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Toads and Frogs
AMPHIBIANS WITHOUT TAILS
The toads and frogs that lived before the Jurassic period probably had long bodies and
long tails. Biologists believe that this body form changed suddenly. The most obvious change
was the disappearance of the tail in the adult. Other, less obvious changes made these animals
better suited to life on the land. Their hind legs developed an extra joint. And their ankle bones
became longer. These changes gave their legs great jumping power. Front legs stayed short.
But this is suited for taking up the shock of landing from a jump. Modern frogs and toads have
wide mouths. Their sticky front-hinged tongues can catch insects with lightning speed. Frogs
and toads have existed for over 200 million years. They live in many places all over the world.
Biologists consider them among the most successful vertebrates.
Frogs and toads are similar in structure. But frogs differ from toads in many ways in
their anatomy and behavior. Of all amphibians, the toad is most able to survive on land. It
leaves the water early in life. It only returns to water to lay eggs. The toad starts life as a small
black tadpole. The tadpole soon grows legs and resorbs its tail. It hops onto land as a small
brown creature with warty skin. Adults of the common toad, Bufo, are usually red-brown on the
top and gray-yellow underneath. There is no truth to the old tale that people catch warts from
toads. Warts are a viral condition that has nothing to do with toads.
Toad
Leopard Frog
Toads often live in areas with loose, moist soil. They dig in and hide from enemies and
summer heat. They can dig quickly with their hind legs. If a toad is unable to bury itself when
disturbed, it may crouch low to the ground and remain still. The color and texture of its skin
provide good camouflage. But poison glands in the skin are a toad’s best defense. These
glands secrete an irritating, bad tasting substance. This causes most animals to leave the toad
alone. But this defense does not keep snakes from eating the toads. Snakes are the toad’s
major enemy.
The toad has been called the “gardener’s friend.” Toads feed upon insects, worms, and
other forms of food harmful to plants. Toads can easily bury themselves in moist, loose garden
soil. It is probably this soil that attracts toads to gardens.
Unlike toads, frogs usually live very near water. You often see them around ponds.
The most common frog in the United States is the leopard frog. These frogs live in almost every
pond, marsh, and roadside ditch. They often travel a long way from water. Sometimes they can
be seen hopping through grassy meadows. They are grayish-green with large dark spots
surrounded by yellow or white rings. This coloration gives them their name. The soft underbelly
of the leopard frog is creamy white. This makes it hard to see the frog when it is in the water.
Its back color blends in with a grassy pond. And its belly blends with the sky when seen from
below.
The bullfrog is names for its loud bellowing sound. It is the most aquatic of frogs. It
seldom leaves the water except to sit on the bank of a lake at night. The color of bullfrogs
ranges from green to nearly yellow. But most of them are greenish-brown. The underbelly is
gray-white with dark splotches. Bullfrogs are excellent swimmers because of their large,
webbed hind feet. Their legs are strong and well developed. Their legs can be 25 centimeters
long. Bullfrogs eat insects, worms, crayfish, and small fishes. Big bullfrogs will sometimes even
eat a small duckling.
The tree frogs of the genus Hyla are interesting amphibians. Most of them have
amazing protective coloration. Several of them have the ability to change their color. Some
members of the genus live in trees. These frogs have a sticky disc on each toe. This enables
them to cling to vertical surfaces. The spring peeper is a member of the genus Hyla. This frog
lives in swamps and bogs rather than in trees.
Peeper eggs are laid in early spring. The tiny tadpoles feed on algae and protozoans.
But the adults help us by eating mosquitos and gnats. A swamp or bog will be filled with the
noise of peepers on a spring or summer night.
THE ECONOMIC IMPORTANCE OF FROGS
A major part of a frog’s diet is insects. This makes frogs valuable because they help
control the insect population. Many states have laws to regulate frog hunting for this reason.
These laws forbid the capture of frogs during the breeding season.
