in West
Africa vary greatly throughout the year. Among the thirteen
species of anurans living in Amazonian Peru, the difference
in food was greatest during the dry season. These
differences are indications of the availability of prey.
Some animals are dormant during the dry season. In some
amphibians, the selection of larger prey is likely when
moisture conditions are heavy.
Amphibians vary considerably in their hunting practices.
The vast majority of anurans and salamanders use vision to
hunt prey. For the species that have developed the
sit-and-wait strategy, vision is important. Once a prey is
sighted, it may be followed for a short distance any then
captured. Sight is also important in identifying kinds of
prey, such as those with a large energy content and or that
may be distasteful or harmful. For example, toads learned
to reject bumblebees by sight alone. Amphibians also use
smell and hearing to hunt prey. Some species of toads can
locate prey just by smell alone. Smell sense is of great
value in tracking the prey, once it is located. Some
amphibians can also detect insects by the sounds they make.
A species of toad, Bufo marinus, is attracted to calling
Amphibians also show major differences in ways of capturing
and in taking the food; however, all terrestrial amphibians
except caecilians use the tongue in capturing prey. Many
caecilians and large anurans use fang like teeth to hold
struggling prey. The tongues of amphibians have glands that
produce a sticky substance, that immobilizes the prey.
Terrestrial caecilians feed primarily on long prey, such as
earthworms located on the ground or in burrows. Prey
capture involves a slow approach towards the prey until
contact is almost made, then the prey is captured by a
In terrestrial salamanders, the tongue plays an important
role in prey capture. The salamander's tongue has a sticky
substance produced by a gland. The salamander sticks out
its tongue and the prey gets stuck to it. Then the
salamander pulls the tongue in the mouth. The tongue can
reach up to eight percent of the length its body. The
entire capture a prey lasts 0.10-0.15 seconds.
Anurans flip their tongues at their prey. Like salamander's
tongue, it is has a sticky substance that is produced by a
gland. The prey gets stuck to the tongue and then the
anuran pulls the tongue back into the mouth. The process is
almost exactly the same as the salamander. The completely
aquatic frogs, known as pipids, do not have tongues.
Therefore, they have an entirely different means catching
their food. They suck in food and water, the water leaves
the mouth before it closes completely.
Amphibians are no different than any other animal because
they can be harmed by a wide variety of predators,
parasites, and diseases. Most caecilians, some salamanders,
and some anurans are known to be near the bottom of the
food chain (Cochran 11). Amphibians are also subject to
Amphibians can be harmed by many parasites. While, most of
the time parasites live without harm. Massive infections
over an amphibian population has been known to cause great
disaster. For example, a parasite called Pleistophora which
is normally a parasite of fish, caused a lethal epidemic of
the toad Bufo bufo in southern England (Cochran 56).
Infestations of the parasite, Carchesium have been known to
clog the gills of tadpoles, causing retardation and death.
Amphibians are prey for a great variety of predators
because they are small and they have soft skin. Because of
their "double life", amphibians encounter predators both in
water and on land. The predators include all classes of
vertebrates and some arthropods. Some small anurans are
even prey to the plant specie, Venus flytrap.
Aquatic eggs of amphibians are mostly the prey of fish and
aquatic invertebrates. The leech is the most common
invertebrate predator of the eggs. Some salamanders feed on
each others' eggs. Larval and adult newts also feed on eggs
of some species of anurans and salamanders.
Terrestrial eggs are eaten by a some groups of anurans and
a variety of insects and vertebrates. They include the
spider, cricket, crabs, and snakes. The snake from the
genus Leptodeira can shape their jaw to part of a clutch of
eggs and devour the rest of clutch because the eggs stick
Generally, amphibians have been classified as defenseless
creatures (Duellman 244). However, they have evolved some
features which will provide some protection. Amphibians
have a variety of encounter behaviors. One example of a
characteristic that has been evolved is escape behavior.
Escape behavior is when a prey senses the presence of a
predator and then attempt to leave the area. A terrestrial
caecilian will dig into the soil quickly when it senses a
predator. When an aquatic caecilian meets a predator, it
will spit a small blob of water. Also, it will produce
large quantities of mucus, which make it very difficult to
hold. Some caecilians are capable of inflicting painful
bites and some have poisonous secretions. A salamander will
get into the position known as the Unken reflex. This is a
immobile posture when the chin and tail are elevated. Toxic
skin secretions come from the glands and cover the
salamander. Salamanders also lash their tails at a
predator. Toxic secretions also cover the tail. Some
salamanders also head-butt their predators. They flex the
head downward and lunge at the predator. Most anurans seem
to rely on escape behavior to avoid predators, but some
frogs are extremely poisonous. Many species of amphibians
have color patterns that will match the environment they
live in, thus providing a camouflage
One of the most interesting amphibian is from the anurans
group. Phyllobates terribilis, a specie of poison-dart
frog, produces one of the most toxic non-protein
substances. The poison is so strong that it can be lethal
to touch. Only 55 out of every 135 species of poison-dart
frogs are known to be toxic. In addition, the frogs have
neon colored skin which warns the predators to stay away.
In the Amazon basin, the natives use the poison-dart frogs
to poison their blow gun darts, which they use for their
hunting for food. To poison their darts, they rub the dart
against the frog's skin (Moffet 98). The poison is
effective for more than a year. Poison-dart frogs range
from a half inch to three inches, in size. They are found
in a small area of lowland rain forest in western Columbia,
where they contribute to human beings survival.
Amphibians have provided and continue to provide scientists
with extraordinary opportunities to study and increase
their knowledge. Specifically, amphibians have contributed
to an understanding of the evolutionary process,
particularly as to how physical features and behavior
patterns can be altered to adapt to changing and diverse
Cochran, Doris. Living Amphibians of the World. New York:
Doubleday & Company Inc., 1962
Duellman, William. Biology of Amphibians. Maryland: The
John Hopkins University Press, 1994.
Moffett, Mark. "Poison-Dart Frogs." National Geographic May
1995: Vol. 187
Radinsky, Leonard. The Evolution of Vertebrate Design.
Chicago: The University of Chicago Press, 1987
Vogel, Zdenek. Reptiles and Amphibians. New York: The
Viking Press, 1964