Bullfrogs hind legs are thought to be a food delicacy. They are raised for this purpose
on farms in marshy areas. Smaller frogs are often used as fish bait. Frogs are a favorite in
biology labs as specimens for dissection. The frog’s internal organs have the same basic
arrangement as the human’s. Thus, dissecting frogs is an excellent introduction to human
anatomy.
Anatomy of the Frog
THE EXTERNAL STRUCTURE
The frog’s body is short, broad, and angular. It lacks the streamlined shape of fishes.
For this reason, frogs do not swim as well as fish do. And the frog’s hopping is not as graceful
as the movement of most land animals. But the frog is able to move about in both of these
environments.
Frog skin is thin, moist, and loose. It is richly supplied with blood vessels. Glands
in the skin secrete mucus. This reaches the skin’s surface through tiny tubes. This skin slime
makes frogs hard to hold. Frog skin lacks protective growths such as the scales of fishes and
reptiles.
ADAPTATIONS OF THE FROG’S LEGS
The frog’s front legs are short and weak. Each foot has four inturned toes with soft,
rounded tips. These feet are not webbed and are not used for swimming. Instead, the front
legs are used to prop up the body when on land. They also break the fall when the frog lands
from a jump. The inner toes, or thumbs, are enlarged on male frogs. These thumb pads
become even larger during the breeding season for males.
The frog’s hind legs are well developed and strong. They are adapted for both
swimming and jumping. The thigh and calf muscles are very powerful. The ankle area and
the toes are long. This forms a foot that is longer than the lower leg. A flexible web membrane
connects the five long toes. This webbing makes each foot a very efficient swimming organ.
When a frog rests on land, its hind legs fold against its body. In this position, a frog is ready to
jump very quickly.
THE HEAD
Frog eyes are very noticeable because they bulge up above the head. The colored iris
surrounds an elongated black pupil opening. Muscles attached to the eyeball rotate the eye in
its socket. The eyes can be pulled into the sockets and pressed against the roof of the mouth.
This helps hold food in the mouth. When a frog’s eyes are pulled down, the upper and lower
eyelids close.
The frog can float just below the water’s surface with only its bulging eyes showing. This
allows the frog to see the surface when hiding in the water. The frog has a third eyelid called
the nicitating membrane. This joins the lower lid. This thin covering keeps the eyeball moist
when a frog is on land. It also protects the eye when the frog is under water.
The nostrils are forward near the top of the head. This allows the frog to breath air when
all but the top of its head is under water.
Frog have no external ears. But they do have eardrums or tympanic membranes.
These are located on the body surface just behind the eyes. The cavity of the middle ear lies
just below the tympanic membrane. Males tympanic membranes are usually darker in color
especially during mating season. A canal connects each middle ear with the mouth cavity.
These canals are called the Eustachian tubes. The inner ears are embedded in the skull.
THE MOUTH – AN INSECT TRAP
The frog’s mouth extends from ear to ear. If you watch a frog catch a fly, you will see
why the mouth is so large. It is a trap for insects. The frog’s thick tongue is attached to the floor
of its mouth at the front. This sticky tongue has two projections at its free end.
To catch an insect, a frog opens its mouth wide and flips its tongue over and outward. If
the aim is good, the insect is caught on the tongue surface and is thrown into the mouth. The
mouth snaps shut quickly. Then the frog swallows the insect. This all happens so fast it is hard
to see. Two teeth project from bones in the roof of the mouth. They are called vomerine teeth.
They aid in holding the prey. Small, cone-shaped maxillary teeth project from the upper jaw.
These also help hold on to prey. Frogs have no teeth on their lower jaws.
Frog’s tongue is well adapted for
Catching insects in that it is both
Flexible and sticky. Note how it is
Attached at the front of the mouth.
The frog’s mouth. Its relatively large size is an
adaptation for obtaining food.
There are various openings inside the frog’s mouth. Internal nostril openings are located
in the roof on either side of the vomerine teeth. Far back on the sides of the roof are openings
of the Eustachian tubes. Openings to the vocal sacs are located at the back of the floor of the
mouth of the male frog. When a male frog croaks, air is forced down these openings into the
vocal sacs. This air forces the sacs to expand between the frog’s ears and shoulders. This
action makes the croak louder and more powerful. When a frog croaks under water, air is
forced from the lungs, over the vocal cords, into the mouth and then back to the lungs. The
throat has two openings. The large gullet openings leads to the stomach. Below the gullet
opening is the slitlike glottis. The glottis leads to the lungs.
THE DIGESTIVE SYSTEM
Adult leopard frogs usually feed on insects and worms. But they can swallow even
larger meals because of their large, elastic gullet. The short gullet leads to a long stomach, an
enlargement of the food tube. The upper end of the stomach is large, and it tapers at the lower
end. At a point called the pylorus, the stomach links up with the coiled, slender small intestine.
At the lower end of the stomach, there is a muscle called the pyloric valve. This valve controls
the movement of food from the stomach into the small intestine.
The small intestine is looped several times. It is supported by a fanlike membrane called
the mesentery. The anterior area of the small intestine, which curves from the pylorus, is the
duodenum. The middle portion is the coiled ileum. The lower end of the small intestine leads
into a short, broad colon. The colon is also called the large intestine. The colon opens into a
cavity called the cloaca. Tubes from the kidneys, the urinary bladder, and the sex organs also
open into the cloaca. Waste materials and eggs or sperms pass out the cloaca through the
cloacal opening.
Tiny gastric glands, in the stomach walls, secrete gastric fluid. Gastric fluid chemically
digests some of the food. A large, three lobed liver partially covers the stomach. The liver
stores digested food products, it also secretes bile and acts as a digestive gland. The
bile collects in the gall bladder, which lies between the middle and right lobes of the liver. From
the gall bladder, bile runs through the bile duct into the upper part of the small intestine. A
second digestive gland, the pancreas, lies inside the curve of the stomach. Pancreatic fluid and
bile pass through the common bile duct into the small intestine. All of the these fluids are
necessary for digestion. Mucous glands in the walls of the stomach and intestine secrete
mucus. Mucus lubricates the passage for food.
Basically, a frog’s digestive system is like those of other vertebrates. It is a long food
tube, generally called the alimentary canal. This canal has special regions for digestion and
absorption of digested food. Increased length of the alimentary canal increases the general
efficiency of these processes.
THE RESPIRATORY SYSTEM
You now know that adult frogs are air breathers. Have you ever wondered how they can
stay under water for a long time? And during the winter, a frog will lie buried in the mud on the
bottom of a pond to hibernate. The skin of the frog is thin. And the skin has a lot of blood
vessels. Thus, the frog’s skin directly absorbs dissolved oxygen from the water. It also
gives off carbon dioxide. As long as a frog stays quiet, this type of respiration is enough to keep
it alive. During hibernation, the frog’s body processes slow down. Therefore, its oxygen need is
very low. But when a frog is active and swimming, it needs more oxygen. The frog then comes
to the surface and breathes air.
We inhale and exhale air by increasing and decreasing the size of our chest cavities.
This is done by moving the ribs and diaphragm. The diaphragm is a muscular partition at the
bottom of the chest cavity. The frog has no diaphragm, and thus no chest cavity. The frog
does not even have ribs. Instead, frogs change the volume and pressure of air in their
mouths. When a frog lowers the floor through the open nostrils. Then, when the floor of the
mouth springs up, air passes out through the nostrils.
The lining of the mouth is also adapted for respiration. It is thin, moist, and has many
blood vessels. Frogs can perform both mouth-breathing and lung-breathing. They may
pump air in and out of their mouths for some time without using their lungs. When the lungs are
used, the nostrils are closed by skin flaps as the floor of the mouth rises. The glottis opens and
air enters the windpipe, or trachea. This leads into the lungs. Then, with the nostrils still closed,
the mouth is trust down. This causes air to pass from the lungs into a partial vacuum. Then a
sudden upthrust of the mouth forces air back into the lungs. After exchanging air between
mouth and lungs a few times, the frog returns to mouth-breathing.
Frogs use their lungs only to assist mouth-breathing. As you might expect, frog lungs
are small compared to lungs of higher vertebrates. Frog lungs have thin-walled sacs that lack
the spongy tissue our lungs have.
THE CIRCULATORY SYSTEM
The circulatory system of the frog is a step more complex than that of the fish.
This represents a step toward the higher vertebrates. One of these advances is the heart. The
frog heart has three chambers. There are two atriums and a muscular ventricle.
Deoxygenated blood flows into the right atrium from various parts of the body. When the lungs
are used, oxygenated blood from the lungs flows into the left atrium. Both atriums contract at
the same time. This forces blood into the ventricle. The ventricle then contracts and pumps
blood out of the heart. The blood leaves through a large vessel that lies against the front side of
the heart. This is called the conus arteriosus. This large vessel immediately divides into two
branches, the right and left truncus arteriosus. Each of these again branches into three arches.
The anterior pair are the carotid arches. These carry blood to the head. The middle pair are the
aortic arches. They transport blood around the right and left sides of the heart. They join below
the liver to form the dorsal aorta. This great artery carries blood to muscles, the digestive
organs and other parts of the body. The posterior pair of arches are called the pulmocataneous
arches. They carry blood to the lungs, skin, and mouth.
The blood that returns to the frog’s heart, after a trip through the body, has lost most of
its oxygen. This blood is loaded with carbon dioxide and other cell wastes. Three large veins
carry blood back to the heart. These are called the venae cavae. They join a triangle-shaped,
thin-walled sac, called the sinus venosus, at the back of the heart. The empties into the right
atrium.
Part of the blood, returning from lower parts of the body flows through the vessels of the
digestive system. There it absorbs digested food. This blood then flows through the hepatic
portal vein which carries it to the liver. The liver picks up some of the food substances carried
by this blood and screens out some of the wastes. From the liver, this blood moves on to the
right atrium.
During each complete circulation, some blood passes through the kidneys. The kidneys
remove some water and nitrogen-containing wastes from cell activity. Pulmonary veins carry
blood from the lungs to the left atrium. This blood is oxygenated when the frog is using its blood
in air-breathing.
Frogs have a three-chambered heart. As you may remember, the fish heart has
only two chambers. Blood passes through the fish heart only once in its trip around the
body. But frog hearts receive blood from both the body and the lungs. And the frog heart
pumps blood to the head and the body, as well as to the various respiratory organs.
THE EXCRETORY SYSTEM OF THE FROG
Frog skin is a vital excretory organ. Most of the carbon dioxide leaving the blood
passes through the frog’s skin rather than through the mouth or lungs. The liver, too,
removes certain wastes. The liver eliminates these wastes with bile or changes them
chemically so the kidneys can remove them. The large intestine eliminates undigested food and
other wastes. But the kidneys are the main excretory organs.
The kidneys are large, dark red organs. They lie on either side of the spine against the
back body wall. Blood flows into kidneys, through the renal arteries and out through the renal
veins. The kidneys filter wastes and collect it as urine. The urine then flows to the cloaca
through tiny tubes, called ureters. Urine may be excreted immediately. But frogs are also able
to force urine from the cloaca through a small opening that leads to the urinary bladder. Here, it
may be stored for a while before being excreted.
THE NERVOUS SYSTEM
The frog brain is more highly developed that the fish brain. Olfactory lobes lie at the
anterior end of the brain. The long lobes of the cerebrum are proportionally larger than those of
the fish. Next are the optic lobes. Behind the optic lobes is the cerebellum. This is small band
of tissue lying at right angles to the long axis of the brain. The medulla oblongata lies at the
back of the frog brain. It joins the short, thick spinal cord. The spinal cord extends down the
frog’s back. The spinal cord is encased in bony vertebrae. As in the fish, spinal nerves branch
from the cord to various parts of the body. In addition, ten pairs of cranial nerves extend from
the brain.
Dorsal and Ventral View of the Nervous System
THE REPRODUCTIVE SYSTEM
In both sexes, the frog’s sex organs are internal. Thus, it is hard to tell the sexes apart,
except during the breeding season. At this time, the thumbs of the males are enlarged and the
“ears” are darker.
Male and female frogs mating. The mass of eggs
In the water indicates external fertilization.
The male reproductive organs are two oval testes. The testes are a creamy-white or
yellow color. They lie in the back, one on each side of the spine, in the anterior region of the
kidneys. Sperm cells develop in the testes. The sperms then pass through tubes, the vasa
efferentia, into the kidneys. When the sperms are discharged, they pass through the ureters
into the cloaca. Some frog species have an enlargement, the seminal vesicle, at the base of
each ureter.
In the female frog, the eggs develop in a pair of large lobed ovaries. The ovaries lie
along the back above the kidneys. During the breeding season, the eggs enlarge and burst
through the thin ovary walls. This frees the eggs into the body cavity. The abdominal muscles
work the eggs toward the anterior. Here funnel-like openings to the oviducts gather the eggs.
Oviducts are long and coiled, and are lined with ciliated cells. The cilia fan the eggs into the
oviduct openings. Oviducts lead into the cloaca. Near the openings to the cloaca, the walls of
the oviduct secrete a jellylike substance. This substance surrounds each egg. There is a
saclike uterus at the base of each oviduct. The eggs are stored in the uterus until they are laid.
Frog Development
FERTILIZATION AND DEVELOPMENT OF THE EGGS
The female leopard frog lays up to 200 eggs between early April and mid-May. When
the eggs are to be laid, the male clasps the female. The male uses its large thumb pads to
press down on the female. This is called amplexus. It helps the female to expel the eggs. As
the eggs pass from the female’s cloaca into the water, the male spreads sperm over them. The
sperm cells reach most of the eggs with this direct external fertilization.
The jelly-like coat that surrounds each egg swells up in the water. This binds the eggs
together in a round mass. In this clump, the eggs look like small beads covered by transparent
jelly. The jelly-like coat protects the eggs from injury and makes it hard for fish to eat
them. It also helps to keep the eggs at a constant temperature. Later, the covering
serves as food for the young tadpoles.
A frog egg is partly black and partly white. The white part is the yolk, a stored food
material. This nourishes the tadpole during development. The dark part is the living protoplasm
of the egg and a dark pigment. The yolk is heavier than the protoplasm. So frog eggs float with
the dark side up. The dark pigment absorbs heat from the sun. The lighter lower half blends in
with the light from the sky. This makes it hard to see the eggs from below. After eight to twenty
days, depending on the weather and temperature, the leopard frog tadpoles hatch. They then
wiggle away from the egg mass.
THE METAMORPHOSIS OF THE FROG- FROM TADPOLE TO ADULT
The hatched tadpole is a tiny, short-bodied creature. It has a little round mouth. It clings
to the egg mass or to a plant. Yolk stored in the body nourishes the tadpole until it starts to
feed. Soon the body grows longer and three pairs of gills develop at the outside of the head.
The tail grows longer and develops a caudal fin. Then the tadpole’s mouth opens, and it begins
to scrape the leaves of water plants with its rough lips.
Not long after the tadpole starts to swim around, its rough lips disappear. A long, coiled
digestive tract develops and the tadpole starts to eat the scum of water plants. Then a flap of
skin grows over the gills, leaving only a small opening on its left side. This allows water to pass
out of the gill chambers. At this stage, the tadpole is like a fish. It has a lateral line, fin, twochambered heart, and a one-circuit circulation. It also has a relatively long intestine coiled in a
spiral.
The change to an adult frog is amazing. First, hind legs appear on the tadpole’s fishlike
body. The front legs start to form at about the same time. But they remain hidden under the
tadpole’s fishlike operculum for a while. When the front legs do appear, the tadpole starts
resorbing its tail. The tadpole does not shed or eat the tail.
Near the end of the metamorphosis, the mouth broadens and teeth develop. As these
external changes happen, internal changes also occur. A saclike chamber, resembling the
swim bladder of a fish, forms behind the tadpole’s throat. This divides into two sacs, which
become the lungs. The heart develops three chambers. The gill arteries change into the
carotids, aortic arches, and pulmoncutaneous arteries. Soon the gills stop functioning and the
tadpole starts swimming to the water’s surface to gulp air. At this time, the tadpoles thin skin
and broad, flat tail still play important roles in respiration.
Even before the tail is completely resorbed, the tadpole starts moving out on the land.
From this point, the tadpole is considered a young frog. The young frog usually takes about a
month to become a full-grown frog.
The metamorphosis of the leopard frog takes from two to three months. Adult leopard
frogs usually appear around July first. Bullfrogs usually spend two winters as tadpoles. And it
may be three years before the adult bullfrog is fully formed.
REGENERATION IN AMPHIBIA
Many amphibians, especially salamanders, can regrow injured or lost body parts. A foot,
a part of a limb, or the tail may be lost in escaping from an enemy. Regeneration can occur
rapidly. In their tadpole stages, frogs and toads also have this capability. But, as the tadpole
matures, the capability disappears. No adult frogs and toads can regenerate body parts.
Frog Behavior
HIBERNATION AND ESTIVATION IN THE FROG
Like fishes and reptiles, frogs are cold-blooded vertebrates. This does not mean
that the blood is always cold. It means that the body temperature varies with the
temperature of the external environment.
With the coming of fall, a frog’s body temperature drops with the temperature of the air.
Soon the frog’s body is too cold to remain active. This is when it buries itself in mud at the
bottom of a pond. Its heart slows down and blood hardly circulates in the vessels. Because it is
so inactive at this time, the frog needs very little oxygen. Enough can be supplied through the
moist skin. Body tissues stay alive by the slow oxidation of stored food. The food is stored in
the liver and in the fat bodies above the kidneys. Nervous activity stops almost completely.
The frog lies in its burrow in a stupor. This is the condition of the frog during hibernation.
Hibernation is a winter rest.
With the coming of spring, the water gradually warms. The frog’s body activities speed
up. And soon the frog wakes from its winter sleep. After a while, it begins the activities of a
normal life again.
The hot summer months bring other problems for the frog. Lacking a way to cool its
body, the frog must escape from extreme heat. It may lie quietly in cool, deep water. Or it
may bury itself in mud at the bottom of a pond. This inactivity during the heat of summer is
called estivation. A small pond may dry up entirely during a mid-summer hot spell. Then the
frog has to bury itself in the mud and estivate. Cool weather eventually returns, and the pond
fills with water again. The frog comes out of estivation and continues normal activities until
hibernation.
SUMMARY
The vertebrate time scale shows an interesting history for amphibians. According to our
reading of fossils, the earliest amphibians date back to the close of the Devonian Period, often
called the Age of Fishes. It was probably then that the early ancestors of amphibians, the
lungfish and lobe-finned fishes, crawled out of the primal seas onto the edges of the land. This
move offered amphibians a new environment. They no longer had to compete with fish. Thus,
amphibians suddenly began to flourish in the Carboniferous and Permian periods. But during
the Mesozoic Era, other vertebrates appeared who were more completely adapted to life on
land. These newer creatures challenged the amphibians and greatly reduced their numbers.
Today there are not many amphibians compared to other land animals, especially the
mammals.
Amphibians are limited to moist areas because they need water to lay eggs and develop
into adults. They represent a kind of “in-between” stage in the evolutionary process. They live
in two worlds, water and land. But fish surpass them in the water. And other vertebrates
surpass them on the land.